Pigs were chosen deliberately. As a biomedical model, swine share striking anatomical and metabolic similarities with humans, particularly in cardiovascular physiology, body fat distribution, and digestive function. Human diet studies are notoriously difficult to control, because participants cheat, under-report, and vary wildly in adherence. Fifteen post-pubertal, non-pregnant, non-lactating gilts, Landrace by Yorkshire crosses averaging around 170 kilograms, were individually penned and paired by weight before being randomly assigned to one of two diets. Seven animals received a conventional, nutrient-balanced maintenance ration built around corn, while eight received a low-carbohydrate, high-fat formulation in which beef tallow supplied the energy normally delivered by grain. Soybean hulls were added to the high-fat diet to match the physical consistency of the control ration, and both diets were formulated to meet National Research Council requirements for swine of this age, sex, and reproductive stage.

The laboratory analysis confirmed the diets differed dramatically. The control diet contained roughly 3.3 percent crude fat and nearly 69 percent non-fiber carbohydrate on a dry matter basis, while the experimental diet contained 22.5 percent crude fat and only about 15.6 percent non-fiber carbohydrate, with neutral detergent fiber soaring to more than 44 percent thanks to the soy hulls. Interestingly, the calculated gross energy of the two rations diverged: the control diet provided approximately 3,993 kilocalories per kilogram, while the high-fat formulation came in at 3,448 kilocalories per kilogram, a discrepancy the authors attribute largely to the indigestible fiber fraction used to dilute the diet. Both rations were fed daily at 1.75 percent of each animal’s body weight, and feed refusals were weighed to calculate actual intake.

The results on body composition were the study’s most eye-catching finding. Control gilts gained weight steadily across the trial, ending roughly seven percent heavier than they started, while the high-fat group maintained a nearly constant body weight throughout. More striking was what happened beneath the skin. Measured by ultrasound adjacent the tenth thoracic vertebra, subcutaneous fat depth fell by 17.7 percent in the high-fat animals over 32 days, while control animals showed a 14 percent increase. At slaughter, the differences inside the body were even more dramatic: control gilts carried 1.74 times more visceral fat, the metabolically dangerous internal belly fat, than their high-fat counterparts, 2,660 grams versus 1,440 grams on average. Crucially, the high-fat animals did not waste away; they simply stopped depositing fat while presumably continuing to build lean tissue, an outcome the researchers interpret as a favorable shift in the ratio of muscle to fat.

Appetite appears to have played a central role. Although both groups were offered the same amount of feed relative to body weight, the high-fat gilts consumed significantly less, averaging 2.36 kilograms per day between days 19 and 32 compared with 3.09 kilograms in the control group. Calculated energy intake told the same story, with the control animals consuming more than 12,300 kilocalories daily against roughly 8,150 for the high-fat group. The authors suggest the animals reached satiety sooner and stopped eating, consistent with human trials reporting appetite suppression on ketogenic diets. They also caution, however, that the enormous soy hull load in the experimental diet may have contributed to early fullness or restricted digestible energy, a confound that future work should disentangle by examining fiber and fat effects on total tract digestibility directly.

Blood chemistry offered reassurance on one of the diet’s most feared risks: acidosis. Blood pH did not differ between treatments, and fasted values for sodium, bicarbonate, and carbon dioxide remained within normal reference ranges, indicating that four weeks on the high-fat regimen did not disrupt acid-base homeostasis. Urine ketone testing revealed that nutritional ketosis was largely absent, with no detectable ketones in the third week and only trace amounts, about 5 milligrams per deciliter, in three animals during the fourth. Yet the body composition changes occurred anyway, suggesting a metabolic shift away from fat storage that operated independently of measurable ketosis. One unexpected trend did emerge: fasting glucose in the high-fat group crept upward over the trial, ending significantly higher than controls at day 32, a pattern the authors tentatively link to early keto-adaptation and the liver’s efforts to protect glycogen reserves when adequate dietary protein fuels gluconeogenesis.

The microscopic analysis of the pancreas produced perhaps the most provocative data. Using immunofluorescent staining for insulin on formalin-fixed tissue sections, the researchers found that the percentage of active beta cells in the high-fat group was 2.81 times greater than in controls, 90.7 percent versus 42.4 percent, and that the area occupied by beta cells within the pancreatic islets was 1.81 times larger. Islet area as a percentage of total pancreatic tissue was also more than doubled. These findings suggest an enhancement of pancreatic architecture and insulin-producing capacity, which stands in contrast to earlier rodent studies reporting that long-term ketogenic diets can shrink beta-cell mass and impair glucose tolerance. The discrepancy may reflect species differences, the short feeding duration, or the moderate degree of ketosis actually achieved.

In skeletal muscle, the story was more selective. The researchers quantified insulin receptor presence by measuring the relative fluorescence intensity of antibody-labeled receptor proteins in cross-sections of the longissimus thoracis, the semimembranosus, and the kidney. Only the semimembranosus, a powerful muscle of locomotion in the hind limb, showed a statistically significant increase in insulin receptor density in the high-fed animals, with relative intensity of 32.8 versus 21.7 in controls. The longissimus showed a numerical but non-significant rise, and the kidney showed no difference. The authors propose that upregulated insulin receptors on muscle fibers may contribute to a muscle-sparing effect, supporting the anabolic influence of insulin on myofibrillar protein synthesis while ketone bodies spare amino acids from being diverted into gluconeogenesis. Notably, proximate analysis found no difference in crude fat content of the liver or longissimus muscle between groups, and although the high-fat animals had heavier livers relative to body weight, there was no biochemical indication of early fatty liver disease.

The researchers are candid about their limitations. The trial used only one sex, mature females, because intact males were unavailable and castrated males are considered irrelevant to human medicine, restricting generalizations to women. The ex vivo tissue analyses relied on just three biological replicates per group, and the 32-day feeding period is short by dietary-intervention standards; some human evidence suggests full metabolic benefits of ketogenic adherence may require twelve weeks or more. Statistical tendencies, including the glucose trajectory and the energy balance calculation, should be viewed with caution given the small sample size. Still, the authors argue the data provide preliminary justification for pursuing the metabolic impact of low-carbohydrate, high-fat diets on obesity using swine as a model, particularly the observation that animals lost fat mass without losing body weight.

For the millions of people experimenting with ketogenic eating, the study offers a cautiously encouraging signal wrapped in important caveats. In a tightly controlled animal model, replacing carbohydrate energy with animal fat reduced the most harmful fat depots, preserved body weight, enhanced markers of pancreatic insulin-producing capacity, and increased insulin receptor expression in a major locomotor muscle, all without measurable ketosis and without disturbing blood chemistry. Whether the same remodeling occurs in humans over months and years, and whether the fiber dilution that accompanied the high-fat ration contributed to the appetite effects, remains to be determined. What the pig study demonstrates is that the diet’s effects on body composition and insulin biology are real, measurable at the tissue level, and more nuanced than the popular narrative of ketosis alone would suggest.

Subject of Research: Effects of a low-carbohydrate, high-fat diet on muscle insulin receptor concentration and body composition in a swine biomedical model

Article Title: Does a low‐carb, high‐fat diet change muscle insulin receptor concentration and body composition in a swine biomedical model?

Article References: Acosta‐Castellanos, N., Tiede, S., Byrd, C. J., Borowicz, P., Young, J. M., & Berg, E. P. (2026). Does a low‐carb, high‐fat diet change muscle insulin receptor concentration and body composition in a swine biomedical model?. Physiological Reports, 14(19), Article e71124. https://doi.org/10.14814/phy2.71124

Image Credits: AI Generated

DOI: 10.14814/phy2.71124

Keywords: ketogenic diet, low-carbohydrate high-fat diet, insulin receptor, body composition, swine model, pancreatic beta cells, visceral fat, beef tallow, nutritional ketosis, skeletal muscle, metabolism, Physiological Reports

The problem the team set out to solve is rooted in the architecture of blood itself. Whole blood is a bi-compartment system: red blood cells occupy a large fraction of blood volume and harbor an enormous reservoir of electrical charge, while the plasma carries the electrolytes clinicians actually measure. The erythrocyte membrane, however, is permeable to water and to selected strong ions, chiefly chloride and lactate. When pH shifts, or when hemoglobin loads or unloads oxygen, these ions migrate across the membrane, changing plasma SID without any net change in the total charge pool of the blood. Such redistribution is a purely physicochemical event, not a primary acid–base disturbance, yet failing to recognize it can lead clinicians to misdiagnose a patient’s acid–base status entirely.

Earlier work by the same group had produced a simplified equation predicting how pH-driven electrolyte shifts alter plasma SID, based on the titratable charges of albumin and hemoglobin, or on the whole-blood buffer power. That model improved bedside interpretation of base excess, but it carried a critical assumption: fully oxygenated blood. In real clinical life, oxygen saturation is anything but constant. Venous blood sampling, extracorporeal gas exchange, and acute changes in pulmonary gas transfer all produce large swings in hemoglobin oxygen saturation, and each swing triggers the Haldane effect through two distinct mechanisms. First, oxygen binding lowers the dissociation constant of certain weak amino acid residues on hemoglobin, reducing its buffering capacity. Second, oxygenated hemoglobin holds chloride less tightly, encouraging chloride and water to move out of the red cell and into the plasma.

To quantify these saturation-dependent effects, the researchers re-analyzed data from an in-vitro study in which venous blood from 20 healthy volunteers, aged 32 to 65, was equilibrated with room air. The design was elegantly simple. Nine-milliliter venous samples were divided into three aliquots: one left undiluted, and two mixed with a buffer solution at varying blood-to-buffer ratios, generating a broad range of hemoglobin and albumin concentrations. Each sample was analyzed for acid–base variables, then exposed to room air through repeated aspiration and rotation over roughly twenty minutes, a procedure that simultaneously oxygenated the blood and washed out carbon dioxide. After equilibration, all samples were re-analyzed, and plasma albumin was measured by centrifugation and immunoassay.

The results were striking. Equilibration with room air raised the oxygen saturation fraction by a median of 0.46, dropped carbon dioxide tension by 29 mmHg, and lifted pH by 0.25 units. Alongside these changes, plasma SID fell by a median of 5.3 mEq/L, driven by both a decrease in strong cations and an increase in strong anions, with sodium declining, chloride rising, and ionized calcium and lactate shifting as well. The drop was more pronounced in less diluted samples, hinting that the protein content of blood, particularly hemoglobin, governs the magnitude of the redistribution. Linear mixed-effects models confirmed that changes in oxygen saturation, changes in carbon dioxide tension, and the whole-blood buffer power were each independently associated with the change in SID.

With these determinants established, the team built and tested predictive equations. An extended model accounted first for pH-dependent charge changes on albumin and hemoglobin, then added a pK-dependent saturation component reflecting oxygen’s effect on hemoglobin’s buffering groups, and finally a chloride-dependent component capturing oxygen’s reduction of hemoglobin’s chloride affinity. Bland–Altman analysis showed that considering pH effects alone underestimated SID changes, with the bias worsening as oxygen saturation rose. Adding the pK-related effect improved matters; adding the chloride-related effect brought the mean bias to just −0.4 mEq/L, with limits of agreement from −2.0 to 1.3 mEq/L, and completely eliminated the dependence of the error on saturation change. In other words, both arms of the Haldane effect must be accounted for to predict plasma SID accurately.

Recognizing that intensive mathematics is impractical at the bedside, the researchers also distilled their findings into a remarkably simple formula. Using a rounded whole-blood buffer power of 1.5 times the sum of hemoglobin and albumin concentrations in grams per deciliter, plus a rounded saturation coefficient equal to hemoglobin concentration divided by four times the change in oxygen saturation, the expected change in SID can be computed with mental arithmetic. This simplified model slightly overestimated SID changes, with a mean bias of 0.8 mEq/L, but the error no longer grew with larger saturation shifts, and the limits of agreement, −0.9 to 2.4 mEq/L, remained within the range considered clinically acceptable. Over the observed range of SID changes approaching 10 mEq/L, the maximal discrepancy between predicted and measured values was roughly 2 mEq/L, comparable to the analytical variability of routine electrolyte measurements.

The simplified equation also carries mechanistic insight. Its rounded saturation coefficient closely matches correction factors proposed decades ago by Siggaard-Andersen and later adopted by Zander to adjust whole-blood base excess for measured oxygen saturation, lending independent support to the new calculations and informing ongoing debate about arterio-venous differences in base excess and SID. The slight overestimation of the simplified model stems from its assumption of a pH-invariant buffer power: in the alkalotic range reached during the experiment, buffer power actually declines as pH moves away from the pK of histidine residues, an effect the extended model captures but the rounded coefficient does not. Conversely, the extended model omits phosphate species, which likely explains its progressive underestimation in undiluted blood rich in hemoglobin and albumin.

The clinical implications could be immediate. Emergency medicine often relies on venous blood gas analysis, in which measured oxygen saturation differs substantially from arterial values, before any arterial catheter is placed. The authors demonstrate with a real clinical case that only when both pH- and saturation-dependent redistribution are considered does the venous sample yield a correct acid–base diagnosis, one later confirmed by arterial blood gas analysis. Neglecting these shifts risks misdiagnosis and potentially harmful management decisions. The study does carry limitations: magnesium and phosphate were not measured, the shared buffer coefficient may falter when albumin and hemoglobin concentrations are extremely imbalanced, and the saturation coefficients were derived from healthy volunteers, so extrapolation to disease states requires caution. Global desaturation in respiratory failure may also dilute SID changes across the entire extracellular space, producing smaller plasma effects than the in-vitro model predicts. Still, the core message stands: the hidden choreography of ions between red cells and plasma, long recognized qualitatively, can now be predicted with a few numbers and a moment of arithmetic, turning a source of diagnostic confusion into a quantifiable, manageable variable.

Subject of Research: Quantifying hemoglobin oxygen saturation effects on plasma strong ion difference via the Haldane effect

Article Title: Quantifying the effects of hemoglobin saturation on plasma strong ion difference during blood oxygenation and decarboxylation

Article References: Giosa, L., Krbec, M., Halamík, J., Bolnberger, A., Busana, M., Brusatori, S., Diemer, T., Kristensen, S. R., Heldeweg, M. L. A., Zadek, F., Rees, S. E., & Duška, F. (2026). Quantifying the effects of hemoglobin saturation on plasma strong ion difference during blood oxygenation and decarboxylation. Physiological Reports, 14(19), Article e71118. https://doi.org/10.14814/phy2.71118

Image Credits: AI Generated

DOI: 10.14814/phy2.71118

Keywords: strong ion difference, Haldane effect, hemoglobin oxygen saturation, acid–base physiology, electrolyte redistribution, red blood cells, chloride shift, buffer power, blood gas analysis, base excess, critical care, physiological Reports

The research team, based at Brigham and Women’s Hospital, followed thirty wild-type pups from three litters through four developmental milestones: postnatal day 0 (P0), day 8 (P8), day 21 (P21), and eight weeks of age. Using the VisualSonics Vevo F2 high-frequency ultrasound system with transducers operating between 25 and 71 MHz, the investigators captured images of hearts beating at rates of 400 to 600 beats per minute with exceptional spatial and temporal resolution. At birth, when a pup weighs barely more than a gram and its left ventricle measures roughly 1.5 millimeters across, imaging was performed without anesthesia or hair removal, with animals gently restrained on a heated platform. From P8 onward, a standardized isoflurane protocol maintained heart rates above 400 beats per minute while body temperature was held at 37 degrees Celsius, minimizing the hemodynamic variability that anesthesia can otherwise introduce.

The structural findings chart an astonishing trajectory of growth. Body weight rose from 1.25 grams at birth to nearly 21 grams at adulthood, while the left ventricular end-diastolic volume expanded almost tenfold, from about 6.4 microliters to 59 microliters. Ventricular walls thickened in parallel, with the systolic anterior wall more than doubling from birth to weaning and continuing to grow into adulthood. Perhaps most intriguing was the behavior of body-weight-indexed left ventricular mass, which surged during the early postnatal and juvenile periods before declining toward adult values. This transient overshoot, the authors note, mirrors the well-described postnatal transition in which cardiomyocytes shift from proliferative expansion to hypertrophic growth, with the heart briefly outpacing the body before mass and body weight rescale proportionally at maturity.

Counterintuitively, conventional indices of pumping performance declined steadily across development. Ejection fraction fell from 69 percent at birth to 60 percent at eight weeks, and fractional shortening followed a similar downward path. Rather than signaling weakening hearts, the authors explain, this decline reflects geometry and loading: as the ventricular cavity expands disproportionately relative to stroke volume, and as systemic afterload rises with maturation, volumetric efficiency measures naturally decrease even as the heart muscle itself grows stronger. Crucially, the strain measurements told a different story. Global circumferential strain, endocardial circumferential strain, and global radial strain all remained statistically stable from P8 through adulthood, indicating that the intrinsic contractile machinery of the myocardium is established early and preserved intact through the storms of growth and remodeling.

Strain analysis, enabled by speckle-tracking software that follows the natural acoustic markers within heart tissue frame by frame, quantifies how much the ventricular wall deforms during each beat. Longitudinal strain captures shortening along the base-to-apex axis, circumferential strain measures shortening around the chamber’s circumference, and radial strain records wall thickening toward the cavity. Because these indices depend on the coordinated behavior of individual contractile units rather than on chamber geometry, they are considered exquisitely sensitive detectors of early dysfunction, often flagging impairment in disease models long before ejection fraction budges. The one strain parameter that did change was myocardial global longitudinal strain, which became significantly more negative, from minus 8.8 percent at P8 to minus 11.9 percent at P21, suggesting selective maturation of the longitudinal contractile pathway during the juvenile period.

Diastolic function, the heart’s capacity to relax and fill, matured along its own timetable. From P8 onward, early and late transmitral filling velocities climbed steadily, with the E/A ratio peaking at day eight before stabilizing. Tissue Doppler velocities at the mitral annulus, including the early relaxation velocity e-prime, the late atrial contraction velocity a-prime, and the systolic annular velocity s-prime, all increased progressively into adulthood. Right ventricular function, tracked through tricuspid annular plane systolic excursion, nearly doubled from 0.40 millimeters at P8 to 0.78 millimeters at eight weeks. Left atrial areas grew in step with the rest of the heart, though fractional area change fluctuated without a consistent directional trend. These trajectories align with known postnatal refinements in calcium-handling proteins such as SERCA2a and ryanodine receptor 2, and with the metabolic switch from glycolysis to fatty-acid oxidation that newborn cardiomyocytes undergo shortly after birth.

One of the study’s most striking discoveries emerged when the data were split by sex. Most conventional parameters showed no sex-specific developmental patterns, but strain imaging revealed a clear divergence in late adolescence. By eight weeks, males exhibited significantly more negative longitudinal strain and higher radial strain than females, and systolic annular velocity showed the same pattern, with males reaching 21.5 millimeters per second versus 17.3 in females. Stroke volume also displayed a significant time-by-sex interaction. The timing is telling: the divergence between P21 and week eight coincides with pubertal maturation and the surge in circulating androgens, and it is consistent with prior work showing that isolated cardiomyocytes from adult male mice contract more vigorously than those from females, likely through testosterone-driven modulation of calcium handling and myofilament function.

Notably, this mouse pattern inverts the human picture, where women typically show greater myocardial strain values than men. The authors point to fundamental species differences as likely explanations: adult mouse ventricles predominantly express the fast-kinet ics alpha myosin heavy chain, whereas human ventricles express the slower beta isoform, and the tenfold difference in resting heart rate between the species fundamentally alters contraction-relaxation dynamics. Sex differences in cardiac function are also dynamic across the lifespan; aging studies suggest the male advantage seen at eight weeks may reverse later in life, with females maintaining better contractility into old age. For researchers translating murine findings to human cardiology, the message is unambiguous: sex- and species-specific normative values are essential, and extrapolating adult or human reference ranges to neonatal and juvenile mice risks misjudging both disease severity and therapeutic efficacy.

The study also functions as a technical manual for a notoriously difficult measurement. The authors document a systematic 12.3 percent overestimation of ejection fraction by short-axis M-mode derived from the Teichholz formula compared with B-mode Simpson’s monoplane volumetry, a bias that persisted at every developmental stage and that they recommend accounting for, particularly in disease models with regional wall motion abnormalities. They further emphasize maintaining normothermia in neonatal pups, which cannot thermoregulate and are prone to hypothermia-induced bradycardia, transferring maternal scent to gloved hands before handling to prevent rejection by dams, positioning the M-mode cursor perpendicular to the septum at the mid-papillary level while avoiding papillary muscle inclusion, and keeping Doppler angles below 45 degrees to prevent velocity underestimation. A printable echocardiography quicksheet summarizing key reference values accompanies the publication as a bench-side tool.

The limitations are candidly acknowledged: the data derive exclusively from Jackson Laboratory C57BL/6 mice, whose developmental trajectories may differ across strains; volumetric parameters rest on geometric assumptions that strain measurements avoid; and the extreme technical constraints of imaging newborn hearts meant that strain, Doppler, and right ventricular assessments could not be performed at P0. Future directions include comprehensive right ventricular and large-vessel characterization, integration with invasive pressure-volume loop analysis, and extending the serial observations into aging to trace how the early-life sex differences evolve across the lifespan. For now, the study stands as the definitive developmental atlas of the laboratory mouse heart, one that promises to sharpen the interpretation of countless cardiac disease models and, by reducing misclassification of phenotypes, to trim both animal usage and cost in preclinical research.

Subject of Research: Postnatal cardiac development and myocardial strain in C57BL/6 mice assessed by high-frequency echocardiography

Article Title: Prospective echocardiographic characterization of postnatal cardiac development in C57BL/6 mice: Strain analysis between birth and adulthood

Article References: Jo, S., Spyropoulos, G., Hart, L. G., Covington, T. A., Pandey, A. K., Michel, T., & Spyropoulos, F. (2026). Prospective echocardiographic characterization of postnatal cardiac development in C57BL/6 mice: Strain analysis between birth and adulthood. Physiological Reports, 14(18), Article e71113. https://doi.org/10.14814/phy2.71113

Image Credits: AI Generated

DOI: 10.14814/phy2.71113

Keywords: echocardiography, strain analysis, C57BL/6 mice, cardiac development, speckle-tracking, ejection fraction, diastolic function, sexual dimorphism, left ventricle, normative values, preclinical imaging, Physiological Reports

The research team, led by investigators at Trinity College Dublin and collaborating institutions, focused on two distinct but related markers of exercise tolerance. The first was peak oxygen uptake, or VO2peak, the maximal amount of oxygen the body can consume during incremental exercise to exhaustion. VO2peak is not merely a laboratory curiosity; it is one of the strongest independent predictors of cardiovascular and all-cause mortality, and people with type 2 diabetes consistently show reductions of roughly twelve to twenty-two percent compared with healthy peers. The second marker was the time constant of the primary phase of the oxygen uptake response, abbreviated as tau VO2p, which describes how quickly oxygen consumption rises toward its steady state when a person transitions from rest to moderate exercise. Slower kinetics mean a larger oxygen deficit at the onset of activity, greater reliance on anaerobic metabolism, and earlier fatigue during everyday tasks such as climbing stairs or carrying groceries.

Participants were stratified into two age cohorts: an early middle-aged group aged thirty-seven to fifty-nine years and a late middle-aged group aged sixty to seventy years. All had been diagnosed with type 2 diabetes for fewer than ten years and had glycated hemoglobin values below ten percent, indicating reasonably well-controlled disease. After an initial treadmill electrocardiogram stress assessment, volunteers were assigned to one of four groups: exercising or non-exercising controls in each age band. The allocation was not random, primarily because of differences in participants’ ability to travel to the training facility, a limitation the authors acknowledge. Those in the exercise arms completed a twelve-week supervised program of up to thirty-six sessions, combining continuous cycling at seventy to eighty percent of maximal heart rate, resistance training for major upper and lower body muscle groups at sixty to seventy percent of one-repetition maximum, core strengthening, flexibility work, and brief high-intensity intervals at approximately ninety percent of maximal heart rate. Adherence was exceptional, with participants averaging thirty-five of a possible thirty-six sessions.

The testing protocol was rigorous. Before and after the intervention, participants performed a graded exercise test on a cycle ergometer to determine VO2peak, peak power output, the power output at the gas exchange threshold, and the oxygen pulse, an indirect marker of stroke volume. In a separate session, they completed six constant-load cycling bouts at a workload corresponding to eighty percent of their individually determined gas exchange threshold, with breath-by-breath gas exchange measured continuously. Cardiac output was assessed by an inert gas rebreathing technique at rest and at thirty and two hundred forty seconds of exercise, allowing the researchers to calculate stroke volume, mean arterial pressure, and systemic vascular conductance. The oxygen uptake data were fitted with monoexponential or biexponential mathematical models to extract the time constant of the primary response, the parameter that captures how efficiently the oxidative system adjusts to a sudden metabolic demand.

The results were unambiguous for the exercising groups. Both the early and late middle-aged training cohorts showed statistically significant increases in absolute and relative VO2peak, rising from roughly twenty-five milliliters per kilogram per minute at baseline to about twenty-eight after training, an improvement of approximately twelve percent. Peak power output, the power output at the gas exchange threshold, and oxygen pulse all rose in parallel, while heart rate at exhaustion was unchanged, indicating genuine physiological adaptation rather than simply greater effort or familiarization with the test. Crucially, the magnitude of these gains did not differ between the two age groups. The non-exercising controls, by contrast, showed no meaningful changes in any of these variables over the twelve weeks, and body mass and HbA1c remained stable across all groups, meaning the fitness improvements could not be attributed to weight loss or better glycaemic control.

The oxygen uptake kinetics data told a similarly encouraging story. The time constant of the primary VO2 response fell from approximately forty seconds at baseline to around thirty-four to thirty-five seconds after training in both exercise groups, representing an acceleration of roughly twelve percent in the late middle-aged cohort and twenty-one percent in the early middle-aged cohort. Although the numerical difference between the age groups was noticeable, statistical testing showed it did not reach significance, and the authors concluded that the improvement in submaximal oxygen kinetics was essentially equivalent across ages. This matters because faster kinetics reduce the oxygen deficit at the start of exercise, limit the accumulation of metabolites that contribute to fatigue, and translate directly into better tolerance of the repetitive, moderate-intensity activities that define daily life, an effect that is particularly relevant for adults in their sixties who may be approaching the threshold of functional decline.

Perhaps the most intriguing mechanistic finding was what did not change. The researchers had hypothesized that faster oxygen uptake kinetics might be driven by improved central cardiovascular delivery of oxygen, reflected in more rapid adjustments of cardiac output, stroke volume, or systemic vascular conductance during the first seconds of exercise. Instead, when they quantified the dynamic responses of these variables by comparing the change at thirty seconds with the change at two hundred forty seconds, they found no significant alterations following training in either age group. The improvements in cardiac output and vascular conductance that were observed at fixed time points largely reflected the higher workloads participants could sustain after training rather than a fundamentally altered cardiovascular response profile. This pattern suggests that the acceleration of oxygen uptake kinetics was more likely mediated by peripheral adaptations, such as enhanced matching of oxygen delivery to metabolic demand within the contracting skeletal muscle, improved vasodilatory capacity, or better dynamic microvascular function, although the study did not directly measure muscle oxygenation.

The clinical implications are considerable. Type 2 diabetes prevalence rises sharply with age, and as populations grow older and more sedentary, the burden of the disease is projected to escalate. Previous work by the same group had shown that middle-aged adults with diabetes in their forties and fifties exhibit slowed oxygen uptake kinetics compared with healthy controls, whereas those in their sixties do not, hinting that age might modify the exercise phenotype of the disease. The new data extend that picture by demonstrating that, regardless of these baseline differences, the responsiveness to structured training, in both maximal and submaximal measures of exercise tolerance, is preserved across early and late middle age. In practical terms, clinicians can reasonably expect a sixty-three-year-old with well-controlled type 2 diabetes to gain as much fitness from a twelve-week combined program as a fifty-four-year-old with the same disease duration, which argues strongly against the assumption that older middle-aged patients are somehow less trainable.

The authors are careful to note the boundaries of their conclusions. The participants had relatively mild, uncomplicated diabetes with mean HbA1c values between 6.3 and 7.0 percent, so the findings may not generalize to people with poorer glycaemic control or advanced disease, who typically show greater impairments in fitness. The age gap between the two cohorts was only about ten years, limiting inferences about broader age-related differences in trainability, and the sex distribution differed between groups, although prior evidence indicates that training adaptations in VO2peak and oxygen kinetics are not influenced by sex in type 2 diabetes. The inert gas rebreathing technique also lacked the temporal resolution to detect subtle changes in central hemodynamics. Even so, the study provides a valuable benchmark for expected training responses in adults with type 2 diabetes in their forties through sixties, and it reinforces a message that exercise physiologists have long championed: the aging body, even one carrying a metabolic disease, retains a striking capacity to grow fitter when given the right stimulus. Whether the same plasticity extends to people over seventy or with longer disease duration remains an open and clinically urgent question.

Subject of Research: Effects of structured exercise training on cardiorespiratory fitness and oxygen uptake kinetics in early versus late middle-aged adults with type 2 diabetes

Article Title: Exercise training improves cardiorespiratory fitness and oxygen uptake kinetics to a similar extent in early and late middle‐aged adults with type 2 diabetes

Article References: Exercise training improves cardiorespiratory fitness and oxygen uptake kinetics to a similar extent in early and late middle‐aged adults with type 2 diabetes. (n.d.). https://doi.org/10.14814/phy2.71107

Image Credits: AI Generated

DOI: 10.14814/phy2.71107

Keywords: type 2 diabetes, exercise training, cardiorespiratory fitness, oxygen uptake kinetics, VO2peak, middle age, cardiac output, skeletal muscle, aging, physical activity, Physiological Reports, vascular function

Researchers at the University of Waterloo recruited twenty-two healthy young adults — eleven women and eleven men — who exercised more than three times per week. Each participant completed a single laboratory session involving three very different exercise tasks: rhythmic handgrip, which activates only the small muscles of the forearm; single-leg extension, which engages the quadriceps and surrounding muscles of one limb; and cycling, a whole-body activity recruiting large muscle groups in both legs. The team, led by Armando J. Aguero, Connor J. Doherty and Paolo B. Dominelli, hypothesized that chemosensitivity would climb in step with the amount of active muscle, so that cycling would produce the largest boost and handgrip the smallest.

To measure peripheral hypercapnic chemosensitivity, the researchers used a clever transient stimulus. On four occasions during each resting and exercising condition, participants unknowingly received two breaths of a gas mixture containing 10 percent carbon dioxide, delivered through a silent manual valve switched by investigators watching real-time ventilatory flow. Because the stimulus lasted only two breaths, it was far too brief to engage the brain’s central chemoreceptors, which respond to slower, sustained changes in carbon dioxide. Instead, the response was attributed to the peripheral chemoreceptors, chiefly the carotid bodies. The sensitivity was then calculated as the change in inspired ventilation divided by the change in end-tidal carbon dioxide pressure, expressed in liters per minute per millimeter of mercury.

The exercise intensities were deliberately kept low. Handgrip was performed against a 2.5-kilogram load at thirty contractions per minute with a one-second-on, one-second-off rhythm. Single-leg extension pushed against a constant five watts of resistance on a custom ergometer at the same cadence. Cycling was performed at forty watts at a self-selected cadence of roughly sixty to eighty revolutions per minute. These modest workloads were chosen to ensure that the metabolic byproducts of exercise would be minimal, reducing the contribution of the muscle metaboreflex — the reflex triggered by metabolite accumulation in working muscle — and allowing the team to focus on neural mechanisms such as central command and mechanoreceptor feedback.

The results confirmed half of the hypothesis. All three exercise modalities produced a significant increase in peripheral hypercapnic chemosensitivity from rest to exercise. Handgrip raised sensitivity by about 11.9 percent, single-leg extension by roughly 52.4 percent, and cycling by approximately 49.2 percent. Heart rate, ventilation, tidal volume and the ventilatory response to the carbon dioxide bolus all rose during the active conditions, with the largest changes seen in the leg exercises. But the critical comparison between single-leg extension and cycling revealed the surprise: despite cycling recruiting far more muscle mass, the two exercises produced statistically indistinguishable increases in chemosensitivity, both in absolute terms and as a percentage change from baseline.

This finding challenges a straightforward assumption about how the body scales its ventilatory response. The researchers had reasoned that larger active muscle mass would demand greater central command — the feedforward signal from the brain’s motor and autonomic centers that drives anticipatory increases in heart rate and ventilation before and during exercise. Central command is known to increase in proportion to both exercise intensity and the amount of muscle recruited, progressing from isolated small muscle groups to combined limb exercise. If central command were the sole driver of chemosensitization, cycling should have outpaced single-leg extension. It did not.

The study’s authors suggest instead that the key factor is the transition from rest to exercise itself, rather than the magnitude of the effort. This interpretation is consistent with their earlier work showing that chemosensitivity rises when cycling begins but plateaus with further increases in workload. Together, the two findings paint a picture in which the act of initiating movement — engaging central command and muscle mechanoreceptors — flips a switch that sensitizes the carotid bodies, but adding more muscle or more intensity does not turn that switch further. The similar responses during single-leg extension and cycling imply that once a threshold of neural activation is crossed, recruiting additional muscle mass adds little to the chemosensory gain.

Mechanoreceptor feedback may also play a role. Type III muscle afferents respond to mechanical stretch and contraction and can augment sympathetic outflow and ventilatory drive early in exercise. Because single-leg extension and cycling stimulate more mechanoreceptors than handgrip, the intermediate rise in chemosensitivity seen in the leg exercises is most consistent with concurrent signaling from both central command and mechanoreceptor feedback. The authors caution, however, that voluntary exercise inherently involves both mechanisms, and their design cannot fully separate the contributions of each. Type IV chemoreceptive afferents, which respond to metabolic stress, are unlikely culprits given the low exercise intensities used and prior evidence that metaboreceptor feedback does not increase chemosensitivity beyond the onset of exercise.

Several methodological considerations shaped the interpretation. Resting chemosensitivity was higher when participants were seated on the cycle ergometer than during handgrip, which the authors attribute to an anticipatory response driven by central command before exercise begins — a phenomenon they had observed previously. Pre-stimulus end-tidal carbon dioxide also differed across conditions: it rose during the leg exercises, creating a mildly hypercapnic environment that would amplify the ventilatory response, but fell during handgrip, which should have blunted the response. Notably, handgrip still produced a significant increase in chemosensitivity despite this hypocapnic baseline, demonstrating that the exercise-related sensitization was robust enough to overcome an attenuating condition. The authors also note that end-tidal carbon dioxide serves as a surrogate for arterial carbon dioxide, a substitution that is reasonable at low exercise intensities where the gradient between the two is minimal.

The broader implications reach into exercise physiology, respiratory medicine and perhaps athletic performance. Understanding how the carotid bodies are sensitized during movement could inform approaches to conditions in which ventilatory control is disordered, and the finding that chemosensitization does not scale with muscle mass simplifies the search for the underlying neural signal. The authors propose that future studies use electrically evoked, non-volitional muscle contractions to activate muscle without engaging central command, or block muscle afferents with intrathecal fentanyl while preserving motor drive, to disentangle the two candidate mechanisms. For now, the message is clear: when it comes to sharpening the body’s carbon dioxide sensors, starting to move matters far more than how much muscle you move with.

Subject of Research: Effect of active muscle mass on peripheral hypercapnic chemosensitivity during exercise in healthy adults

Article Title: Greater muscle mass does not affect peripheral hypercapnic chemosensitivity during exercise in healthy adults

Article References: Aguero, A. J., Doherty, C. J., & Dominelli, P. B. (2026). Greater muscle mass does not affect peripheral hypercapnic chemosensitivity during exercise in healthy adults. Physiological Reports, 14(17), Article e71102. https://doi.org/10.14814/phy2.71102

Image Credits: AI Generated

DOI: 10.14814/phy2.71102

Keywords: peripheral hypercapnic chemosensitivity, carotid bodies, exercise physiology, central command, mechanoreceptor feedback, ventilatory control, carbon dioxide sensitivity, rhythmic handgrip, cycling, muscle afferents, Physiological Reports, respiratory physiology

The research team, led by investigators at the Pain & Fatigue Study Center in the Department of Neurology, drew participants from an NIH-funded parent study designed to examine the relationship between cardiopulmonary exercise testing and post-exertional malaise, the hallmark symptom of ME/CFS. Between August 2021 and July 2025, forty-nine adults aged 25 to 60 years who met the 1994 Fukuda case definition completed both assessments. To ensure a substantial current symptom burden, participants were required to report moderate-to-severe ratings on post-exertional malaise and on at least three of seven additional symptoms, including sore throat, headache, myalgia, and unrefreshing sleep. Individuals with medical conditions that could explain the fatigue, including anemia identified by routine laboratory testing, were excluded, as were those taking medications that blunt cardiovascular responses and those with certain psychiatric diagnoses.

The orthostatic assessment relied on the 10-minute lean test, a protocol originally developed by NASA to detect orthostatic intolerance in astronauts returning from spaceflight. After at least ten minutes of supine rest, baseline blood pressure, heart rate, respiratory rate, and end-tidal carbon dioxide were recorded in triplicate at one-minute intervals using a portable capnograph and pulse oximeter. Participants then stood with feet together roughly six to eight inches from a wall and leaned against it without moving for ten minutes, while the same parameters were captured every minute. This test captures several distinct physiological abnormalities: postural orthostatic tachycardia syndrome, defined as a heart rate rise of at least 30 beats per minute or an absolute heart rate of at least 120 beats per minute; supine hypocapnia, defined as any baseline end-tidal carbon dioxide below 34 mmHg; and the postural orthostatic syndrome of hypocapnia, in which carbon dioxide falls below that threshold only upon standing.

Blood volume was measured with the semiautomated Daxor BVA-100 system, a technique regarded as a reference standard for quantitative intravascular volume assessment. After a baseline blood sample was drawn, human serum albumin labeled with iodine-131 was injected intravenously and allowed to equilibrate. Five additional samples were collected at 15, 21, 27, 33, and 39 minutes to correct for the gradual leakage of albumin out of the vasculature, and plasma volume at time zero was extrapolated from at least three samples with a coefficient of variation of no more than 3.9 percent. Total blood volume was calculated as the sum of plasma volume and red blood cell volume, and all values were normalized to each participant’s ideal body size. Total blood volume was then categorized as hypovolemic if it deviated more than 8 percent below the predicted value, normovolemic within 8 percent, and hypervolemic above 8 percent, thresholds consistent with prior ME/CFS blood volume research.

The results painted a picture of remarkable physiological heterogeneity. Seventeen participants, or 35 percent, were classified as hypovolemic, twenty-one as normovolemic, and eleven as hypervolemic. Red blood cell volume told a similar story: nearly half of the cohort, twenty-four participants, had a red cell volume deficit more than 10 percent below ideal, a phenotype that, when routine hemoglobin and hematocrit values are normal, can be considered an occult red cell mass deficit rather than conventional anemia. Yet when the researchers cross-tabulated these volume categories against lean-test outcomes, no significant associations emerged. Fifty-five percent of participants exhibited at least one orthostatic abnormality, with hypocapnic responses proving at least as common as tachycardia: twenty participants showed the postural orthostatic syndrome of hypocapnia, six showed supine hypocapnia, six met criteria for POTS, and four had both POSH and POTS. The distribution of these abnormalities, however, was statistically indistinguishable across hypovolemic, normovolemic, and hypervolemic groups, and the same held true for red blood cell volume categories.

These findings directly confront a body of earlier work. Multiple previous studies have reported reduced total or compartmental blood volumes, smaller cardiac chambers, and diminished stroke volume in subsets of ME/CFS patients, and one pilot study using a dual-isotope technique found lower blood volumes among patients whose day-to-day symptoms suggested orthostatic intolerance. That earlier study, however, relied on clinical history rather than contemporaneous physiological testing, leaving open the possibility that subjective symptom reports and objective orthostatic physiology diverge. By combining quantitative blood volume analysis with a standardized lean test incorporating capnography, the Mount Sinai team demonstrated that even objectively confirmed orthostatic abnormalities do not track with measured volume status. In other words, a patient can be quantitatively hypovolemic yet pass the lean test, or have entirely normal blood volume yet exhibit marked hypocapnia or tachycardia upon standing.

The null result does not mean blood volume is clinically irrelevant. Identifying hypovolemia in an individual patient may still matter, because volume expansion remains a logical therapeutic target for that subgroup. But the new data help explain why pharmacological volume expansion has disappointed in controlled trials: randomized studies of fludrocortisone, a mineralocorticoid that promotes sodium and water retention, showed limited benefit in ME/CFS, implying that correcting hypovolemia alone is often insufficient. The authors argue that orthostatic intolerance in this illness likely arises from multiple converging mechanisms that static volume measurements cannot capture. Among the candidates are impaired venous constriction and excessive venous capacitance, which promote gravitational pooling of blood in the legs; autonomic dysregulation, evidenced by reduced vagal modulation and altered heart rate variability at rest, during sleep, and during orthostatic challenge; and abnormalities of neurohumoral control, including the renin-aldosterone paradox described in POTS and small-fiber neuropathy identified in subsets of patients.

Cerebral perfusion adds another critical dimension. Previous controlled work has shown that upright posture can substantially reduce cerebral blood flow in ME/CFS patients even when heart rate and blood pressure remain within conventional limits, and that this cerebral blood flow reduction during tilt testing is independent of deconditioning as defined by exercise testing. Deconditioning itself, which can reduce plasma volume and ventricular mass over weeks of bed rest, is unlikely to account for the high prevalence of orthostatic intolerance in this population, since patients show greater cerebral blood flow reductions than healthy, sedentary controls. Taken together, these observations support a model in which orthostatic pathology in ME/CFS reflects dynamic failures of autonomic, vascular, respiratory, and cerebral hemodynamic regulation rather than a simple deficit in the amount of blood in the circulation.

The study has limitations that the authors acknowledge candidly. The sample of forty-nine participants, recruited from a single tertiary care center and limited to patients well enough to undergo testing, constrains statistical power and may underrepresent the most severely affected patients. The eight-day interval between the lean test and the blood volume assessment, together with the lack of standardization of acute hydration and salt intake, may have introduced temporal variability in both volume and hemodynamic measures. Anxiety-related hyperventilation and other breathing-pattern abnormalities could not be excluded as contributors to the hypocapnic findings, because state anxiety and tidal volume were not assessed. The 10-minute lean test also imposes milder orthostatic stress than head-up tilt, so hypovolemia-driven instability might conceivably emerge only under more prolonged or severe challenge. The absence of a healthy control group further precluded direct comparison of volume patterns between patients and volunteers.

Even with those caveats, the message for the field is clear and consequential. Orthostatic intolerance was highly prevalent in this cohort, yet neither total blood volume nor red blood cell volume category predicted who would exhibit supine hypocapnia, POSH, or POTS on standardized testing. The authors call for multicenter studies that incorporate dynamic measurements of cerebral blood flow, respiratory pattern, and carbon dioxide regulation during graded orthostatic stress, integrated with blood volume status, to build comprehensive pathophysiological models of the illness. Such phenotype-directed approaches, they argue, will be essential for developing rational therapies that address the actual mechanisms driving orthostatic symptoms, rather than assuming that a single measurable deficit, however intuitively appealing, holds the key to one of ME/CFS’s most disabling features.

Subject of Research: The relationship between measured blood volume, red blood cell volume, and orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome

Article Title: Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome

Article References: Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome. (n.d.). https://doi.org/10.14814/phy2.71087

Image Credits: AI Generated

DOI: 10.14814/phy2.71087

Keywords: ME/CFS, orthostatic intolerance, blood volume, red blood cell volume, POTS, hypocapnia, lean test, capnography, autonomic dysfunction, cerebral blood flow, hypovolemia, Physiological Reports

The research, published in Physiological Reports, tackled a problem that has long frustrated both clinicians and preclinical scientists. Standard practice after spinal cord injury prioritizes medical stabilization and catheterization, pushing comprehensive urodynamic testing weeks or months down the road. By the time detailed bladder measurements are taken, a critical therapeutic window may already have closed. Compounding the problem, the bladder is not a single organ behaving in a single way; it is a synchronized neuromuscular system whose output depends on coordinated interactions among pressure, volume, timing, and filling rate. Reading any one of those numbers in isolation, the study’s authors argue, misses the multivariate signature that actually carries prognostic information.

To capture that signature, the team, led by Zachary Danziger of Florida International University, built an unusually rigorous monitoring setup. Twenty adult female Sprague-Dawley rats received bladder catheters; ten of them then underwent complete transection of the spinal cord at the ninth thoracic segment. All animals were housed in custom metabolic cages fitted with laser-etched wire mesh floors that let urine pass instantly to digital scales while deflecting solid waste, allowing around-the-clock automated recording of every voiding event. Implanted catheters connected to pressure transducers sampled intravesical pressure at 40 hertz. The result was a continuous, uninterrupted record of bladder pressure and voided volume for roughly 26 days after injury, spanning more than 5,000 voiding events across the group, a dataset with no real precedent in preclinical neurogenic bladder research.

The longitudinal data told a story of gradual but incomplete recovery. After the initial spinal shock phase, spinally mediated voiding reflexes re-emerged, and average voiding efficiency crept upward while post-void residual volumes declined. But even after a month, the injured group as a whole remained significantly worse than intact controls, with lower voiding efficiency and higher residual urine. More striking was the spread between individual animals. When the researchers applied an unsupervised clustering algorithm called DBSCAN to chronic-phase measurements of voiding efficiency and residual volume, three distinct recovery phenotypes emerged from the ten injured rats: a good recovery group of four animals reaching efficiency near 0.71 with residuals around 0.21 milliliters, a moderate group of five animals at roughly 0.45 efficiency and 0.78 milliliters of residual, and a single poor-recovery animal at 0.34 efficiency with nearly two milliliters retained after each void.

Crucially, even the best-recovering animals had not returned to normal. Intact rats showed clear day-night differences in voiding, emptying more efficiently during their rest phase, a pattern partly governed by circadian regulation of vasopressin and urine production. In every injured rat, regardless of phenotype, that diurnal variation was abolished and never returned over the four-week observation window. The good-recovery animals had achieved efficient emptying, but they did so through an involuntary spinal reflex circuit that resembles the primitive neonatal pattern of micturition rather than the supraspinally controlled system that existed before injury. Efficient voiding, in other words, can mask a fundamental change in the mechanism producing it.

The heart of the study lies in its predictive analysis. The researchers asked whether measurements taken in the first week after injury, days 3 through 10, could forecast which chronic phenotype each rat would occupy. Using a generalized linear model with a logit link function, constrained to at most three terms to avoid overfitting, they searched across thousands of candidate predictor combinations, including squared terms and interactions among six acute-phase urodynamic parameters. The winning model combined two interaction terms: the product of acute voided volume and acute voiding efficiency, and the product of acute ureter flow rate and the square of acute voiding efficiency. On the fitted data, this compact equation explained more than 90 percent of the variance in chronic voiding efficiency, and leave-one-out cross-validation, in which the model was retrained ten times with one rat withheld each time, confirmed that it generalized, achieving an R-squared of 0.84 on unseen animals.

That predictive power came from interactions rather than any single measurement, and this may explain why conventional clinical assessment has struggled to prognosticate early. During the acute phase, the urodynamic parameters of rats destined for good, moderate, or poor recovery were statistically indistinguishable when compared one at a time. Yet a principal components analysis revealed that each animal’s starting position along the dominant axis of multivariate variation, a dimension dominated by residual volume, efficiency, and filling rate, correlated with its eventual phenotype. Individual parameters told no story; the constellation of parameters told nearly the whole one. The finding suggests that redundancy in the urodynamic parameter space is considerable, which is good news for translation: a carefully chosen subset of clinically accessible measures might suffice for prediction without exhaustive monitoring.

The study also uncovered a temporal split in how bladder function reorganizes. Storage parameters, bladder capacity and ureter flow rate, established phenotype-specific values within the acute phase and then remained essentially stable for the rest of the month. Voiding parameters, by contrast, evolved continuously across weeks, consistent with progressive reorganization of spinal reflex circuitry. The authors interpret this split as evidence that two processes on different timescales shape outcome: rapid structural and molecular changes in the bladder wall, including epithelial barrier failure and inflammatory remodeling that begin within hours of injury, and slower neural plasticity in the spinal cord. Acute ureter flow rate, which reflects renal filtration rather than bladder physiology, varied systematically across phenotypes, hinting that systemic physiological differences at the time of injury may also contribute to prognosis through mechanisms that remain to be determined.

Pressure dynamics added a further layer. Non-voiding contractions, transient pressure rises unaccompanied by urine output, separated the phenotypes at every phase of recovery. Good-recovery animals averaged fewer than one such contraction per filling interval in the acute phase, while moderate-recovery animals averaged more than eight, a difference that persisted through the intermediate and chronic phases. Maximum voiding pressure diverged later, with good-recovery animals showing significantly lower pressures by the intermediate phase. Because inter-void intervals did not differ between groups, the contraction-count differences reflect genuine differences in detrusor instability rather than differences in how long the bladder had to misbehave.

The authors are careful to frame the work as preliminary and hypothesis-generating. Only female rats were studied to limit biological variability, and known sex differences in post-injury bladder dysfunction mean that validation in male and mixed cohorts is essential. Continuous metabolic cage housing also precluded simultaneous recording of external urethral sphincter activity, leaving detrusor-sphincter coordination unverified electrophysiologically. Still, the conceptual advance is substantial: this is, to the authors’ knowledge, the first continuous longitudinal characterization of post-injury bladder reorganization in a rat model, and it demonstrates that chronic outcome is not determined by injury level alone but by patterns of multivariate urodynamic interaction established early in recovery. If those patterns can be captured with clinically practical measurements in human patients, the window in which the spinal cord and bladder are most adaptable could finally become a window in which clinicians act, directing antispasmodic therapy, catheterization strategy, or emerging neuromodulation approaches to the patients who need them most, before irreversible secondary damage takes hold.

Subject of Research: Prediction of chronic neurogenic bladder recovery after spinal cord injury using early longitudinal urodynamics and machine learning in a rat model

Article Title: Early urodynamic profiles predict lower urinary tract recovery after spinal cord injury in female rats

Article References: Afrashteh, B., Khan, S., Adury, R. Z., Angoli, D., & Danziger, Z. C. (2026). Early urodynamic profiles predict lower urinary tract recovery after spinal cord injury in female rats. Physiological Reports, 14(17), Article e71101. https://doi.org/10.14814/phy2.71101

Image Credits: AI Generated

DOI: 10.14814/phy2.71101

Keywords: spinal cord injury, neurogenic bladder, urodynamics, voiding efficiency, machine learning, generalized linear model, rat model, bladder recovery, micturition, DBSCAN clustering, principal components analysis, Physiological Reports

The research emerged from a larger 12-week randomized controlled trial designed to test whether high-intensity combined training could match traditional training in military-style fitness outcomes. Participants were young adults aged 18 to 27 from the Birmingham, Alabama area, all of whom were sedentary before enrollment. One group, labeled TRAD, performed 30 minutes of cycling at 70 percent of heart rate reserve followed by a full-body resistance routine of squats, presses, rows, and curls at three sets of 13 repetitions. The other group, called HITT, completed ten rounds of a maximal 30-second on/off circuit featuring box jumps, burpees, kettlebell swings, cycling and rowing sprints, battle ropes, and wall balls, then finished the same resistance exercises in superset form at lower volume and shorter rests. Both groups trained in the early morning after an overnight fast, and researchers drew blood before exercise, immediately after, three hours later, and again at 24 hours.

The analytical scale of the study is striking. Using untargeted metabolomics with both reverse-phase and HILIC chromatography in positive and negative ion modes, the team detected 10,793 serum compounds, annotated 5,215 of them, and consolidated these into 3,243 non-redundant features. After statistical filtering with a false discovery rate threshold of 0.10, 2,052 compounds changed significantly in at least one comparison, including 738 annotated endogenous metabolites spanning amino acids, lipids, acyl carnitines, nucleotides, and biogenic amines. Partial least squares-discriminant analysis showed that the serum metabolome clustered distinctly at the immediate and three-hour timepoints in both groups, before settling back toward baseline by 24 hours — a molecular signature of exertion that fades over roughly a day.

Within each group, the response was enormous. In the traditional group, 683 metabolites shifted below the significance threshold at one or more timepoints, compared with 585 in the high-intensity group. Immediately after exercise, both groups showed roughly equal numbers of rising and falling metabolites, but by three hours the picture skewed heavily toward accumulation: 333 upregulated metabolites in TRAD and 284 in HITT, dominated by fatty acids, conjugated fatty amines, and ketones. By 24 hours, the traditional group still had 183 altered metabolites while the high-intensity group had 86, indicating that the longer session left a deeper and more persistent biochemical footprint. The researchers attribute this to the sustained metabolic demand of continuous endurance work and the roughly doubled duration of the traditional prescription.

The timing of specific metabolite classes tells a coherent physiological story. Immediately after exercise, the blood was enriched for the active substrates of ATP generation — glucose, pyruvate, citrate, aconitate, and fumarate — along with products of purine catabolism such as hypoxanthine and uridine monophosphate. Working skeletal muscle, which cannot express xanthine oxidase, releases these purine intermediates into circulation, where the liver salvages them back into the ATP pool or converts them to uric acid. This salvage pathway is more energy-efficient than building nucleotides from scratch, and its appearance in the blood mirrors findings from earlier studies of moderate steady-state cycling and resistance exercise alone.

Three hours later, the emphasis shifted to recovery and fuel replenishment. Both groups showed elevated levels of 12,13-diHOME, an adipose-derived exerkine known to stimulate fatty acid uptake in skeletal muscle, alongside palmitoylcarnitine, which ferries fatty acid chains across the mitochondrial inner membrane for beta-oxidation, and a broad suite of long-chain fatty acids. Ninety-seven metabolites rose in both groups at this timepoint, most of them lipid-related, representing the largest shared signature in the study. The authors interpret this coordinated lipid mobilization as systemic replenishment of energy stores drained by the bout — the metabolic equivalent of refueling after a long drive.

Only a handful of metabolites distinguished the two training modes directly. Four met the false discovery threshold: capryloylglycine was lower in the traditional group immediately after exercise; hydroxynorleucine and acetylcholine were higher in that group at three hours; and D-mannose was lower in the traditional group at three hours. The acetylcholine finding is particularly intriguing because high-intensity exercise has previously been linked to reduced circulating choline and acetylcholine, yet choline itself fell in both groups here, suggesting that another mechanism — perhaps reduced acetyl-CoA availability or diminished choline acetyltransferase activity — drove the drop in the high-intensity group. The mannose difference may reflect the liver responding to glycolytic stress, since epinephrine-driven glycogenolysis releases mannose that cells can phosphorylate and channel into glycolysis or glycogen synthesis.

Some of the distinguishing metabolites remain biochemical mysteries. Capryloylglycine, a conjugate of the medium-chain fatty acid caprylic acid and glycine, has been described as a principal pancreatic metabolite in pigs and is elevated in the muscle of older adults, and genetic defects in mitochondrial beta-oxidation raise its levels — but why a traditional combined session would lower it acutely is unknown, since glycine availability was similar between groups. Hydroxynorleucine does not even appear as an identified entry in the Human Metabolome Database. The authors are candid that these findings need replication and mechanistic follow-up before any physiological meaning can be assigned.

Unbiased clustering across time revealed that most response patterns were shared between the two prescriptions, with key metabolites such as glucose, lactate, cortisol, palmitoylcarnitine, and glutathione rising or falling in parallel. Yet each mode also produced unique signatures: the traditional group showed a distinct cluster containing succinate that fell equally at both early timepoints, while the high-intensity group had a cluster with ornithine, glycine, and citrulline that dropped immediately and then climbed steadily through 24 hours. These idiosyncratic patterns, layered on top of the shared core response, are the clearest evidence yet that exercise dose and structure write distinguishable chemical signatures into the blood.

The study has caveats worth noting. Participants consumed a standardized protein drink after the immediate post-exercise blood draw to mimic real-world training, which could have contributed to some of the three-hour changes, though the lipid and amino acid patterns closely match those of fasted studies. All participants were untrained, so unaccustomed exertion itself likely drove much of the response, and the design covaried for sex rather than testing sex-specific effects, which the team has reported separately. Still, as the first timecourse study of the acute circulating metabolomic response to combined endurance and resistance exercise, the work establishes that a 45-minute high-intensity session and a 90-minute traditional session converge on the same fundamental biology — fuel burning, purine salvage, lipid mobilization — while leaving behind subtly different molecular fingerprints. For time-pressed exercisers and military planners alike, that suggests the shorter workout may deliver much of the same metabolic conversation, just in a more compressed dialect.

Subject of Research: Acute serum metabolomic responses to traditional versus high-intensity combined endurance and resistance exercise in young adults

Article Title: Serum metabolomics signatures after an acute bout of combined traditional or high‐intensity tactical training in young adults

Article References: Graham, Z. A., Pathak, K. V., Garcia‐Mansfield, K., Lavin, K. M., Torres, A. R., McAdam, J. S., Broderick, T., Pirrotte, P., & Bamman, M. M. (2026). Serum metabolomics signatures after an acute bout of combined traditional or high‐intensity tactical training in young adults. Physiological Reports, 14(18), Article e71094. https://doi.org/10.14814/phy2.71094

Image Credits: AI Generated

DOI: 10.14814/phy2.71094

Keywords: metabolomics, exercise physiology, high-intensity interval training, resistance training, serum metabolites, combined exercise, fatty acid oxidation, purine salvage, recovery, military fitness, young adults, Physiological Reports