The research team, led by Aneta Teległów of the University of Physical Culture in Krakow together with colleagues from Jagiellonian University Medical College and the University of Agriculture in Krakow, focused on hemorheology, the study of how easily blood flows through vessels. Blood is not a simple liquid like water; it is a suspension of red cells, white cells, and platelets in plasma, and its viscosity depends on hematocrit, plasma protein concentration, red cell deformability, and aggregation. When any of these properties shift, the heart must work harder to push blood through the microcirculation, and the risk of thrombosis can rise.

Previous work had suggested reasons for concern. Cold exposure triggers peripheral vasoconstriction, raises arterial pressure, and has been linked in older studies to increases in platelet counts, red cell counts, and plasma fibrinogen, a key clotting protein. Strenuous exercise itself is known to transiently raise blood viscosity through hemoconcentration and to activate platelets. The researchers wanted to know whether combining the two stresses, maximal exertion and severe cold, would produce an amplified hemorheological response that could help explain why cardiovascular events cluster during winter sports and snow shoveling.

Thirty healthy, non-smoking male students aged around twenty-two were recruited and randomly assigned to one of two groups. The experimental group performed an incremental endurance shuttle-run test inside a climate chamber held at −10 °C, while the control group ran the identical protocol outdoors at 18 °C. The test began at eight kilometers per hour and increased by half a kilometer per hour each minute, with participants running twenty-meter shuttles until exhaustion. Fasting blood samples were drawn from an antecubital vein before and immediately after the run, and the researchers measured whole blood and plasma viscosity, hematocrit, red cell deformability and aggregation, fibrinogen concentration, platelet count and activation markers, and white cell counts.

The results were striking for what they did not show. Both groups displayed the expected exercise-induced hemoconcentration: plasma volume fell, hematocrit and hemoglobin rose, and platelet numbers increased, consistent with the well-documented splenic release of stored platelets during intense effort. Yet the magnitude of these changes was essentially the same whether the men had sprinted in deep cold or in mild air. Fibrinogen concentrations did not rise in either group, and there was no significant difference between the cold-exposed and control groups in any of the measured clotting or rheological variables.

Plasma viscosity, often considered a sensitive marker of cardiovascular risk, was unchanged by the cold exposure itself. The authors note that this finding aligns with some earlier reports but contradicts others that described viscosity increases during cold exposure at rest. One possible explanation is that the participants wore insulating clothing during the cold trial, blunting the skin-level cold stimulus, and that the short duration of the exercise bout was insufficient to trigger the fluid shifts and inflammatory responses that longer exposures produce. The young age and excellent health of the volunteers may also have allowed their compensatory mechanisms, including rapid plasma volume regulation, to fully offset any cold-induced hemoconcentration.

The study’s authors are careful to frame the findings within their limits. The cohort consisted exclusively of young, healthy, recreationally active men, a group whose cardiovascular systems respond flexibly to combined stress. The results cannot be extrapolated to older adults, people with coronary artery disease, or people with hypertension, in whom cold-induced vasoconstriction and impaired adaptive responses are known to raise event risk. Indeed, epidemiological data consistently show that winter cardiovascular mortality spikes precisely in populations whose hemorheological and vascular reserve is compromised.

Nevertheless, the work carries a practical message for the growing number of people who exercise outdoors in winter. For healthy young individuals, a brief high-intensity session in temperatures around −10 °C, dressed appropriately, does not appear to push blood rheology into a prothrombotic state beyond what the exercise itself already causes. The platelet increase observed in both groups reflects a normal, transient physiological response to maximal exertion rather than a cold-specific hazard, and it resolves as plasma volume normalizes.

The researchers suggest that future studies should examine longer cold exposures, female participants, older adults, and clinical populations to map where the protective compensatory mechanisms begin to fail. They also propose measuring additional markers such as catecholamines and inflammatory cytokines to clarify the signaling pathways linking cold, exertion, and blood rheology. Until then, the message for winter runners and athletes is cautiously reassuring: the deep cold of a January morning is unlikely to turn a hard sprint into a clotting crisis for a healthy young body, though anyone with cardiovascular risk factors should still treat cold-weather exertion with respect.

Subject of Research: Effects of acute sprint exercise in cold versus thermoneutral conditions on blood hemorheology in young men

Article Title: Acute hemorheological responses to an incremental shuttle-run test at -10 °C versus 18 °C in young healthy men

Article References: Teległów, A., Rembiasz, K., Pobędza, J., Cebula, Ł., Dziechciowski, Z., Czerwiński, A., Leśniowski, J., Marchewka, J., & Mika, P. (2026). Acute hemorheological responses to an incremental shuttle-run test at -10 °C versus 18 °C in young healthy men. Frontiers in Physiology, 17, Article 1935076. https://doi.org/10.3389/fphys.2026.1935076

Image Credits: AI Generated

DOI: 10.3389/fphys.2026.1935076

Keywords: hemorheology, blood viscosity, cold exposure, exercise physiology, platelets, fibrinogen, cardiovascular risk, shuttle-run test, plasma volume, winter sports, Acute, hemorheological

Allulose, sometimes labeled D-allulose or D-psicose, is a rare sugar that occurs naturally in small quantities in foods such as figs, raisins, and jackfruit. Chemically, it is a C-3 epimer of fructose, meaning it shares fructose’s molecular formula but differs in the spatial orientation of a single hydroxyl group. That tiny structural difference has large metabolic consequences: allulose is poorly digested in the small intestine, contributes negligible calories, and does not meaningfully raise blood glucose or insulin. It has attracted growing attention as a bulk sweetener for sugar-reduced beverages and foods, and recent work has explored how gut microbes can even utilize it as a substrate. Yet while its glycemic profile is well documented, rigorous human evidence that allulose can actively improve rehydration, rather than simply avoid harming it, has been scarce.

The new study, led by Hye Rim Kim and Jinsoo Noh of Samyang Corporation together with colleagues at Yonsei University in South Korea, set out to fill that gap with a carefully controlled human trial. Fifteen healthy men completed the protocol, which used a randomized, double-blind, crossover design widely regarded as the gold standard for nutrition interventions. In a crossover trial, each participant serves as his own control, experiencing both experimental conditions in randomly assigned order with a washout period in between. This design minimizes the noise created by individual differences in sweat rate, fitness, and baseline hydration, which are substantial in fluid-balance research. Neither the participants nor the investigators knew which beverage was consumed on a given trial day, protecting the results from expectation effects.

The experimental challenge was deliberately physiological. Each participant first underwent exercise-induced dehydration, losing body water through physical exertion in a controlled setting, before consuming one of two electrolyte-matched beverages. The drinks were identical in their electrolyte content, so the only meaningful variable was the sweetener: one contained allulose, while the control beverage was sweetened with sucralose, a non-nutritive sweetener that provides sweetness without any carbohydrate transport activity. By matching electrolytes and holding volume constant, the researchers isolated the specific contribution of allulose to the rehydration process. The team then tracked an extensive panel of outcomes, including plasma volume recovery, fluid retention, thirst ratings, body water composition measured by bioelectrical impedance analysis, urine specific gravity, blood glucose, and body temperature.

The headline result was striking. Plasma volume, the liquid component of blood that supports cardiovascular function and thermoregulation during exercise, recovered far more completely in the allulose condition. Participants who drank the allulose beverage showed a plasma volume recovery of roughly 28 percent relative to their post-exercise deficit, compared with only about 4 percent after the sucralose drink, a difference the authors report as statistically significant at p = 0.0001. Fluid retention told a similar story: the allulose drink allowed the body to hold onto approximately 91.5 percent of the ingested fluid, whereas the sucralose control retained about 80.6 percent, a gap of more than ten percentage points that reached significance at p = 0.0023. In practical terms, the allulose beverage did not just quench thirst; it kept the water inside the body where it could do physiological work.

Subjective thirst followed the objective measurements. Participants reported a significantly greater reduction in thirst after the allulose beverage, with a mean thirst reduction score of 6.47 compared with 4.60 for the control, significant at p = 0.0012. This matters because thirst is a surprisingly poor guide to true rehydration. People often stop drinking once oral sensations fade, long before plasma volume and cellular hydration are fully restored, a phenomenon that contributes to chronic mild dehydration in athletes and ordinary people alike. A beverage that both relieves thirst and measurably improves fluid retention addresses both the behavioral and the physiological sides of the problem. Notably, the researchers found no glycemic cost: blood glucose was not adversely affected, and there was no thermoregulatory penalty, meaning the sugar did not interfere with the body’s temperature control after exercise.

Why would a poorly digested sugar enhance fluid retention at all? The most plausible mechanism, which the authors situate within existing literature, involves sodium-glucose cotransport. The intestinal transporter SGLT1 couples the uptake of glucose and sodium, and in doing so creates an osmotic gradient that pulls water across the gut lining. Research on this transporter has shown it is an extraordinarily efficient facilitator of passive water movement, which is precisely why oral rehydration solutions for illness rely on glucose and sodium in balanced proportions. Although allulose is only partially absorbed, some transport activity appears sufficient to augment water uptake and, critically, to help retain absorbed fluid within the vascular and cellular compartments rather than letting the kidneys excrete it rapidly. The kidney’s own sodium-glucose cotransport systems may also contribute to keeping filtered water in the body.

The study’s measurement strategy adds credibility to these conclusions. Bioelectrical impedance analysis allowed the team to distinguish total body water, extracellular water, and intracellular water, offering a window into where the retained fluid actually resided. Urine specific gravity served as a standard clinical index of hydration status, reflecting how concentrated the kidneys are making urine; a lower value after rehydration indicates the body is conserving water rather than dumping it. Combining these objective markers with plasma volume estimates and subjective thirst scales gives a more complete picture than any single measure could, and the consistency across outcomes strengthens the case that the effect is real rather than a statistical artifact.

Several caveats deserve attention. The sample comprised fifteen healthy men, so the findings cannot yet be generalized to women, older adults, or clinical populations, and the modest sample size, while typical for crossover metabolic studies, means the confidence intervals around the effect estimates are wide. The trial was funded by Samyang Corporation, which provided the study material, and most of the authors are affiliated with the company; the disclosure is transparent, but independent replication will be important before the results are treated as settled. The comparison sweetener, sucralose, was a sensible inert control, but future work comparing allulose directly against glucose-based sports drinks would clarify whether it matches or exceeds the gold standard, not merely whether it beats a non-transported sweetener. Doses, timing, and effects during exercise rather than after it also remain open questions.

Even with those limitations, the implications are compelling. Sports nutrition guidelines have long emphasized that fluid replacement after exercise should account for electrolyte losses and that carbohydrate can aid absorption, but the caloric arithmetic has always been awkward for the vast majority of people who exercise for health rather than performance. Sugar-sweetened beverages are firmly implicated in the global rise of obesity and type 2 diabetes, while purely non-nutritive sweeteners have faced persistent questions about metabolic effects. Allulose occupies a genuinely different position: it tastes and behaves like a sugar in formulation, it is metabolically stable and largely excreted or fermented rather than converted to energy, and this trial now provides human evidence that it can actively enhance plasma volume recovery and fluid retention after dehydration. If larger independent trials confirm these findings, the rehydration aisle could be reshaped by a rare sugar that most consumers have never heard of, one that helps the body hold onto water without holding onto calories.

Subject of Research: Effects of allulose on post-exercise rehydration and plasma volume recovery in humans

Article Title: Allulose enhances plasma volume recovery and fluid retention after exercise-induced dehydration: a randomized, double-blind, crossover trial

Article References: Kim, H. R., Noh, J., Park, J., Hwang, H., Han, J., & Byun, S. (2026). Allulose enhances plasma volume recovery and fluid retention after exercise-induced dehydration: a randomized, double-blind, crossover trial. Food Science and Biotechnology. https://doi.org/10.1007/s10068-026-02329-z

Image Credits: AI Generated

DOI: 10.1007/s10068-026-02329-z

Keywords: allulose, rehydration, dehydration, plasma volume, fluid retention, exercise, rare sugar, sports nutrition, sucralose, crossover trial, sodium-glucose cotransport, low-calorie sweetener

The study, led by researchers including Edgar Cristancho-Mejía and Walter Schmidt, recruited healthy, non-smoking adults who had lived at their respective altitudes for at least two years. Participants came from six locations spanning 970 to 2600 meters, including regions around Bogotá in the Central Eastern Andes and the lower Western Andes near Tuluá and Cali. Crucially, none of the participants were endurance athletes, eliminating a major confounding factor that has plagued earlier comparisons between highland populations and sea-level dwellers. The cohort included 133 men and 148 women, with ancestry distributions reflecting the regional population mix of African, European, and Native American heritage.

Measuring hemoglobin mass directly is technically demanding, and the team used the optimized carbon monoxide rebreathing method, widely regarded as the gold standard. Participants inhaled a precisely calculated dose of carbon monoxide over two minutes, and the researchers tracked how much of it bound to hemoglobin in the blood, allowing an accurate calculation of total hemoglobin mass. From this, together with venous hemoglobin concentration and hematocrit, the team derived red cell volume, plasma volume, and total blood volume. Blood samples drawn from a cubital vein provided hemoglobin concentration, hematocrit, and oxygen saturation values for each participant.

The results revealed a clear and unexpected divergence in thresholds. Hemoglobin mass rose significantly only in residents living at 2180 meters or higher, with the effect becoming firmly established at 2600 meters. Plasma volume, by contrast, was already significantly reduced at 1730 meters—roughly 450 meters lower than the erythropoietic threshold. Between the lowest and highest study locations, hemoglobin concentration increased by about 1.0 grams per deciliter in men and 1.3 grams per deciliter in women, while hematocrit rose by roughly 3.2 to 3.7 percentage points. Relative to lean body mass, hemoglobin mass was 5.8 percent higher in men and 8.9 percent higher in women at 2600 meters compared with 970 meters, while plasma volume per kilogram of lean mass fell by about 6 percent in both sexes.

The physiological logic behind these two thresholds traces back to the shape of the oxygen dissociation curve. This sigmoid curve allows arterial blood to remain nearly fully saturated with oxygen up to altitudes of roughly 2000 meters, meaning the kidneys—the primary sensors of oxygen availability—detect little reason to stimulate red cell production. Below that ceiling, the flat upper portion of the curve buffers the modest drop in ambient oxygen pressure. Once saturation begins to fall more steeply, however, the kidney responds by releasing erythropoietin, triggering the bone marrow to expand hemoglobin mass. Plasma volume regulation operates on a different logic entirely: it responds to the overall oxygen-carrying deficit rather than to a specific saturation threshold, and it does so within days rather than weeks.

When the researchers combined their data with earlier measurements taken at 420 and 3800 meters in Bolivia using the same methodology, a striking pattern emerged. Hemoglobin mass increased slowly and modestly up to about 2600 meters, then climbed sharply at higher elevations, reaching values 27 percent above near-sea-level levels at 3800 meters. Notably, within the moderate-altitude range of the current study, there was no significant relationship between hemoglobin mass and oxygen saturation—suggesting that up to roughly 2600 meters, altitude exerts only a gentle influence on erythropoiesis. Above that point, the diminishing oxygen supply becomes the dominant driver, most likely registered by oxygen-sensing mechanisms in the kidneys.

To quantify how much each component contributed to the observed rise in hemoglobin concentration, the team performed multiple regression analyses. The calculations showed that every 100-gram increase in hemoglobin mass raised concentration by 1.59 grams per deciliter, while every liter of plasma volume loss raised it by 3.07 grams per deciliter. Applying these coefficients to the altitude-related changes, the researchers found that of the 1.2 grams per deciliter elevation in hemoglobin concentration at 2600 meters, approximately 0.56 grams per deciliter was attributable to increased hemoglobin mass and 0.61 grams per deciliter to reduced plasma volume—a near-perfect split. When the team applied an alternative correction for the Fåhraeus effect, which accounts for differences between central and peripheral hematocrit, the plasma volume contribution grew even larger.

The findings carry immediate clinical weight. Hemoglobin concentration is one of the most commonly ordered diagnostic measurements in medicine, and anemia thresholds are already adjusted for altitude. Yet the hemoconcentration caused by plasma volume contraction at moderate elevations has never been factored into those thresholds. If a substantial fraction of the altitude-related rise in hemoglobin reflects a shrunken liquid blood volume rather than genuinely enhanced oxygen-carrying capacity, then current anemia criteria may misclassify residents of towns like Bogotá at 2600 meters. The study also resonates with recent work on high-altitude populations worldwide: data from Sherpa communities in the Himalayas suggest their lower hemoglobin concentrations compared with Andean highlanders stem largely from larger plasma volumes rather than smaller hemoglobin masses, underscoring that the ratio of red cells to plasma, not red cell count alone, defines the blood’s oxygen transport profile.

The authors acknowledge limitations. Sample sizes at individual altitudes were modest, and the high interindividual variability in hemoglobin mass—driven largely by differences in lean body mass—means the altitude effect, though statistically robust, is just one of several factors shaping any person’s blood profile. Ambient temperature may also play a role, since the lowland study sites were considerably warmer, and heat acclimatization is known to expand plasma volume. Plasma volume itself was calculated indirectly rather than measured directly, and the Fåhraeus correction introduces some uncertainty, though the researchers showed that an altitude-adjusted correction strengthened rather than weakened their conclusions. Nevertheless, the study provides the first systematic evidence that the blood’s response to moderate altitude unfolds in two distinct stages: plasma contracts first, at around 1730 meters, and the erythropoietic machinery follows at roughly 2180 meters. For the roughly 5 percent of humanity living above 1500 meters, that distinction may soon matter at the doctor’s office as much as on the mountain.

Subject of Research: Altitude thresholds for hemoglobin mass and plasma volume regulation and their effects on hemoglobin concentration in Andean residents

Article Title: Altitude thresholds for hemoglobin and plasma volume regulation in adult residents of different altitudes and their impact on hemoglobin concentration

Article References: Cristancho‐Mejía, E., Trompetero‐Gonzalez, A. C., Benavides‐Pinzón, W. F., & Schmidt, W. F.-J. (2026). Altitude thresholds for hemoglobin and plasma volume regulation in adult residents of different altitudes and their impact on hemoglobin concentration. Physiological Reports, 14(19), Article e71108. https://doi.org/10.14814/phy2.71108

Image Credits: AI Generated

DOI: 10.14814/phy2.71108

Keywords: hemoglobin mass, plasma volume, altitude, hypoxia, erythropoiesis, Andes, Colombia, anemia thresholds, oxygen saturation, blood volume, CO rebreathing, physiology