The research team, led by Taira Kajisa and Toshiyuki Sakai of Toyo University, equipped 20 male athletes classified as Tier 3 Highly Trained/National with FreeStyle Libre 2 continuous glucose monitoring (CGM) sensors. These devices measure glucose in the interstitial fluid just beneath the skin, providing a reading every 15 minutes without needles during exercise. The cohort contributed 36 race-event datasets spanning track running, road relays, half-marathons, cross country, and race walking, generating 16,360 individual glucose data points collected from three days before to three days after each race.

The study design addressed a critical confound: carbohydrate intake. Athletes, all living in a team dormitory with standardized meals, completed solid carbohydrate consumption at least 120 minutes before race starts, and event regulations prohibited carbohydrate-containing fluids during races. This means the glucose elevations observed mid-race could not be explained by recent fueling. Fifteen of the athletes also wore sensors during high-intensity training sessions prescribed at the identical race pace, on matching course types, within eight days of the competition, enabling a rare within-subject comparison.

The statistical picture was unambiguous. Mean glucose during the pre-race hour climbed to 8.3 ± 1.5 millimolar, well above the cohort’s overall mean of 6.1 millimolar, and then surged to a competition peak of 11.2 ± 2.2 millimolar, nearly double typical resting values. After the finish line, glucose dropped sharply, falling to 6.7 millimolar within an hour. A repeated-measures analysis of variance confirmed a powerful effect of race phase, and paired comparisons adjusted with the Holm-Bonferroni procedure showed the jump from pre-race to during-race glucose carried a large effect size, with Cohen’s dz of 1.45.

The within-subject comparison between competition and training proved the study’s centerpiece. Peak interstitial glucose reached 12.91 ± 1.93 millimolar in races versus 11.02 ± 1.18 millimolar in matched training bouts, a mean difference of 1.89 millimolar with a 95 percent confidence interval of 1.06 to 2.72. The paired t-test yielded p = 0.000131, and the geometric mean ratio of 1.166 translates to approximately 17 percent higher competitive peaks. Exploratory analyses showed the difference was not driven by race distance: short events under 10 kilometers showed +2.17 millimolar, mid-distance events +2.15, and longer events +1.40, a gradient that actually runs opposite to what a pure intensity-difference explanation would predict.

What could drive this competition-specific hyperglycemia? The authors frame their findings within classical stress physiology. Competitive arousal activates the sympathetic nervous system, releasing epinephrine and norepinephrine, which stimulate the liver to break down glycogen and dump glucose into the bloodstream. Causal support exists in the literature: in adrenalectomized humans, adrenaline infusion during exercise directly increases hepatic glucose production. Crucially, muscle glycogen stores, though five times larger than the liver’s roughly 100 grams, cannot release glucose into blood, so the racing hyperglycemia must reflect hepatic mobilization, not muscle fuel leaking into circulation.

A preliminary validation experiment underscored the technique’s reliability. A sub-elite marathoner (personal best under three hours) ran 5 kilometers fasted at 4 minutes per kilometer while wearing both a CGM and periodically sampling finger-prick blood glucose. CGM tracked capillary glucose closely, with the expected lag of about 15 minutes and a modest average underestimate of roughly 1.5 millimolar. During an 18-day monitoring window surrounding a full marathon, this athlete’s glucose spiked above 10 millimolar just before the start, peaked at 11.7 millimolar mid-race, and stayed elevated through the night, while easy practice runs kept glucose hovering near 5.4 millimolar.

The researchers are careful, refreshingly so, about what the data cannot prove. Internal physiological load was not measured; heart rate, power, and blood lactate went unrecorded, and pace was prescribed rather than verified during training. The authors acknowledge they cannot definitively attribute the competition-training difference to psychological stress rather than subtle differences in effort. A single bipolar pre-race questionnaire item on arousal showed no significant link to glucose, but the team itself argues the measure was inadequate, calling instead for validated instruments like the Competitive State Anxiety Inventory-2 and direct measurements of plasma or salivary catecholamines and cortisol.

The practical implications could reshape how endurance athletes prepare. Athletes whose glucose already soars before the gun may not need aggressive pre-race carbohydrate supplementation, suggesting individualized monitoring could personalize fueling strategies. Coaches might deliberately build competition-like stress into training sessions to acclimate athletes’ metabolism to the competitive milieu, since exogenous carbohydrate intake cannot reproduce the endocrine signature of real racing. And the sharp post-race glucose drop within an hour highlights a narrow recovery window when prompt glycogen replenishment matters most.

No athlete in the study experienced intra-race hypoglycemia despite the extraordinary glucose turnover, a reassuring finding given the high metabolic demands documented. The cohort’s limitations, all male, all collegiate, all from a single university squad, mean generalization to female athletes, masters competitors, or world-class Tier 4-5 performers requires caution. Yet the study stands as a proof of concept that wearable CGM technology can capture the metabolic fingerprint of competition itself, in the chaotic real world where athletic destinies are decided rather than in sanitized laboratory conditions. Future work pairing glucose data with heart rate variability, psychometric stress scales, and hormone sampling may finally untangle how much of the racing surge comes from the mind rather than the muscles.

Subject of Research: Interstitial glucose responses to sanctioned competition versus high-intensity training in highly trained male collegiate endurance athletes measured by continuous glucose monitoring

Article Title: Interstitial glucose during sanctioned competition and during high‐intensity training in highly trained collegiate male endurance athletes: A real‐world continuous glucose monitoring study

Article References: Kajisa, T., & Sakai, T. (2026). Interstitial glucose during sanctioned competition and during high‐intensity training in highly trained collegiate male endurance athletes: A real‐world continuous glucose monitoring study. Physiological Reports, 14(17), Article e71099. https://doi.org/10.14814/phy2.71099

Image Credits: AI Generated

DOI: 10.14814/phy2.71099

Keywords: continuous glucose monitoring, endurance athletes, competition stress, hepatic glucose output, catecholamines, exercise metabolism, running, race walking, glycogen, sports physiology, interstitial glucose, athletic performance