Abstract / Summary
Abstract To characterize in-race interstitial glucose dynamics in highly trained male distance runners using continuous glucose monitoring (CGM), and to test whether the direction of the glucose–performance association differs between actual competition and race-pace training. Fifty-nine highly trained (McKay Tier 3) male distance runners from three collegiate squads wore flash CGM during 203 race-pace training sessions and 116 officially sanctioned races. Glucose was modelled with linear mixed-effects models including the athlete as a random intercept. Performance was defined in both conditions as attainment of a pre-established coach–athlete goal time, and additionally on a continuous rating scale within each condition. Because the two conditions were contributed by largely different athletes, the analysis was repeated in the 14 athletes observed in both. Glucose ran far higher in competition than in training (during-exercise average 193.1 ± 39.1 vs. 127.8 ± 27.3 mg/dL), and the separation was already present 2 h before the event began. The association with performance ran in opposite directions in the two contexts: athletes who met their goal time had lower glucose than those who did not during training (− 20.3 mg/dL, 95% CI − 29.9 to − 10.8), but higher glucose during competition (+ 33.3 mg/dL, 95% CI 19.2 to 47.4), an interaction of + 52.4 mg/dL (95% CI 36.1 to 68.8). About half of that difference was established before the start, and the interaction remained after adjustment for pre-exercise glucose (+ 28.5 mg/dL, 95% CI 16.4 to 40.6). It also held in the 14 athletes observed in both conditions (+ 41.9 mg/dL, 95% CI 13.8 to 70.0), although their competition-side association alone was not significant. In-race carbohydrate intake was not measured; excluding the races in which it was permitted did not materially alter the association, and neither did adjustment for event duration. Competition is accompanied by a large interstitial glucose elevation, and glucose relates to goal attainment in opposite directions in competition and in training. These data are consistent with a stress-related increase in hepatic glucose mobilization but cannot separate it from differences in exercise intensity, pre-event nutrition or other competition-specific factors, and the two conditions were contributed by largely different athletes. CGM nonetheless makes in-competition metabolic physiology directly observable in the field.