Alterations in glycemic control and glucose tolerance following overtraining in endurance athletes

Previous research has demonstrated attenuated exercising carbohydrate oxidation and circulating glucose when endurance athletes are functionally overreached; however, whether free-living glycemic control is altered with overreaching is unknown. Trained endurance athletes (M:8, F:2) completed a 5-week training protocol composed of three phases: 1 week of reduced training, 3 weeks of high-intensity overtraining, and 1 week of recovery training. Participants wore continuous glucose monitors (CGM) to collect minute-by-minute interstitial glucose concentrations, and data were analyzed for weeks 1 (PRE), 4 (POST), and 5 (REC). A fasted 75 g oral glucose tolerance test (OGTT) was performed at the end of each phase with repeated capillary sampling to measure glucose concentrations. CGM-derived 24 h average glucose, overnight average glucose (12  am to 6  am), and daily time spent in low (<3.9 mmol/L), normal (3.9-5.0 mmol/L), and medium-high (5.1-7.8 mmol/L) glucose ranges were not different across training phases. The proportion of time with elevated interstitial glucose (>7.8 mmol/L) was elevated at REC (3.5 ± 1.1%) relative to PRE (2.4 ± 1.9%, P = 0.05) and POST (2.1 ± 1.0%, P = 0.021; ηp² = 0.66). The daily mean amplitude of glycemic excursions also increased at REC (2.6 ± 0.2 mmol/L) relative to PRE (2.3 ± 0.3 mmol/L, P = 0.05) and POST (2.2 ± 0.3 mmol/L, P = 0.008; ηp² = 0.41). Capillary glucose area-under-the-curve was nonsignificantly reduced from PRE (852.5 ± 87.6 mmol/L·min) to POST (811.4 ± 77.5 mmol/L·min, P = 0.06), and elevated from POST-to-REC (866.3 ± 79.8 mmol/L·min, P = 0.02; ηp² = 0.3) during the OGTT. Glycemic control and glucose tolerance are unchanged with overtraining; however, following a week of recovery there is greater time spent with elevated glucose and greater glucose variability suggesting disrupted glycemic control during recovery.