Effects of hypohydration and oral or intravenous saline rehydration on glucose and insulin responses to carbohydrate feeding prior to exercise in the heat

This study assessed the plasma glucose (PG) and insulin (INS) responses, during exercise in the heat, to carbohydrate (CHO) ingestion in a hypohydrated state versus oral or intravenous (IV) rehydration with saline solutions. It also compared the efficacy of the different methods of rehydration in alleviating the detrimental effects of hypohydration on subsequent exercise in a hot environment.^ Eight male subjects (age, 21.8 $\pm$ 1.0 yr; mass 74.6 $\pm$ 2.5 kg; VO$\sb{\rm 2max}$, 57.3 $\pm$ 1.3 ml$\cdot$kg$\sp{-1}\cdot$min$\sp{-1}$) volunteered for this study. On five separate days, each subject performed moderate exercise in a 33$\sp\circ$C environment until a 4% body weight loss was achieved. Subjects then underwent one of four treatments over 45 min: rehydration (25ml$\cdot$kg$\sp{-1}$) with 0.9% IV saline (9IV), 0.45% IV saline (45IV), 0.45% oral saline (ORAL) or no fluid (NF). After a 55 min rest, subjects ingested 1g$\cdot$kg$\sp{-1}$ of a commercially available solid glucose-sucrose CHO food. A fifth trial (n = 3) involved ORAL rehydration but a placebo gelatin food was consumed instead of CHO (NC). Following this, subjects underwent an exercise-heat tolerance test (EX) (walking on treadmill, 50% VO$\sb{\rm 2max}$, T$\sb{\rm amb}$ = 36$\sp\circ$C) for up to 90 min.^ Exercise duration in NF was significantly shorter (p $\u3c$ 0.05) than in 9IV, 45IV, ORAL and NC(58 $\pm$ 8 min vs. 76 $\pm$ 6, 77 $\pm$ 5, 86 $\pm$ 2, and 69 $\pm$ 9 min respectively), but there were no differences between rehydration methods or due to CHO consumption. During exercise, Rectal temperatures were significantly higher in the NF trial throughout EX compared to all other trials. However, increased rectal temperature did not appear to limit exercise tolerance. PG increased significantly (p $\u3c$.05) over pre-exercise (PREX) after 15 min but had returned to PREX levels by 45 min in 9IV, 45IV, NC, and ORAL whereas it remained elevated for 75 min in NF. These differences persisted even when corrected for changes in plasma volume. No significant differences were noted in INS response. However, a strong correlation between INS and PG was present in 9IV, 45IV, NC and ORAL (r =.65 to.77, p $\u3c$.05) but not in NF (r =.38, p $\u3e$.05). The change in norepinephrine concentration during EX was significantly greater (p $\u3c$ 0.05) in NF and NC than in the other three trials.^ It was concluded that no differences existed in the efficacy of the different rehydration methods to prolong exercise tolerance in the heat. However, NF resulted in decreased exercise endurance. In addition the $-$4% hypohydration level was associated with elevated PG levels for a more prolonged time during exercise, suggesting an impairment of glucose disposal caused by hypohydration and/or hyperosmolality.