Blood glucose responses to carbohydrate feeding prior to exercise in the heat: Effects of hypohydration and rehydration

This study assessed the plasma glucose (PG) and hormonal responses to carbohydrate ingestion, prior to exercise in the heat, in a hypohydrated state versus partial rehydration with intravenous solutions. On separate days, 8 subjects (21.0 ± 1.8 years; 57.3 ± 3.7 ml · kg^-1 min^-1) exercised at 50% V̇O_2max in a 33 °C environment until a 4% body weight loss was achieved. Following this, subjects were rehydrated (25 ml · kg^-1) with either: 0.45% IV saline (45IV), 0.9% IV saline (9IV), or no fluid (NF). Subjects then ingested 1 g · kg^-1 of carbohydrate and underwent an exercise test (treadmill walking, 50% V̇O_2max, 36 °C) for up to 90 min. Compared to pre-exercise level (294 mg · dl^-1), PG increased significantly (>124 mg · dl^-1) at 15 min of the exercise test in all trials and remained significantly elevated for 75 min in NF, 30 min more than in the 2 rehydration trials. Although serum Insulin increased significantly at 15 min of exercise in the 45IV trial (7.2 ± 1.2 vs. 23.7 ± 4.7 μIU · ml^-1), no significant differences between trials were observed. Peak plasma norepinephrine was significantly higher in NF (640 ± 66 pg · ml^-1) compared to the 45IV and 9IV trials (472 ± 55 and 474 ± 52 pg · ml^-1, respectively). In conclusion, ingestion of a small solid carbohydrate load prior to exercise in the 4% hypohydration level resulted in prolonged high PG concentration compared to partial IV rehydration.