DRINK COMPOSITION AND THE ELECTROLYTE BALANCE OF CHILDREN EXERCISING IN THE HEAT

Twelve 9- to 12-year-old children (6 boys, 6 girls) performed four exe rcise-in-heat (35 degrees C, 45% RH) trials which differed in the comp osition of the fluids they drank. In each trial, subjects cycled for o ne 20-min and two 15-min bouts at 50% peak VO2 with 10-min rest period s in between. In a fourth bout, they cycled at 90% peak VO2 until exha ustion. Drinks had the same grape flavor and were assigned in a double -blind design and in a Latin-square order. Subjects drank 7 ml . kg(-1 ). h(-1) to keep them euhydrated. Three of the drinks had 6% carbohydr ates (CHO), with different [Na+]: 0, 8.8, 18.5 mmol . l(-1) and one dr ink had neither CHO nor Na+ (WATER). Among drink trials, there were no differences in the increase in rectal temperature, HR, or performance time to exhaustion. Despite the larger Na+ deficit induced by the Na free drinks compared with the Na+ drinks (11.8 + 1.4 vs 5.7 +/- 0.9 m mol . h(-1)), neither plasma [Na+] nor osmolality were affected. These results suggest that electrolyte, as in the above conditions, did not affect electrolyte balance, thermoregulatory responses, or aerobic pe rformance of children exercising in the heat. The greater Na+ deficit induced by ion-free drinks was of minor biological importance.