Preventing dehydration in children with cystic fibrosis who exercise in the heat

Purpose: In healthy children who exercise in the heat, the addition of flav or, carbohydrate, and 18 mmol.L-1 NaCl to water induced a major increase in voluntary drink intake compared with the intake of unflavored water. This increase was sufficient to prevent voluntary dehydration. We hypothesized t hat, to achieve a similar effect in children with cystic fibrosis (CF), who se NaCl losses in sweat are markedly excessive, the drink should include an NaCl concentration higher than 18 mmol.L-1. Methods: Eleven subjects with CF (6 girls, 5 boys, ages 10.9-19.5 yr) attended three 3-h sessions of inte rmittent exercise of moderate intensity (four 20-min bouts), at 35 degrees C, 50% relative humidity. Either water (W), flavored water (FW), or a 30 mm ol.L-1 NaCl plus 6% carbohydrate solution (Na30) was offered od libitum, in a counterbalanced sequence. Six subjects performed an additional session i n which they drank a 50 mmol.L-1 NaCl-6% CHO solution (Na50). Results: Ther e was no significant drink effect on body fluid balance, core temperature, heart rate, or serum electrolytes with W, FW or Na30. Serum osmolality decr eased throughout the sessions from 290.6 +/- 1.1 (mean +/- SEM) to 281.3 +/ - 1.2 mmol.kg(-1) (P < 0.0005), serum sodium from 143.1 +/- 0.5 to 141.1 +/ - 0.7 mmol.L-1 (P = 0.01) and serum chloride from 109.1 +/- 0.5 to 107.5 +/ - 0.5 mmol.L-1 (P < 0.001). In contrast, the 50 mmol.L-1 NaCl drink induced a near significant (P = 0.08) higher fluid intake, and it significantly am eliorated the rate of progressive dehydration. Conclusions: The marked loss of NaCl in the sweat of CF patients may induce an hypo-osmolar state in th e serum, even when the drink contains 30 mmol.L-1 NaCl. This may diminish t he thirst drive triggered by hypothalamic osmoreceptors and may lead to vol untary dehydration. A flavored drink with an even higher salt content (50 m mol.L-1), however, enhances drinking and attenuates the voluntary dehydrati on.