Serial analyses of sweat NaCl concentrations by iontophoresis in children with cystic fibrosis (CF) commonly show unexplained fluctuations. Some children with previously normal values subsequently develop elevated levels. Some allergic children have values in the high normal range during acute respiratory or gastrointestinal infections and these levels decrease upon recovery. We have measured serially sweat NaCl levels in 22 children with chronic intestinal problems and/or chronic pulmonary disease who had had sweat Cl concentrations greater than 60 mEq/L. Tests were done before and after periods of 2-13 days on an elemental diet, Vivonex, 1800-2400 cal/day with 2-3 gm Nad/day, and after varying periods on diets excluding foods to which children are commonly sensitive. Daily NaCl intake was kept in a similar range on all diets. Seven of 22 had a striking decrease in sweat Cl to normal (range 13-60 mEq/L) on one or more diets. (The one patient with a low value of 60 mEq/L had levels on other diets over 100 mEq/L.) Three of these had initial values of 80, 84 and 85 mEq/L. In 4 patients normal concentrations persisted weeks to months while on a limited diet. Four of the 7 were shown by complement (C3) alteration following food challenges to be sensitive to foods. These results support the concept that all children with elevated sweat Cl levels do not have CF and suggest that sweat NaCl concentrations may be a function of diet composition even when salt intake is kept constant.

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