Dietary salt restriction improves pulmonary function in exercise-induced asthma

PURPOSE: Exercise-induced asthma (EIA) occurs in approximately 90% of persons with asthma. The mechanism has not been delineated. Epidemiological studies have suggested that dietary salt may play a role in airway responsiveness. Therefore, the purpose of this study was to determine the influence of both elevated and restricted salt diets on pulmonary function in subjects with EIA. METHODS: Eight subjects with EIA and eight subjects without EIA (control) participated in a double-blind crossover study. Pulmonary function was determined pre- and post-exercise challenge before and after 2 wk on a normal salt, sodium chloride, diet (NSD), a low salt diet (LSD), and a high salt diet (HSD). A 1-wk washout occurred between diets. RESULTS: Diet had no effect on preexercise pulmonary function values in either group and had no effect on postexercise pulmonary function values in control subjects. However, LSD improved and HSD worsened postexercise pulmonary function values in EIA subjects. Forced expiratory volume in 1 s (FEV1) decreased by at least 10% in EIA subjects with exercise. In EIA subjects, FEV1 decreased by 14+/-6% on LSD, 20+/-7% on NSD, and 24+/-6% on HSD at 15 min postexercise. Similar patterns were observed for forced vital capacity and peak expiratory flow rates. Although LSD did not normalize pulmonary function in EIA, it did improve it. CONCLUSIONS: These data suggest that individuals with EIA might benefit from lower salt diets.