Responses to moderate and low sodium diets during exercise-heat acclimation.

This investigation examined whether low sodium (Na+) (LNA; 68 mEq Na+.d-1) or moderate Na+ (MNA; 137 mEq Na+.d-1) intake allowed humans to maintain health, exercise, and physiologic function during 10 days of prolonged exercise-heat acclimation (HA). Seventeen volunteers, ages 19 to 21, consumed either LNA (n = 8) or MNA (n = 9) during HA (41 degrees C, 21% RH; treadmill walking for 30 min.h-1, 8 h.d-1 at 5.6 km.h-1, 5% grade), which resulted in significantly reduced heart rate, rectal temperature, and urine Na+ for both groups. There were few between-diet differences in any variables measured. Mean plasma volume in LNA expanded significantly less than in MNA by Days 11 and 15, but reached the MNA level on Day 17 (+12.3 vs. +12.4%). The absence of heat illness, the presence of normal physiologic responses, and the total distance walked indicated successful and similar HA with both levels of dietary Na+.