Impact of 3-day high and low dietary sodium intake on sodium status in response to exertional-heat stress: a double-blind randomized control trial.

Purpose: To determine the impact of altering dietary sodium intake for 3 days preceding exercise on sweat sodium concentration [Na ⁺ ], and cardiovascular and thermoregulatory variables.
Methods: Fifteen male endurance athletes (runners n = 8, cyclists n = 7) consumed a low (LNa, 15 mg kg ^-1  day ^-1 ) or high (HNa, 100 mg kg ^-1  day ^-1 ) sodium diet, or their usual free-living diet [UDiet, 46 (37-56) mg kg ^-1  day ^-1 ] for 3 days in a double-blind, randomized cross-over design, collecting excreted urine (UNa) and refraining from exercise. On day 4, they completed 2 h running at 55% [Formula: see text]O _2max or cycling at 55% maximum aerobic power in T _amb 35 °C. Pre- and post-exercise blood samples were collected, and sweat from five sites using absorbent patches along the exercise protocol.
Results: UNa on days 2-3 pre-exercise [mean (95% CI) LNa 16 (12-19) mg kg ^-1  day ^-1 , UDiet 46 (37-56) mg kg ^-1  day ^-1 , HNa 79 (72-85) mg kg ^-1  day ^-1 ; p < 0.001] and pre-exercise aldosterone [LNa 240 (193-286) mg kg ^-1  day ^-1 , UDiet 170 (116-224) mg kg ^-1  day ^-1 , HNa 141 (111-171) mg kg ^-1  day ^-1 ; p = 0.001] reflected sodium intake as expected. Pre-exercise total body water was greater following HNa compared to LNa (p < 0.05), but not UDiet. Estimated whole-body sweat [Na ⁺ ] following UDiet was 10-11% higher than LNa and 10-12% lower than HNa (p < 0.001), and correlated with pre-exercise aldosterone (1st h r =  - 0.568, 2nd h r =  - 0.675; p < 0.01). Rectal temperature rose more quickly in LNa vs HNa (40-70 min; p < 0.05), but was similar at the conclusion of exercise, and no significant differences in heart rate or perceived exertion were observed.
Conclusions: Three day altered sodium intake influenced urinary sodium excretion and sweat [Na ⁺ ], and the rise in rectal temperature, but had no effect on perceived exertion during moderate-intensity exercise in hot ambient conditions.