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

Abstract

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% $\dot{V}$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.