Effects of Oral Sodium Supplementation on Indices of Thermoregulation in Trained, Endurance Athletes

Guidelines recommend the consumption of sodium during exercise to replace losses in sweat; however, the effects of sodium on thermoregulation are less clear. To determine the effects of high-dose sodium supplementation on indices of thermoregulation and related outcomes, 11 endurance athletes participated in a double-blind, randomized-sequence, crossover study in which they underwent 2-hrs of endurance exercise at 60% heart rate reserve with 1800 mg of sodium supplementation (SS) during one trial and placebo (PL) during the other trial. A progressive intensity time-to-exhaustion test was performed after the 2-hr steady state exercise as an assessment of exercise performance. Sweat rate was calculated from changes in body weight, accounting for fluid intake and urinary losses. Ratings of perceived exertion (RPE) and heat stress were assessed using verbal numeric scales. Cardiovascular drift was determined from the rise in HR during the 2-hr steady state exercise test. Skin temperature was measured with an infrared thermometer. Dehydration occurred in both SS and PL trials, as evidenced by substantial weight loss (2.03 ± 0.43% and 2.27 ± 0.70%, respectively; p = 0.261 between trials). Sweat rate was 1015.53 ± 239.10 ml·hr-1 during the SS trial and 1053.60±278.24 ml/hr during the PL trial, with no difference between trials (p = 0.459). Heat stress ratings indicated moderate heat stress (“warm/hot” ratings) but were not different between trials (p = 0.825). Time to exhaustion during the SS trial was 6.88 ± 3.88 minutes and during the PL trial averaged 6.96 ± 3.61 minutes, but did not differ between trials (p = 0.919). Cardiovascular drift, skin temperature, and RPE did not differ between trials (all p > 0.05). High-dose sodium supplementation does not appear to impact thermoregulation, cardiovascular drift, or physical performance in trained, endurance athletes. However, in light of the possibility that high sodium intakes might have other adverse effects, such as hypertension, it is our recommendation that athletes interpret professional recommendations for sodium needs during exercise with caution. Key points Based on current professional recommendations to replace sodium losses in sweat during exercise, some endurance athletes consume salt or other electrolyte supplements containing sodium during training and competition, however the effects of sodium on thermoregulation are less clear. High-dose sodium supplementation does not appear to impact thermoregulation, cardiovascular drift, or physical performance in trained, endurance athletes. The possibility remains that high sodium intakes might have other adverse effects. It is our recommendation that athletes interpret professional recommendations for sodium needs during exercise with caution. Key words: Salt, sweat, hydration, heat stress, heart rate, electrolyte Introduction The physiological effects of decreased plasma volume and low serum sodium concentration have been studied extensively in endurance performance. It is clear that appropriate hydration plays a role in thermoregulation and cardiovascular changes during exercise (von Duvillard et al., 2004). Although increases in sodium intake are encouraged to offset sodium losses in the form of sweat (ACSM exercise and fluid replacement guidelines recommend 20-30 mEq sodium·L-1 (~460-690 mg·L-1) fluid during exercise), high serum sodium concentrations reduce sweat rates and thus, could impair thermoregulation (Cosgrove and Black, 2013; Sawka et al., 2007; Shibasaki et al., 2009). Based on recommendations and perceptions, many endurance athletes consume salt supplements while exercising, despite the possibility that salt may impair thermoregulation during prolonged endurance exercise. During endurance exercise, sweat rates of athletes typically average 1.0-1.5 L of fluid per hour. The amount of sodium lost during exercise averages 0.8 grams per liter of sweat, but can vary with genetics, diet, heat acclimatization and hydration status (Sawka et al., 2007). One proposed mechanism by which sodium alters sweat rate is due to its impact on active cutaneous vasodilation through changes in plasma osmolality. As core body temperature increases during exercise, skin blood flow and sweat rate increase. Subsequently, heat dissipation occurs. Increased plasma osmolality, either by ingestion of sodium or dehydration, raises the core body temperature threshold at which sweating occurs, delaying cutaneous vasodilation and lowering sweat rate (Wendt et al., 2007). Previous studies have addressed the effects of pre-exercise sodium loading on sweat rate and core body temperature, however the results were conflicting. One study showed no significant differences in sweat rate or core temperature during cycling with an isotonic sodium load vs. a hypotonic placebo in male cyclists (Coles and Luetkemeier, 2005). Two other studies with nearly identical protocols found that a concentrated sodium beverage delayed the rise in core body temperature, and lowered sweat rate when compared to a low sodium beverage in female cyclists, but the same effect was not seen with male cyclists (Sims et al., 2007a; 2007b). The primary purpose of this research study was to determine the effects of high-dose salt consumption during long-duration endurance exercise on indices of thermoregulation in trained, endurance athletes. Earlier research studies focused on largely on pre-exercise sodium intake and used lower doses of sodium supplementation (~300-700 mg·hr-1). Additionally, the method of sodium delivery was typically through oral fluid ingestion or intravenous injections in a laboratory setting, as opposed to tablets or capsules, which more closely resemble real-world practices. Because thermoregulation impacts other areas of endurance performance, a secondary purpose of the study was to examine the effects of salt supplementation on cardiovascular drift, perceived rating of exertion and time to exhaustion. It was hypothesized that high-dose salt supplementation during 2 hours of endurance exercise would decrease sweat rate, increase perceived heat stress, decrease the magnitude of cardiovascular drift, decrease the rating of perceived exertion, and increase time to exhaustion.