High-Dose Sodium Supplementation on Hydration and Thermoregulatory Responses while Cycling in the Heat

Sodium is commonly included in sports beverages to promote hydration and support carbohydrate (CHO) absorption, typically at concentrations of ~270-400mg. Recently, high-dose sodium supplements, often approaching 1000mg and lacking CHO, have entered the market with minimal evidence to support their use. PURPOSE: Investigate how hydration and thermoregulatory responses change during cycling in the heat when consuming a supplement containing 810mg of sodium. METHODS: Twenty recreationally active participants (15 males, 5 females; age = 23.4 ± 2.8) completed a 2-h bout of cycling in an environmental chamber (37.3℃, 59.1 ± 4.4% relative humidity) on two occasions separated by > 7 days. Participants were randomly provided a large-dose sodium supplement (810mg/532 mL of water) before one trial. Participants were instructed to consume 266mL 1-h pre-trial and consumed the remaining 266mL within the first hour of cycling. Pre- and post-cycling urine samples and body mass measurements were collected to determine acute hydration status. Water and a sports beverage were provided ad libitum and fluid intake was recorded every 30-min during cycling. Paired samples t-tests and repeated measures ANOVA were used to analyze differences between condition and time. Significance level was set at p < 0.05. RESULTS: For all time points and between conditions, there were no significant differences found for urine specific gravity (Usg), plasma volume change (ΔPV), or percentage of body mass loss (%BM) pre- to post-cycling (p > 0.05). Pre-cycling Usg was 1.013 ± 0.007 (control) and 1.017 ± 0.014 (supplement). Post-cycling, Usg was 1.021 ± 0.020 (control) and 1.017 ± 0.009 (supplement). For both trials, average ΔPV was 1.0 ± 0.2. For %BM losses, the control group lost an average of -0.5 ± 1.2%BM while the supplement group lost -0.2 ± 1.0%BM. Core temperature (Tc) was not significantly different between conditions. CONCLUSION: High-dose sodium supplementation does not appear to significantly alter Usg, %BM, ΔPV, or Tc while cycling for 2-hours in a hot, humid environment. Although both groups remained euhydrated (Usg < 1.025) by the end of each trial, Usg remained stable in the supplement group, while the control group exhibited a modest Usg increase after 2-h of cycling.