There is increasing evidence that sodium is accumulating in the skin and muscle where it can be stored without being osmotically active. This might suggest that sweat is involved in the control of the human sodium balance, but this has received little attention thus far. The aim of this study was to assess whether changes in sodium intake induce parallel changes in the sodium concentration of sweat in healthy subjects.
Design and method:
In this crossover study we measured the sodium concentration of sweat (Na sweat) in 12 healthy normotensive volunteers (aged 33.8 ± 11.8 years, 16.7% male) under two levels of dietary sodium intake. Participants followed a high salt (HS) diet (6 gr of salt per day added to their normal diet during 5 days) and, one month later, a low salt (LS) diet (5 days of their normal diet). After each diet, a 24 h urine collection was performed, and Na sweat was measured using a standardized pilocarpine test and the Macroduct sweat collection system. Differences in concentration between the two diets were tested using paired t-tests, and the relationship between 24-hour urinary sodium (Na urine) and Na sweat was tested using an analysis of repeated measures of correlation (Rmcorr, R package).
Sweat sodium concentration (Na sweat) increased from 39.9 ± 22.5 mmol/l under LS to 49.7 ± 19.9 mmol/l under HS conditions (p = 0.05), whereas urinary sodium excretion (Na urine) increased from 43.2 ± 30.2 mmol to 230.7 ± 89.1 mmol (p < 0.001). The changes in Na sweat correlated significantly within changes in Na urine (rm = 0.71, p < 0.001, see Figure 1). Sodium concentration in sweat also correlated with the chloride concentration (rm = 0.9, p < 0.001). The increase in dietary sodium intake was associated with an increase in body weight (rm = 0.63, p < 0.001), without changes of blood pressure (systolic rm = 0.1, p = 0.55, diastolic rm = 0.05, p = 0.8).
The sodium concentration of sweat is higher under high salt conditions, suggesting that sweat might play a role in the maintenance of sodium balance in humans. Yet, the Na concentration in the sweat is highly variable within subjects and the diet-induced change is relatively modest in terms of amount.