Several studies have demonstrated regional variations in the solute content of sweat, but these studies have involved small subject numbers (n = 6, Lemon et al. 1986; n = 10, Patterson et al. 2000). The present study measured the electrolyte content of sweat from different anatomical sites in a larger subject population.

With Ethics Committee approval, 30 healthy young males drawn from two professional football clubs gave written consent to participate. Measurements were made during a 90-minute training session carried out as part of pre-season training. Regional sweat collections were made at the forearm, chest, back and thigh by application of absorbent gauze swabs which were covered with a non-porous adhesive film (Tegaderm dressing and pad, 3M, Loughborough). Before application the area was cleaned with distilled water and dried with a gauze swab. Body mass was measured before and after training and the mass of drinks consumed was also measured to estimate total sweat loss. Sweat volume in the patch was determined gravimetrically. Sodium and potassium concentration was measured by flame photometry and chloride concentration by coulometric titration. Statistical analysis was by one way ANOVA followed by the Tukey test as appropriate. Data are presented as mean ± S.D.

Mean whole body sweat rate was 1.43 ± 0.25l/h, with a wide range between individuals (1.03-1.89l/h). The sweat sodium (Na), potassium (K) and chloride (Cl) concentrations differed between the sample sites tested. The highest sweat Na concentration was measured at the chest (54 ± 21 mmol/l) and this was significantly greater than that at the arm and thigh (42 ± 16 and 35 ± 11 mmol/l; P = 0.000). Further, the thigh sweat Na concentration was lower than that on the back (49 ± 19 mmol/l). A very similar picture was found for sweat Cl concentration. The highest sweat Cl concentration was measured at the chest (53 ± 14 mmol/l): this was significantly greater than that at the arm and thigh (39 ± 11 and 32 ± 9 mmol/l; P = 0.000). Further, the thigh sweat Cl concentration was lower than that on the back (49 ± 14 mmol/l). However, for the sweat K concentrations, the results demonstrate the opposite effect. The highest sweat K concentration was measured at the thigh (6.7 ± 2.5 mmol/l) and this was significantly greater than that at the chest and back (5.4 ± 1.3 and 4.6 ± 1.2 mmol/l; P = 0.000). The arm sweat K concentration (6.4 ± 1.6 6 mmol/l) was higher than that on the back. The greatest sweat Na concentrations were therefore recorded from the trunk of the body while the greatest sweat K concentrations were recorded from the peripheral sites.

These results indicate a substantial regional variation in sweat electrolyte concentration. The K concentration cannot be explained by the effect of differences in sweating rate on ductal reabsorption. This is however likely to account for the differences in Na and Cl concentrations.