Intra-individual Variability In Sweat Responses Of Division I Football Players During Fall Training Camp

Sweat testing is commonly used to quantify fluid and electrolyte losses during training, however, intra-individual variability in sweat responses are not well established. PURPOSE: Investigate day-to-day variability in individual sweat responses and identify potential modulators for intra- and inter-individual variability. METHODS: Nine NCAA Division I football players participated in the study during 8 outdoor sessions in Fall training camp. Technical absorbent sweat patches were adhered to the lower back for 20-30 minutes. Local sweat rate (LSR) was calculated as the weight difference in absorbent patch divided by the product of the patch area and time adhered (g·cm-2·hour-1). Sweat electrolytes (Na+, K+, and Cl-) were analyzed using ion selective probes. Wet-bulb globe temperature was measured during practice and Catapult global positioning trackers quantified training load. Within 3 weeks of starting the study, body composition was assessed via Dual Energy X-ray Absorptiometry. RESULTS: During 7 of the 8 sessions, mean temperature and relative humidity (RH) were 35.2 ± 0.9 °C and 71.7 ± 3.5%, respectively. Mean intra-individual coefficients of variation for the 7 sessions were 23.6 ± 11.9% for LSR, 12.4 ± 6.3% for sweat Na+, 7.1 ± 4.7% for K+, and 10.2 ± 5.9% for Cl-. The range of LSR between individuals was 0.41 g·cm-2·hour-1, and the range within-individuals varied from 0.03 to 0.16 g·cm-2·hour-1. An effect of temperature and RH was found on individual LSR (P < 0.01 for both), but not sweat electrolytes (P > 0.05 for all). Sweat electrolytes were negatively correlated with body fat percent (r ≤ -0.29, P ≤ 0.03), but not lean mass. Conversely, LSR was positively correlated with lean mass (r = 0.43, P ≤ 0.01), but not body fat percent. Player load was positively correlated with sweat Na+ and Cl- concentrations (r > 0.39, P ≤ 0.01). CONCLUSION: Given the inter-individual range in sweat responses, sweat testing is necessary to guide personalized rehydration recommendations. Since responses may be largely driven by body composition, testing during a single session may be sufficient to estimate guidelines for fluid and electrolyte replacement. In less trained, unacclimated individuals or more variable environmental conditions, multiple sweat tests may be required to account for additional confounding variables.