Thermoregulatory Responses and Hydration Practices in Heat-Acclimatized Adolescents During Preseason High School Football

Context: Previous researchers have not investigated the thermoregulatory responses to multiple consecutive days of American football in adolescents. Objective: To examine the thermoregulatory and hydration responses of high school players during formal preseason football practices. Design: Observational study. Setting: Players practiced outdoors in late August once per day on days 1 through 5, twice per day on days 6 and 7, and once per day on days 8 through 10. Maximum wet bulb globe temperature averaged 23 +/- 4 degrees C. Patients or Other Participants: Twenty-five heat-acclimatized adolescent boys (age = 15 +/- 1 years, height = 180 +/- 8 cm, mass = 81.4 +/- 15.8 kg, body fat = 12 +/- 5%, Tanner stage = 4 +/- 1). Main Outcome Measure(s): We observed participants within and across preseason practices of football. Measures included gastrointestinal temperature (T-GI), urine osmolality, sweat rate, forearm sweat composition, fluid consumption, testosterone to cortisol ratio, perceptual measures of thirst, perceptual measures of thermal sensation, a modified Environmental Symptoms Questionnaire, and knowledge questionnaires assessing the participants' understanding of heat illnesses and hydration. Results were analyzed for differences across time and were compared between younger (14-15 years, n = 13) and older (16-17 years, n = 12) participants. Results: Maximum daily T-GI values remained less than 40 degrees C and were correlated with maximum wet bulb globe temperature (r = 0.59, P =.009). Average urine osmolality indicated that participants generally experienced minimal to moderate hypo-hydration before (881 +/- 285 mOsmol/kg) and after (856 +/- 259 mOsmol/kg) each practice as a result of replacing approximately two-thirds of their sweat losses during exercise but inadequately rehydrating between practices. Age did not affect most variables; however, sweat rate was lower in younger participants (0.6 +/- 0.2 L/h) than in older participants (0.8 +/- 0.1 L/h) (F-1,F-18 = 8.774, P = .008). Conclusions: Previously heat-acclimatized adolescent boys (T-GI < 40 degrees C) can safely complete the initial days of preseason football practice in moderate environmental conditions using well-designed practice guidelines. Adolescent boys replaced most sweat lost during practice but remained mildly hypohydrated throughout data collection, indicating inadequate hydration habits when they were not at practice.