Responses of lean and obese boys to repeated summer exercise in the heat bouts

PURPOSE: To determine the degree of natural acclimatization and artificially induced acclimation-related changes during repeated exercise in the heat bouts in seven lean and seven obese 9- to 12-yr-old boys during summer months. METHODS: Beginning at random times during the summer, subjects underwent a 70-min exercise (30% VO _2max) in the heat exposure (38°C, 50% relative humidity) on six separate days. RESULTS: On day 1, obese children were less naturally acclimatized as indicated by significantly higher baseline core temperatures (T _c) (obese = 37.62 ± 0.06 vs lean = 37.41 ± 0.06; P < 0.004). By day 6 versus day 1, significant reductions in baseline T _c were evident in both groups (obese = 37.41 ± 0.04 vs lean = 37.18 ± 0.04; both P < 0.05). Baseline T _c in obese subjects by day 6 was similar to that of lean subjects on day 1. Daily reductions in exercise T _c were evident in both groups (final exercising T _c day 1 vs day 6: obese = 38.15 ± 0.05 vs 37.89 ± 0.05; lean = 38.17 ± 0.09 vs 37.72 ± 0.06°C; both P < 0.001), occurring at a significantly slower rate in obese subjects (final exercise T _c day 6 - day 1: obese vs lean = -0.26 ± 0.04 vs -0.45 ± 0.08°C; P < 0.05). Significant reductions in exercising heart rate (HR) occurred in the lean but not the obese subjects by day 6 (final exercising HR day 1 vs day 6: obese = 132 ± 3 vs 131 ± 3, P > 0.05; lean = 138 ± 3 vs 127 ± 3 bpm; P < 0.001). CONCLUSIONS: During summer months, obese children are less naturally heat-acclimatized and subsequently acclimate at a slower rate.