The influence of regular exercise with differences of exercise mode and environment during exercise on core temperature: a meta‐analysis

Abstract

PurposePrevious meta‐analyses indicated that core temperature decreased as a result of regular exercise. However, no meta‐analyses examined the effect of regular exercise on core temperature with a consideration of exercise mode and environment during exercise. The purpose of this study is to review an influence of exercise mode and environment during exercise on core temperature using a meta‐analysis.MethodsA literatures search was conducted using PubMed, and literatures performing regular exercise and reporting temperature in rectum, esophagus, or gastrointestine as core temperature were collected. Differences in the core temperature at rest or the end of exercise tests between pre‐ and post‐intervention were converted into standardized g values as Hedges. These values were pooled, and a difference among studies (i.e., heterogeneity) was assessed using an I2 value. A simple meta‐regression analysis was performed in order to evaluate the relationship between changes in the core temperature and the variables regarding exercise mode and environment during exercise. The differences in the g value in core temperature served as objective variable and heat index (℃) calculated from temperature and relative humidity according to the U.S. National Weather Service as exercise environment, exercise intensity (percentage of maximal oxygen consumption [% VO2max]), exercise time (min/session), and the duration of intervention (days) were selected as explanatory variables, and the regression coefficient, the intercept, and then the R2 value were calculated.ResultsForty‐two studies were analyzed (476 subjects). The heat index ranged from 24.5 to 62.6℃ (median, 43.1℃). For the exercise mode, the exercise intensity ranged from 40 to 80% VO2max (median, 55% VO2max), the exercise time ranged from 30 to 120 min/session (median, 90 min/session), and the duration of intervention ranged from 4 to 63 days (median, 10 days). Results of the meta‐analyses indicated that the g values decreased significantly at the end of exercise tests (‐0.87 [95% CI: ‐1.03 to ‐0.71]) and rest (‐0.01 [95% CI: ‐0.03 to ‐0.01]), and these contained heterogeneity (I2=52.0% and I2=51.0%, respectively). A result of the meta‐regression analysis indicated that the increase in the heat index was significantly associated with a decrease in the g values at the end of exercise test (regression coefficient, ‐0.02 [95% CI: ‐0.04 to ‐0.002]; intercept, ‐0.02 [95% CI: ‐0.77 to 0.73]; R2=22.7%), but heterogeneity was not alleviated (I2=46.4%). All explanatory variables were not significantly associated with a change in the g values at rest, and heterogeneities were not alleviated.ConclusionsCore temperature is decreased with regular exercise, and the effect is depended in part on heat index during exercise. However, one of the limitations is that no randomized controlled trial was involved, thus further studies are required to investigate the influence of exercise mode and environment during exercise on core temperature.