Vigorous exercise and the population distribution of body weight

Background: While the benefits of vigorous exercise on body weight and regional adiposity are well established, whether these benefits affect equally the highest and lowest portions of the weight distribution have not been previously reported. The impact of exercise on the more extreme body weights and body circumferences is clinically important because these values represent individuals at greatest health risk. Method: Self-reported weights and body circumferences from a cross-sectional sample of 7288 male and 2326 female runners were divided into five strata, according to the distances run per week and within each stratum the 5th, 10th, 25th, 50th, 75th, 90th and 95th percentiles were determined. Least-squares regression was then employed at each percentile to determine the dose - response relationship between running distance and adiposity as determined by body mass index (BMI) and self-reported circumferences of the waist, hip and chest. Results: Per kilometer run per week, the associated decline for BMI was three-fold greater at the 95th than at the 5th percentile in men, and six-fold greater at the 95th than the 5th percentile in women ( all P<0.001). Reported waist circumference also declined more sharply at the 95th percentile than at the 5th percentile in men ( - 0.13 +/- 0.02 vs - 0.06 +/- 0.01 cm per km/week) and women ( - 0.18 +/- 0.04 vs - 0.05 +/- 0.01 cm per km/week). In women, both hip and chest circumferences declined more sharply per kilometer run at the 95th percentile than at the 5th percentile. Conclusions: These results are consistent with the hypothesis that running promotes the greatest weight loss specifically in those individuals who have the most to gain from losing weight. Comparisons based on average BMI or average body circumferences are likely to underestimate the health benefits of running because of the J-shaped relationship between adiposity and mortality. Whether the observed cross-sectional associations are primarily due to exercise-induced weight loss or self-selection remains to be determined.