Background: Public health policies for preventing obesity need guidelines for active individuals who are at risk due to exercise recidivism. Methods: Changes in adiposity were compared to the running distances at baseline and follow-up in men and women whose reported exercise increased (N=4,632 and 1,953, respectively) or decreased (17,280 and 5,970, respectively) during 7.7 years of follow-up. Results: Per Delta km/wk, decreases in running distance caused over four-fold greater weight gain between 0-8 km/wk (slope+-SE, males: -0.068+ -0.005 kg/m2, females: -0.080+-0.01 kg/m2) than between 32-48 km/wk (-0.017+-0.002 and -0.010+-0.005 kg/m2, respectively). In contrast, increases in running distance produced the smallest weight losses between 0-8 km/wk and statistically significant weight loss only above 16 km/wk in males and 32 km/wk in females. Above 32 km/wk (30 kcal/kg) in men and 16 km/wk (15 kcal/kg) in women, weight loss from increasing exercise was equal to or greater than weight gained with decreasing exercise, otherwise weight gain exceeded weight loss. Substantial weight gain occurred in runners who quit running, which would be mostly retained with resumed activity. Conclusion: Public health recommendations should warn against the risks of irreversible weight gain with exercise cessation. Weight gained due to reductions in exercise below 30 kcal/kg in men and 15 kcal/kg in women may not be reversed by resuming prior activity. Current IOM guidelines (i.e., maintain total energy expenditure at 160 percent of basal) agree with the men s exercise threshold for symmetric weight change with changing exercise levels.

Context and Objective: Vigorous exercise, alcohol and weight loss are all known to increase HDL-cholesterol, however, it is not known whether these interventions raise low HDL as effectively as has been demonstrated for normal HDL. Design: Physican-supplied medical data from 7,288 male and 2,359 female runners were divided into five strata according to their self-reported usual running distance, reported alcohol intake, body mass index (BMI) or waist circumference. Within each stratum, the 5th, 10th, 25th, 50th, 75th, 90th, and 95th percentiles for HDL-cholesterol were then determined. Bootstrap resampling of least-squares regression was applied to determine the cross-sectional relationships between these factors and each percentile of the HDL-cholesterol distribution. Results: In both sexes, the rise in HDL-cholesterol per unit of vigorous exercise or alcohol intake was at least twice as great at the 95th percentile as at the 5th percentile of the HDL-distribution. There was also a significant graded increase in the slopes relating exercise (km run) and alcohol intake to HDL between the 5th and the 95th percentile. Men's HDL-cholesterol decreased in association with fatness (BMI and waist circumference) more sharply at the 95th than at the 5th percentile of the HDL-distribution. Conclusions: Although exercise, alcohol and adiposity were all related to HDL-cholesterol, the elevation in HDL per km run or ounce of alcohol consumed, and reduction in HDL per kg of body weight (men only), was least when HDL was low and greatest when HDL was high. These cross-sectional relationships support the hypothesis that men and women who have low HDL-cholesterol will be less responsive to exercise and alcohol (and weight loss in men) as compared to those who have high HDL-cholesterol.

Although cross-sectional studies show active individuals are leaner than their sedentary counterparts, it remains to be determined to what extent this is due to initially leaner men and women choosing to exercise longer and more intensely (self-selection bias). In this report walking volume (weekly distance) and intensity (speed) were compared to current BMI (BMIcurrent) and BMI at the start of walking (BMIstarting) in 20,353 women and 5,174 men who had walked regularly for exercise for 7.2 and 10.6 years, respectively. The relationships of BMIcurrent and BMIstarting with distance and intensity were nonlinear (convex). On average, BMIstarting explained >70 percent of the association between BMIcurrent and intensity, and 40 percent and 17 percent of the association between BMIcurrent and distance in women and men, respectively. Although the declines in BMIcurrent with distance and intensity were greater among fatter than leaner individuals, the portions attributable to BMIstarting remained relatively constant regardless of fatness. Thus self-selection bias accounts for most of the decline in BMI with walking intensity and smaller albeit significant proportions of the decline with distance. This demonstration of self-selection is germane to other cross-sectional comparisons in epidemiological research, given self-selection is unlikely to be limited to weight or peculiar to physical activity.

Background: Body weight increases with aging. Short-term, longitudinal exercise training studies suggest that increasing exercise produces acute weight loss, but it is not clear if the maintenance of long-term, vigorous exercise attenuates age-related weight gain in proportion to the exercise dose. Methods: Prospective study of 6,119 male and 2,221 female runners whose running distance changed less than 5 km/wk between their baseline and follow-up survey 7 years later. Results: On average, men who ran modest (0-24 km/wk), intermediate (24-48 km/wk) or prolonged distances (>_48 km/wk) all gained weight throughage 64, however, those who ran ?48 km/wk had one-half the average annual weight gain of those who ran <24 km/wk. Age-related weight gain, and its reduction by running, were both greater in younger than older men. In contrast, men s gain in waist circumference with age, and its reduction by running, were the same in older and younger men. Women increased their body weight and waist and hip circumferences over time, regardless of age, which was also reduced in proportion to running distance. In both sexes, running did not attenuate weight gain uniformly, but rather disproportionately prevented more extreme increases. Conclusion: Men and women who remain vigorously active gain less weight as they age and the reduction is in proportion to the exercise dose.

Background: The predicted health benefits of becoming physically active or fit will be exaggerated if health outcomes cause fitness and activity rather than the converse in prospective and cross-sectional epidemiological studies. Objective: Assess whether the relationships of adiposity to fitness and activity are explained by adiposity prior to exercising. Design: Cross-sectional study of physical fitness (running speed during 10km foot race) and physical activity (weekly running distance) to current BMI (BMIcurrent) and BMI at the start of running (BMIstarting) in 44,370 male and 25,252 female participants of the National Runners' Health Study. Results: BMIstarting explained all of the association between fitness and BMIcurrent in both sexes, but less than a third of the association between physical activity and BMIcurrent in men. In women, BMIstarting accounted for 58 percent of the association between BMIcurrent and activity levels. The 95th percentile of BMIcurrent showed substantially greater declines with fitness and activity levels than the 5th percentile of BMIcurrent in men (i.e., the negative slope for 95th percentile was 2.6-fold greater than the 5th percentile for fitness and 3-fold greater for activity) and women (6-fold and 3.4-fold greater, respectively). At all percentiles, the regression slopes relating BMIstarting to fitness were comparable or greater (more negative) than the slopes relating BMIcurrent to fitness, whereas the converse was true for activity. Conclusion: Self-selection bias accounts for all of the association between fitness and adiposity and probably a portion of other health outcomes, but has less affect on associations involving physical activity

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