Population studies show that body weight increases in men through late middle age. The middle-age weight gain is coincidental to an increase in intra-abdominal fat, which is thought to play a major role in the development of insulin resistance, hyperlipidemia, and hypertension. To assess whether middle-age weight gain occurs regardless of activity level, we examined the association of adiposity with age in participants of the National Runners' Health Study. This included 4,769 male runners, aged 18-49 years old. Body mass index (BMI) is an index of body fatness used in population studies when direct measurement of body fat is impractical. It is calculated by dividing an individual's weight (in kilograms) by the square of their height (in meters). A BMI under 25 is considered ideal, a BMI between 25 and 30 is moderately overweight, and a BMI of 30 or more is severely overweight. A BMI of 25 corresponds to 184 pounds in a six foot man and 150 pounds in a 5 foot 4 inch woman. We and other researchers have shown that runners who run longer distances each week are leaner than short-distance runners. There is a statistically significant decrease in BMI for every 10 mile per week increment in weekly mileage (graph not shown). Runners' waist lines provide an estimate of upper body adiposity, presumably intra-abdominal fat. The bars in this graph show the average waist line by weekly mileage. Every 10 mile per week increment in weekly mileage was associated with a significant apparent reduction in the waist line.