Body mass index (BMI) is an index of body fatness that is used in population studies when the 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 have studied the influence of diet on body fatness (adiposity) in runners. Data collected by us in 7,059 men suggest that adiposity is less likely to be affected by diet as men become more physically active. The figure to the left depicts the calculated fattening effects of diets characterized by red meat content. For example, the bar furthest to the left is the estimated impact on BMI of each weekly serving of red meat in men who ran under 15 miles per week. Compared to a man consuming no red meat, one who consumed 1 serving per week would be expected to have a BMI that was 0.12 kg/m2 higher, and one consuming 7 servings a week would be expected to have a BMI that was 0.84 kg/m2 higher. For a six foot man, 7 servings of meat corresponds to about a six pound difference in weight. The bars for men who ran 15-30, 30-45 and over 45 miles/week are progressively shorter, suggesting that as the amount of vigorous activity increases, the fattening effects of diets high in red meat are diminished. Thus, a runner averaging over 45 miles per week is expected to be only half as sensitive to the fattening effects of diets enriched with red meat as a low mileage runner.