To lose a pants size, run longer.

A new study of over 60,000 active men, ages 18 to 85, finds that distance run per week affects waist circumference more than it does body mass index (BMI).

Exercise is associated with reductions in abdominal fat. But the authors also propose that it is the transformation of Type 2b muscle fibers to Type 2a with which exercise has been correlated that reduces waist circumference. Waist circumference has been purported to be negatively correlated with Type 2a muscle fibers and positively correlated with Type 2b--much more so than BMI has been shown to be.

Nevertheless, distance running reduces BMI as well. In the present study, those who ran over 64 kilometers (40 miles) per week had 11% lower BMI and 8% smaller waist circumference than those running less than 16 kilometers (10 miles) per week. Simply put, adding distance increases caloric expenditure, and this is an effective means of staving off undesired weight gain as we age.

From middle age onward, weight management becomes more difficult. The study, which was conducted by the Life Sciences Division of the Lawrence Berkeley Laboratory in Berkeley, CA, found that BMI and waist circumference increased with age at all running distances. However, the increase diminished by running further. And the heaviest men have the most to gain, so to speak. It was the 90th percentile of weight class that saw the biggest drop in BMI. The difference between this group and the 10th percentile, or leanest among the men, was over two and a half times greater for BMI versus distance run per week than for BMI versus age, which suggests that the difference in the way these two weight-extremes responded is a property of the effect of greater distance and not an attribute of BMI.

(Med. Sci. Sports & Exerc., 2005, Vol. 37, No. 8, pp. 1329-1337)