In the bottle before you is a pill, a marvel of modern medicine that will regulate gene transcription throughout your body, helping prevent heart disease, stroke, diabetes, obesity, and 12 kinds of cancer - plus gallstones and diverticulitis. Expect the pill to improve your strength and balance as well as your blood lipid profile. Your bones will become stronger.

You'll grow new capillaries in your heart, your skeletal muscles, and your brain, improving blood flow and the delivery of oxygen and nutrients. Your attention span will increase. If you have arthritis, your symptoms will improve. The pill will help you regulate your appetite and you'll probably find you prefer healthier foods. You'll feel better, younger even, and you will test younger according to a variety of physiologic measures. Your blood volume will increase, and you'll burn fats better. Even your immune system will be stimulated. There is just one catch. There's no such pill. The prescription is exercise.

Written by Jonathan Shaw, Harvard Magazine, March-April 2004.