Our genes demand exercise, and that's at odds with our modern lifestyle, which increasingly does not. Evolutionary biologists theorize that a genetic hunger for physical activity comes from our hunter-gatherer ancestors who survived to pass on their DNA to us, as their sluggish comrades starved to death, were annihilated by predators, or simply flunked out in the mating game. But for the 40 to 50 million sedentary Americans who carry the greatest burden of chronic disease and for the rest of us who are at risk, this theory has taken on lifesaving importance. Researchers have recently discovered a bevy of genes, ones geared to make healthier, sturdier, longer-living humans, that do their job only if stimulated by an appropriate dose of physical activity.

More than 140 exercise-related genes are awakened if the body gets off the couch and engages in physical activity. These sleeping beauties make proteins with wide-ranging benefits to body metabolism, muscle mass, fat deposition, blood vessels, and immune function. As C. Ronald Kahn, endocrinologist and president of the Joslin Diabetes Center in Boston, says, "If you don't exercise, you 'dysregulate' your genes." Among other things, such miscues lead to the health scourge of our immobile age--a skyrocketing epidemic of type 2 diabetes. Kahn further points out that inactivity is every bit as important a diabetes risk factor as is poor diet. With only modest exercise, for example, genes are turned on that cause muscle to take up glucose even without insulin, lowering blood sugar. Thus, exercise helps patients with diabetes to better control their sugar levels. And among those at risk for diabetes, physical activity significantly reduces their chances of getting the disease in the first place.

The good news for most of us is that you don't have to be a hunter-gatherer or a trained athlete or look good in spandex to unlock the secrets of your own genes. The formula for modest activity is 30 minutes of exercise five days a week. If you are one of those people who get hives at the mere thought of jogging, good old-fashioned walking will do just fine. There is nothing more natural; you need no trainer or coach, no club memberships. Rain or shine, indoors or outdoors, everyone can do it--and set the terms. And for those who envy those sleek joggers who whisk by us in the park, there's some consolation. Walking is as good as jogging, and even the most intense walking routine is unlikely to harm your joints or stress out your immune system as some rigorous exercise regimens sometimes do.

Step lively. To get sluggish Americans up and moving, we must remember that you can only change what you can measure. That's as true for cholesterol levels and blood pressure recordings in preventive medicine as it is for miles, laps, and times in competitive sports. Similarly, for the average Joe or Jane, we need a concrete gauge of exercise performance to both motivate and monitor. The limitation of preaching "30 minutes, five days a week" is that we can't tell if it's enough for each individual without knowing that person's pace and baseline activity. Recognizing this, the President's Council on Physical Fitness and Sports in 2002 pushed for the routine use of a simple but elegant measure--steps per day tallied by your very own pedometer. Pedometers may not be gene chips, but they have a gold-star past. The energetic Leonardo da Vinci is credited with designing the first pedometer in the 15th century, and the ever inquisitive Thomas Jefferson brought one to the States from France in the late 1700s.