"Rest is precisely what aging people do not need," says William Evans of the U.S. Department of Agriculture’s Human Nutrition Research Center on Aging at Tufts University.

And he is not just talking about people in their nineties. Starting in middle age, we begin to gain fat and lose muscle, strength, bone, and aerobic capacity. Our risk of heart disease, diabetes, high blood pressure, and osteoporosis climbs. We are headed for what Evans calls The Disability Zone.

A healthy diet can help reduce those risks, but exercise is equally, and in some cases more - important. Evans is not talking about aerobic exercise. The single most critical step to not just retard, but to reverse the aging process, he says, is strength training.

HERE ARE 10 DETERMINANTS OF AGING YOU CAN CONTROL

1. Your Muscle Mass. The rate at which you lose muscle accelerates after age 45. Use your muscles frequently and they will not shrink. Push them to the limits of their capacity and they will grow... no matter what your age.
2. Your Strength. Starting after age of 30-perhaps as early as 20-people lose both muscle cells and the nerves that tell them to contract. To rebuild strength, you must exercise. No pain, no gain.
3. Your Basal Metabolic Rate (BMR). The number of calories you burn at rest (BMR) drops by 100 calories per decade, starting at age 20. That is because older people need less oxygen to fuel their smaller muscle mass. Build those muscles and you burn more calories.
4. Your Body Fat Percentage. The average 25-year -old woman is 25 percent fat. The average 65-year-old woman is 43 percent fat. Cutting calories can help you lose fat, but unless you exercise, you will also loose muscle.
5. Your Aerobic capacity. If you are the average man of 30, your aerobic capacity has already started to decline. By age 65, it could be 30-40 percent smaller than it was when you were 20. As you grow older, muscle mass becomes more important for maintaining your aerobic capacity. Consequently, weight lifting is just as vital as walking, running or cycling.
6. Your Blood Sugar Tolerance. When it is low, you have a higher risk of heart disease and diabetes. Maintain your tolerance with a low-fat, high fiber diet and exercise, which lowers body-fat and increases your muscles response to insulin.
7. Your Cholesterol/HDL Ratio. It should be 4.5 or lower. Exercise, losing body fat, and smoking cessation can raise HDL.
8. Your Blood Pressure. Studies show that people who exercise have a lower risk of hypertension.
9. Your Bone Density. Weight bearing exercise (such as walking, running, cycling and weight lifting) can reduce the rate at which you lose bone.
10. Your Body Temperature Regulation. Older people are more susceptible to dehydration and injuries caused by heat or cold. Aerobic exercise can improve at last one way your body regulates its temperature, your ability to sweat.

THE MOST IMPORTANT: Your independence and your dignity. The National Academy of Sciences claims that if we could postpone institutionalization by just one month, it would save $3 billion in Medicare and Medicaid.

I would like to end with one of my favorite clichés: "Exercise may not add days to your life, but it definitely adds life to your days."