A crucial first step toward making the lifestyle changes essential to extending your health span understands these 10 biomarkers - each sensitive to your exercise and activity level - that determine your biological age:


The average middle-aged person's problem is not excess weight as much as it is excess body fat coupled with too little muscle. Simply losing weight is the wrong goal; the key is changing your ratio of body fat to muscle. People with a greater ratio of muscle to fat enjoy a higher metabolism and don't have to worry as much about gaining weight or about how much they eat - that active tissue burns more calories.

Most people's muscle mass declines with age, and the rate of loss accelerates after age 45. The amount of muscle you have - and that you retain - is determined in part simply by how much you use your muscles. If you use your muscles frequently, you can maintain their strength. But if you push your muscles to the limits of their capacity by exercise, you can actually increase their strength - no matter your age.


As you age, you lose whole "motor units" - sets of muscles and nerves that work together to make your body go. For example, between ages 30 and 70, it's been estimated, people typically experience a 20% decrease in the number of motor units in the thigh.

Aging also brings a decline in "fast-twitch" muscle fibers - the kind you use to lift heavy objects or to do high-intensity, sprint-type activities. This slows you down and partially accounts for age-related loss of strength.

But a decline in muscle strength and size is not inevitable. Landmark research at Tufts in the 1980s debunked the conventional wisdom that age somehow decreases the ability of muscles to benefit and get bigger from strength training, such as weight lifting. So don't think that you're "too old" for strength training. The fact is that you can regain muscle mass and strength, no matter what your age, 18 or 80, or what shape you're in now.


This simply means the rate of your body chemistry - your metabolism - when you're at rest. Like other essential measures, your BMR declines with age and you burn fewer calories when your body isn't in motion. From about age 20 on, every decade that you age reduces the amount of calories you need to maintain a constant body weight by roughly 100 calories. So a typical 70-year-old needs 500 fewer daily calories than a 20-year-old.

But if you keep eating the way you did when you were 20 - or even 40 - you'll put on weight unless you start burning more calories. It can be a vicious cycle: too many calories, too little exertion, reduced musculature and a declining metabolic rate, all adding up to more fat. That's why you need to exercise to increase your muscle and turn back the clock on your BMR.


Even if your body weight hasn't gone up that much as you've gotten older, you've probably gained fat. As your musculature shrinks, fat tissue accumulates. Since muscle tissue weighs more than body fat, that number on your bathroom scale can be deceiving. Instead of focusing on losing weight, concentrate on gaining muscle and shedding fat. While it's not the same as your body fat percentage, your Body Mass Index (BMI) can still be a useful tool in measuring your risk for developing the chronic diseases associated with aging. You can calculate your BMI with this formula:

                                                                 Weight (in kgs)
                                      BMI =

---

                                                                  Height (in mts)²

While BMI is often used as a shorthand measure of overweight or obesity, it's important to recognize that your "ideal BMI" - the figure associated with the lowest risk of chronic disease or mortality - changes with age: For the elderly, a slightly higher BMI seems to be protective; think of elderly people you know who seem to be "wasting away" and are vulnerable to disease. Professional athletes can also get away with a higher BMI, because their body-fat levels are so low. The rest of us, though, need to watch it.



You should also watch your waistline, as studies have shown that the distribution of body fat may be an even better predictor of the risk of chronic disease and mortality. It's healthier to be shaped like a pear - with more body fat stored below the hips - than like an apple, with fat stored above the hips. Developing a pot belly, in short, increases your risk. Recently, researchers using data from the worldwide Interheart study found that waist-to-hip ratio was three times more effective than BMI in predicting cardiovascular risk. To figure your own ratio, simply divide your waist measurement by your hip measurement; so, for example, a 36-inch waist and 40-inch hips would be a ratio of 0.9. Anything over 0.85 for women and 0.9 for men indicates greater risk for heart disease, according to these findings.

If you combine a high BMI with a high waist-to-hip ratio, your risk is magnified. This chart from the NHLBI shows how risk for cardiovascular disease, type-2 diabetes and hypertension increases as BMI and waist circumference grow:

So what can you do if you don't like what this biomarker is telling you? The best way to attack excess body fat is a combination of exercise and moderate caloric restriction. By adding exercise to a "weight-loss" diet, you'll maintain muscle mass while you lose fat tissue, raise your metabolic rate and, of course, burn more calories.


How much oxygen can your body process within a given time? That's your aerobic capacity, for which you need healthy lungs, a strong heart and an effective vascular network. Here's another biomarker that naturally declines with age in most people: In both men and women, aerobic capacity at age 65 is typically 30-40% less than in young adults. But older people who exercise regularly lose less of their aerobic capacity.

Researchers have found, however, that while both young and older people benefit from regular aerobic exercise - the kind that makes you huff and puff - the positive changes in older people come almost entirely in the muscles' ability to utilize oxygen ("oxidative capacity"), rather than in the heart of cardiovascular system. That's another reason why you need strengthening exercises as well as aerobic activity: When you build muscle, you create more muscle cells to consume oxygen. The more demand for oxygen from your muscles, the greater your utilization of oxygen and your aerobic capacity.


"Glucose tolerance" means your body’s ability to control blood sugar (glucose). Aging takes a toll on your ability to use this sugar from your bloodstream, though you probably don't realize it — like hypertension, impaired glucose tolerance displays no symptoms. By age 70, some 30% of women and 20% of men have an abnormal glucose-tolerance curve, increasing their risk of developing type-2 diabetes. This creeping blood-sugar intolerance can be among the most devastating changes associated with aging.

While aging is one factor in the changing of your glucose metabolism for the worse, so are three factors you can control:

* Increased body fat

* Inactivity

* A diet high in fat. Combining a proper diet, low in fat and high in raw vegetables and whole grains, with regular workouts can often transform what was previously an insufficient amount of insulin - which stimulates muscle cells to utilize glucose from the blood - into an adequate amount. Strength-training exercises are especially critical to reinvigorating your body's glucose tolerance and lowering your diabetes risk; besides helping to lower body fat, strength-building exercise has been shown to increase your muscles' insulin sensitivity.


You probably already know that not all cholesterol in the blood is bad for you; that are why you'll often see HDL-cholesterol labeled "good" and LDL-cholesterol parenthetically explained as "bad". So it stands to reason that a low total cholesterol level offers no guaranteed protection against heart disease. Your goal, rather, should be to raise HDL-cholesterol while lowering LDL- cholesterol levels in your blood.

An easy way to compute how you're doing is to figure your total cholesterol/HDL ratio: Simply divide your total cholesterol number by your HDL count. For middle-aged and older men and women, the total cholesterol/HDL ratio goal should be 4.5 or lower.

As you age, your HDL level tends to remain constant, while the harmful components of blood cholesterol increase - boosting your total cholesterol and thus your total cholesterol/ HDL ratio. You can work to lower the harmful LDL by changing your diet, especially by reducing the amount of saturated fat you consume. But dietary changes can significantly improve only one side of the equation; to boost your HDL levels, you need to also exercise and lower your body fat. Aerobic exercise in particular seems to be effective in raising HDL-cholesterol levels.


The importance of controlling your blood pressure should come as no surprise. What may surprise you is that an increase in blood pressure with age is not inevitable: Many populations around the world show no increase in blood pressure with age. You can help defend against hypertension by reducing the salt in your diet. But regular, vigorous exercise can also help prevent and even treat high blood pressure.


With age, the mineral content of your bones declines, leaving you with a weaker, less dense, more brittle skeleton. On average, a person loses approximately 1 % of bone mass per year. When this loss reaches the point where your fracture risk is substantially higher, it's called "osteoporosis," a condition that can affect men as well as women. But osteoporosis isn't an inevitable consequence of aging.

The effectiveness of calcium supplements against osteoporosis - the subject of a great deal of publicity in recent years - remains a matter of scientific inquiry. But there's another way to fight osteoporosis that's received less attention, even though it may be more effective, whatever your calcium intake: Exercise.

Early evidence of the importance of physical activity to bone strength came from studies of the effects of a lack of activity: Subjects who spent two weeks in complete bed rest showed as much bone calcium loss as ordinarily seen from a full year of aging - a 50-fold increase in the normal rate of bone mineral loss. That accelerated loss stopped when bed rest patients were made to stand for some time every day, even if they didn't walk, apparently simply from the effect of the stress of gravity on their bones.

Stress repeatedly placed on a bone causes it to grow stronger. A number of studies have shown that a prolonged span of weight-bearing exercise - such as walking or running - can reduce the rate of bone loss. Research at Tufts has further shown that exercise may help foster the body's calcium absorption.

The evidence strongly suggests, in short, that a brisk daily walk can be a crucial factor in preventing the development of osteoporosis.


Your body comes with a built-in thermostat, which works to cool you off by sweating when you're hot or by shivering when you're cold. But this important thermo-regulatory ability diminishes with age, due to a complex set of body changes:

* Older people have a reduced sensation of thirst, so they often fail to replenish fluids lost by sweating.

* Older people require a warmer internal temperature before they start sweating.

* Reduced kidney function contributes to dehydration and thermoregulatory problems.

* Lower fitness and aerobicl capacity mean that older people have a lower rate of sweating.

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