Body composition is not something you do, like 10 push-ups or 50 sit-ups. Body composition is something you are, but it has a lot to do with what you do. Basically, Your body is composed of two types of tissues known as fat weight and lean weight. Fat weight is the fat stored in fat cells throughout the body. Lean weight includes all other tissues, such as organs, bones, blood, skin, and muscle. Approximately half of our lean weight is muscle which, along with fat, is most likely to change during our adult years.

As we age, we typically lose about five pounds of muscle and add about 15 pounds of fat every decade of life. While this represents a 10-pound change in bodyweight, it is actually a 20-pound change in body composition. The muscle loss adversely affects our physical function and personal appearance. Perhaps more importantly, it results in a reduced metabolic rate that facilitates fat gain. This is because every pound of muscle loss reduces our resting metabolism by at least 35 calories per day. Assuming we eat approximately the same amount of food, calories that were previously used for muscle maintenance are now placed into fat storage, resulting in creeping obesity.

Excess body fat is a major health risk associated with many medical problems including low back pain, type II diabetes, various forms of cancer, high blood pressure, and heart disease. Most people understand this, and half of all Americans are presently on low calorie diet plans to reduce unwanted fat. Unfortunately, dieting alone has a dismal record of success, with over 90 percent of dieters regaining all of this fat weight within one year. Even worse, about one-quarter of the weight lost through dieting is muscle, further reducing this vital tissue and resting metabolic rate. No wonder a return to normal and necessary eating behavior results in fat regain.

Because the deterioration in body composition is a two-fold problem (too little muscle and too much fat), restoration of desirable body composition requires a dual solution (muscle replacement and fat reduction). Obviously, regular exercise is essential for replacing muscle tissue. However, only strength training is effective for this purpose. Endurance exercise is ideal for improving cardiovascular fitness, but it neither builds muscle nor prevents the loss of muscle during our adult years.

So step one in attaining a more desirable body composition is a basic program of strength exercise. Our research reveals excellent results from two or three weekly training sessions of 25 minutes each. This is all the time it takes to complete one set of 12 different exercises that address all of your major muscle groups. Each set is performed at a slow movement speed through a full movement range with a weightload that permits between eight and 12 repetitions. When you can do 12 good repetitions the resistance should be increased by one to five pounds.

Combining this simple strength training protocol with 25 minutes of endurance exercise (treadmill walking, stationary cycling, etc.) is an excellent approach for enhancing body composition. In one of our studies, almost 300 men and women performed this combination exercise program for a period of eight weeks. On average the participants added three pounds of muscle and lost nine pounds of fat, for a six-pound reduction in bodyweight and a 12-pound improvement in body composition. These beneficial changes were accomplished without strict dietary intervention, but everyone received heart-healthy eating guidelines and sample menu plans.

In all probability, the three-pronged approach is best for permanent weight management and optimal body composition. The most important component is strength training (two 25-minute sessions per week are sufficient) to replace muscle, raise resting metabolic rate, and improve physical function. The second component is endurance exercise (three 25-minute sessions per week are recommended) to reduce fat stores and increase cardiovascular fitness. The third component is a commitment to better eating habits and sound nutrition, which typically requires more food rather than less. This is because the recommended foods (grains, vegetables, fruits, lean meats, and low-fat dairy products) generally have fewer calories per serving than the less-nutritious foods that they replace (popular fast foods, fried foods, fat foods, and snack foods).

The results of our summer research study support the three-piece plan for a variety of personal benefits besides better body composition. In addition to adding muscle and losing fat, the 87 participants in our Keeping Fit Program achieved significant increases in their muscle strength, performance power, and static balance, and attained significant decreases in their waist girth and hip girth. They also realized a one-third inch increase in height due to improved posture resulting from stronger lower back, upper back, and neck muscles.

Wayne L. Westcott, Ph.D., is fitness research director at the South Shore YMCA in Quincy, MA., and author of several books including the new releases Building Strength and Stamina and Strength Training Past 50.