BMI stands for body mass index and is determined by your height and weight. The formula for calculating BMI is your body weight in kilograms divided by your height in meters squared.

Although the equation requires mathematical manipulations, the result is a number based on height and weight. Consequently, BMI addresses body size but not body composition.

The recommended range for a healthy body mass index is 18.5 through 25. However, this reading does not indicate what percentage of your body mass is fat (adipose tissue) and what percentage is lean (muscle, bone, blood, organ and connective tissue).

For example, a 6-foot tall man with 30 pounds of fat weight and 170 pounds of lean weight has the same BMI as a 6-foot tall man with 60 pounds of fat weight and 140 pounds of lean weight, because both individuals have identical body heights and weights.

Obviously, the first man has a much healthier body composition than the second man. With 30 pounds of fat weight, the first man is 15 percent body fat, which is very desirable for males. With 60 pounds of fat weight the second man is 30 percent fat, which is highly undesirable for males from both a health and fitness perspective.

As another example, Tom weighs the same 160 pounds at age 65 that he did at age 25. Assuming his height hasnít changed, his BMI would be exactly the same as it was 40 years earlier. However, if Tom did not perform regular resistance exercise, he has actually lost more than 20 pounds of muscle and added 20 pounds of fat for a 40-pound undesirable change in his body composition. Unfortunately, BMI does not provide this important health-related information.

I favor body composition assessments over BMI calculations for a variety of reasons. Body composition assessments determine what percentage of your body weight is fat weight and what percentage is lean weight. Once these percentages are known, you can make more sense out of body weight changes.

For example, letís say Mary weighs 100 pounds and is 25 percent fat. She therefore has 25 pounds of fat weight and 75 pounds of lean weight. After a three-month program of strength and endurance exercise, Mary still weighs 100 pounds. However, her new body composition assessment reveals she is only 20 percent fat. That means she now has 20 pounds of fat weight and 80 pounds of lean weight.

Although her weight has not changed, she has actually lost 5 pounds of fat and added 5 pounds of muscle for a 10-pound improvement in her body composition.

Body composition assessments are also helpful to dieters who lose weight too quickly because of an unhealthy reduction in lean tissues.

For example, Linda starts her diet plan at 150 pounds and 30 percent body fat. She therefore has 45 pounds of fat and 105 pounds of lean weight. After one month of dieting she weighs 130 pounds, and her new body composition is 25 percent body fat. Her new fat weight is 32 pounds, for a 13-pound fat loss, which is good.

However, her new lean weight in 98 pounds for a 7-pound muscle loss, which is not good. More than one-third of Lindaís weight loss has been muscle tissue, which is an unhealthy consequence of losing weight too quickly.

Linda must change her eating habits to prevent further muscle loss, which can lead to serious health problems. She should also begin a sensible strength training program to regain the essential muscle tissue that she has lost through insufficient caloric consumption.

If Linda had used BMI calculations, she may not have realized that her diet program was actually more harmful than helpful, seriously reducing her muscle mass and metabolic rate. This is one reason I recommend dieters have periodic percent fat assessments.

Most YMCAs and fitness centers can measure your body fat quickly and easily through ultrasound, electrical impedanceor skinfold caliper.

By measuring your body composition, you can accurately track your progress toward an ideal fat level, generally acknowledged as less than 15 percent for men and less than 25 percent for women.

Wayne L. Westcott, Ph.D., teaches exercise science at Quincy (Massachusetts) College and consults for the South Shore YMCA.