When the long hours you spend in training pay off with low body fat and toned sculpted muscles, you feel pretty good about yourself. That is, until you assess your body mass index, or BMI. For athletes and fitness enthusiasts, BMI can categorize you as overweight or obese, even though you are in better shape and have lower body fat than your non-athletic friends.
According to the Centers for Disease Control and Prevention, or CDC, BMI is a measure of weight adjusted for height, calculated by dividing your weight in kilograms with your height in meters squared. BMI measures excess weight rather than excess fat, and provides an easy, non-invasive and inexpensive means of classifying individuals into weight categories. The CDC concedes that BMI does not calculate your body fat and should not be used as a diagnostic tool to assess health, but rather as an indicator of potential health problems.
If you are athletic, you are likely to be more muscular and have higher bone mineral density than a sedentary or non-athletic person, and that adds up to extra pounds. When you put your weight into the BMI equation, you may get a value greater than 25 percent, putting you in the overweight category, or perhaps greater than 30 percent, classifying you as obese. The CDC admits that some athletic individuals may have a high BMI but a low percent body fat.
Kinesiology professor Sue Beckham, PhD of the University of Texas at Arlington, asserts that BMI is not useful in assessing athletic muscular individuals and is not a good indicator of changes in body composition. A 2007 study of male and female college athletes published in "Medicine and Science in Sports and Medicine" concluded that BMI incorrectly classifies athletes with normal body fat as overweight and that separate standards should be established for athletic populations.
BMI versus FFM
Your fat free mass, or FFM, includes your muscles, bones, connective tissue and other not-fat components of your body mass. According to exercise scientist Len Kravitz, PhD of the University of New Mexico, we all need a certain amount of fat to maintain good health, and women need more than men. When you subtract your FFM from your total body mass, you get your fat mass. Body composition is a measurement that compares your fat mass to your total body mass, expressed as percent body fat.
Better than BMI
Body composition gives an athletic individual a more accurate profile than BMI of health status in relation to weight because you are measuring fat and not just weight. The "gold standard" of assessing body composition is underwater weighing, which can be expensive and inconvenient. Less expensive and more convenient methods are skin fold measurements, and bioelectric impedance using a hand-held device. Of the two, skinfolds provide a more accurate assessment for athletes, according to Kravitz, because bioelectric impedance tends to overestimate percent body fat in very lean individuals. A qualified fitness professional can do a skinfold assessment for you. Desirable athletic ranges of body fat are 5 to 13 percent for men, and 12 to 22 percent for women. Optimal fitness values are 12 to 18 percent for men, and 16 to 25 percent for women.
- Centers for Disease Control and Prevention: About BMI for Adults
- Centers for Disease Control and Prevention: Body Mass Index: Considerations for Practitioners
- Medicine and Science in Sports and Exercise: Body Mass Index as a Predictor of Percent Fat in College Athletes and Non-Athletes
- University of New Mexico: Getting a Grip on Body Composition
- University of Texas at Arlington: Body Composition