Chapter 6 – Body Composition
Body Mass Index
In addition to body composition and waist/hip circumferences, measuring body mass has also been used as an effective method to assess health risks. Body Mass Index (BMI) is a measurement of height (m2) and weight (kg) suggesting that a person’s weight should be proportional to his or her height.
Evaluating Body Mass Using Body Mass Index
- Convert weight to kilograms: If your weight is in pounds, divide it by 2.2 to convert it to kilograms.
- Convert height to meters: If your height is in feet and inches, multiply your height in feet by 0.3048 and your height in inches by 0.0254, then add the results.
- Square your height: Multiply your height in meters by itself.
- Divide weight by height squared: Divide your weight in kilograms by your height squared.
| Classification | BMI range – kg/m2 |
| Severe Thinness | < 16 |
| Moderate Thinness | 16 – 17 |
| Mild Thinness | 17 – 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese Class I | 30 – 35 |
| Obese Class II | 35 – 40 |
| Obese Class III | > 40 |
For example, based on the BMI scale, a female with a height of 5’6” should not weigh more than 155 lbs. If her weight exceeded 155 lbs., she would be categorized as “overweight.”
Criticism of Body Mass Index
Among several criticisms, the BMI method has been faulted for not distinguishing between fat mass and fat-free mass, since only the overall weight is taken into account. For athletes, who may be more massive as a result of larger muscles, this criticism holds true. For example, a professional football player who weighs 215 pounds and stands at 6’3” would have the exact same BMI as a relatively sedentary arm-chair quarterback who also weighs 215 pounds with the same height. This discrepancy also exists when applying BMI to the senior population. As age increases, muscle mass declines. Seniors who have experienced years of muscle mass decline but increased body fat may maintain a constant weight despite having a very different body composition.
Other criticisms of using BMI as a health risk assessment tool include its failure to take age or gender into account. As discussed previously, females naturally have more body fat yet are classified in the same context as males. Because this measurement is so widely used by physicians, patients continue to express concerns about the validity of BMI as an indication of fatness.
Regardless of the criticisms, BMI as used for the general population, has been shown to be a reasonable predictor of health outcomes. At its core, it is not intended to be an estimate of body composition, i.e., measure fat mass and fat-free mass. Instead, it is intended to be used as an estimate of healthy/unhealthy levels of body fat. When used as a means of tracking weight changes over time it can be a valuable tool in predicting health and for recommending lifestyle modifications.11
