The Body Mass Index, or BMI, is a calculation that helps assess whether an individual is at risk for diseases related to weight. While it does not directly measure body fat, BMI does show a strong correlation to the amount of fat on a person's body based on his weight compared to his height. BMI makes no adjustment for body type, but several factors must be considered.
Athletes
BMI is a calculation of total body mass, which includes both fat and muscle. Since no distinction is made between the two, athletes may have a high BMI but still have a low amount of body fat since their exercise regimen results in lean muscle, which weighs more than fat. The Centers for Disease Control and Prevention note that, while BMI in the "overweight" range may not necessarily mean a person has a high amount of body fat, a BMI in the "obese" range almost certainly is an indication of "increased levels of body fatness."
Men and Women
Men and women obviously differ in body type, and this difference carries into BMI results as well. At the same BMI, women are likely to have more body fat than men. While no adjustment is made in BMI ranges to account for this fact, the result of BMI calculations is meant to be an assessment tool and not a diagnostic one. More factors must be considered, like the sex of the individual, family history, diet, physical activity and even other assessment tests.
Other Factors
The National Heart, Lung and Blood Institute adds a second measurement to BMI scores that in some degree accounts for body shape. The agency recommends measuring waistlines to determine whether a person is more prone to fat around the waist instead of at the hips. This measurement does not change or adjust an individual's BMI, but it does indicate whether a person is more at risk for heart disease and type 2 diabetes as people with more fat around the middle tend to be. According to the Institute, risks rise quickly for waist measurements over 35 inches for women and over 40 inches for men.
Children and teens under the age of 20 are still growing and their bodies change rapidly. Calculations to determine BMI are the same as for adults, but the results factor in a child's age and sex as these characteristics greatly influence whether child and teen BMI results fall in overweight or obese categories.
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