Body Mass Index

Body mass index (BMI) is a calculation of a person's weight compared to his or her height. It is also known as the Quetelet index, named after Adolphe Quetelet, who developed the formula in the mid-1800s. BMI is calculated by taking a person's weight (in kilograms) [Page 273]divided by the person's height squared (in meters). Originally, BMI was thought to be a good estimation of the amount of fatty tissue within a person, but some limitations do exist.

The range of indices can vary from about 15 (which is starvation) to over 40 (morbidly obese). This range is further subdivided into underweight, normal weight, overweight, obese, and morbidly obese. The definition of underweight is a BMI less than 18.5, while a BMI of greater than 25 is considered overweight. A person with a BMI between 18.5 and 25 is classified as normal weight. BMI above 30 is considered obese, and above 40 is morbidly obese. These ranges are only applicable to adults, and may vary due to ethnicity.

In children, BMI is calculated in the same manner; however, the use of the index is different compared to adults. BMI in children should be compared by age and sex for interpretation. It is calculated and compared in a percentile fashion rather than the index number alone. Children above the 95th percentile for their age and sex are considered overweight, and children who have a BMI greater than the 85th percentile have a greater risk of becoming overweight. Those who are under the 5th percentile for age and sex are considered underweight.

While BMI is an easy and inexpensive measurement, it can be a fairly inaccurate measurement of body fat due to the fact that it does not take into account muscle mass, bone structure, fat percentage, or body type. This is why a professional football player will be regarded as overweight or obese, and a marathon runner as underweight. Even with these limitations, BMI is acceptable for monitoring weight trends in inactive and overweight persons. This is because while monitoring BMI, we know that the person's height is unchanged. In the United States, BMI is also used for defining those with eating disorders. According to the Diagnostic and Statistical Manual, one of the criteria for the diagnosis of anorexia nervosa is a BMI below 17.5. Nutritional status can also be determined for these people if a BMI trend is followed.

In the elderly population, there can be a misleading trend of decreasing BMI. This is because as one ages, the amount of muscle mass starts to decrease, while the amount of fatty tissue is increasing. Because muscle is denser than fat, a decreasing BMI does not mean a decrease in fatty tissue. In fact, in the elderly population, it is more likely that they will gain fatty tissue.