BMI has been used by the WHO
as the standard for recording obesity statistics since the early 1980s. In the United States, BMI is also used as a measure of underweight, owing to advocacy on behalf of those suffering with eating disorders, such as anorexia nervosa
and bulimia nervosa
BMI can be calculated quickly and without expensive equipment. However, BMI categories do not take into account many factors such as frame size
The categories also fail to account for varying proportions of fat, bone, cartilage, water weight, and more.
Despite this, BMI categories are regularly regarded as a satisfactory tool for measuring whether sedentary individuals are “underweight”, “overweight” or “obese” with various exemptions, such as: athletes, children, the elderly, and the infirm.
One basic problem, especially in athletes, is that muscle weight contributes to BMI. Some professional athletes would be “overweight” or “obese” according to their BMI, despite them carrying little fat, unless the number at which they are considered “overweight” or “obese” is adjusted upward in some modified version of the calculation. In children and the elderly, differences in bone density and, thus, in the proportion of bone to total weight can mean the number at which these people are considered underweight should be adjusted downward.
The medical establishment has generally acknowledged some major shortcomings of BMI.
Because the BMI formula depends only upon weight and height, its assumptions about the distribution between lean mass and adipose tissue are not always exact. BMI sometimes overestimates adiposity
on those with more lean body mass (e.g., athletes) while greatly under-estimating excess adiposity on those with less lean body mass. A study in June, 2008 by Romero-Corral et al. examined 13,601 subjects from the United States’ Third National Health and Nutrition Examination Survey (NHANES III)
and found that BMI-defined obesity was present in 21% of men and 31% of women. Using body fat percentages
(BF%), however, BF%-defined obesity was found in 50% of men and 62% of women. While BMI-defined obesity showed high specificity
(95% of men and 99% of women presenting BMI-defined obesity also presented BF%-defined obesity), BMI showed poor sensitivity
(BMI only identified 36% of the men and 49% of the women who presented BF%-defined obesity).