Body mass index (BMI) is a common method used to assess body weight relative to height. The reason it is so commonly employed is because it is a simple, indirect measure of body composition because it correlates highly with body fat in most people, however, the inherent flaws in the method limit the accuracy of the assessment. The weight excess or deficiency may, in part, be accounted for by body fat, although other factors such as muscularity also affect BMI significantly (see below).
Individuals with a BMI over 27 are considered to be at a significantly greater risk of health problems than those under 27. Female athletes who have very low BMIs (18 and below) may be at risk of developing irregular menstrual cycles; additionally, females with low overall body weights may also be at greater risk of developing osteoporosis.
Standard weight scales can delineate a person's total weight, but can't determine the lean-to-fat ratio of that weight. Height and weight charts were traditionally used as guidelines (see Body Mass Index) to determine if people are overweight, but because they don't take into account a person's body composition, such methods as BMI are woefully inaccurate. For example, a person who stands 5'8" tall and weighs 230 pounds, but has only 8% body fat would be considered obese by the BMI standard (36.31 BMI).
In studies by the National Center for Health Statistics,
- BMI values less than 18.5 are considered underweight.
- BMI values from 18.5 to 24.9 are healthy.
- Overweight is defined as a body mass index of 25.0 to less than 30.0. A BMI of about 25 kg/m2 corresponds to about 10 percent over ideal body weight. People with BMIs in this range have an increased risk of heart and blood vessel disease.
- Obesity is defined as a BMI of 30.0 or greater (based on NIH guidelines) — about 30 pounds or more overweight. People with *BMIs of 30 or more are at higher risk of cardiovascular disease.
- Extreme obesity is defined as a BMI of 40 or greater.
Some well-trained people with dense muscle mass may have a high BMI score but very little body fat. For them, the waist circumference, the skinfold thickness or more direct methods of measuring body fat may be more useful measures.
Weight in kilograms is divided by height in meters squared (kg/m2). To calculate BMI value in US units of measure, multiply weight in pounds by 703, divide by height in inches, then divide again by height in inches.
A frequent use of the BMI is to assess how much an individual's body weight departs from what is normal or desirable for a person of his or her height. The World Health Organization (WHO) regard a BMI of less than 18.5 as underweight and may indicate malnutrition, an eating disorder, or other health problems, while a BMI greater than 25 is considered overweight and above 30 is considered obese. These ranges of BMI values are valid only as statistical categories when applied to adults, and do not predict health.
|Category||BMI range - kg/m2|
|Severely underweight/Anorexic||less than 16.5|
|Underweight||from 16.5 to 18.5|
|Normal||from 18.5 to 25|
|Overweight||from 25 to 30|
|Obese Class I||from 30 to 35|
|Obese Class II||from 35 to 40|
|Severely Obese||from 40 to 45|
|Morbidly Obese||from 45 to 50|
|Super Obese||from 50 to 60|
|Hyper Obese||above 60|
The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of American men and 49% of women have BMIs over 25. Extreme obesity — a BMI of 40 or more — was found in 2% of the men and 4% of the women. The newest survey in 2007 indicates a continuation of the increase in BMI, 63% of Americans are overweight, with 26% now in the obese category. There are differing opinions on the threshold for being underweight in females, doctors quote anything from 18.5 to 20 as being the lowest weight, the most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the health risks involved, with a BMI <17.5 being an informal criterion for the diagnosis of anorexia nervosa.