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body mass index

The obesity guideline from the National Heart, Lung, and Blood Institute of the National Institutes of Health proposes that practitioners use body mass index (BMI) to assess subjects because the index is simple, correlates to fatness, and applies to both men and women.

Body mass index is a relationship between weight and height that is associated with body fat and health risk.

The equation used to calculate body mass index is as follows:

 Body Mass Index = Weight in kilograms / (Height in meters)2

Converted to English, the equation is as follows:

 Body Mass Index = Weight in pounds / (Height in inches)2 * 704.5

NHLBI Body Mass Index Calculator   NHLBI Body Mass Index Table

Body Mass Index Classifications
The National Heart, Lung, and Blood Institute proposes the following classifications of health using body mass index:

 Body Mass Index Classification Less than 18.5 Underweight 18.5 to 24.9 Normal 25.0 to 29.9 Overweight 30.0 to 34.9 Obese Class I 35.0 to 39.9 Obese Class II 40.0 or greater Extremely Obese

Health Risks
Research has identified the health risk associated with a wide range of body mass index values:

Limitations
While a useful tool, using BMI to assess a subject's weight has limitations. For example, it does not take frame size into account, so some individuals may be considered overweight even if they do not have excessive body fat. As a result, there are several types of individuals for which BMI is not always a good predictor of overweight, including:

 • Children and teens, because BMI ranges are based on adult heights. • Competitive athletes and bodybuilders, because heavier muscle weight may skew results. • Pregnant or nursing women, because they need more fat reserves than usual. • People over 65, because even BMI values of 29 may not be unhealthy at this age.

 References: Bedogni G, et al. Is body mass index a measure of adiposity in elderly women? Obesity Research, 2001 Jan;9(1):17-20. Gallagher DG, et al. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. American Journal of Clinical Nutrition, 2000;72:694-701. Guo SS, et al. Body mass index during childhood, adolescence and young adulthood in relation to adult overweight and adiposity: the Fels Longitudinal Study. International Journal of Obesity and Related Metabolic Disorders, 2000 Dec;24(12):1628-35. Micozzi MS, Harris TM. Age variations in the relation of body mass indices to estimates of body fat and muscle mass. American Journal of Physical Anthropology, 1990 Mar;81(3):375-9. Pasco JA. Body fat estimated from anthropometric and electrical impedance measurements. Human Nutrition: Clinical Nutrition, 1985 Sep;39(5):365-9.