Calculating BMI is simple, quick, and inexpensive—but it does have limitations. One problem with using BMI as a measurement tool is that very muscular people may fall into the “overweight” category when they are actually healthy and fit. Another problem with using BMI is that people who have lost muscle mass, such as the elderly, may be in the “healthy weight” category—according to their BMI—when they actually have reduced nutritional reserves. BMI, therefore, is useful as a general guideline to monitor trends in the population, but by itself is not diagnostic of an individual patient’s health status. Further evaluation of a patient should be performed to determine his or her weight status and associated health risks.

For more information on measuring overweight and obesity, see Weight and Waist Measurement: Tools for Adults.

Why do statistics about overweight and obesity differ?

Previous studies in the United States have used the 1959 or the 1983 Metropolitan Life Insurance tables of desirable weight-for-height as the reference for overweight.[3] More recently, many Government agencies and scientific health organizations have estimated overweight using data from a series of cross-sectional surveys called the National Health Examination Surveys (NHES) and the National Health and Nutrition Examination Surveys (NHANES). The National Center for Health Statistics (NCHS) of the Centers for Disease Control and Prevention (CDC) conducted these surveys. Each had three cycles: NHES I, II, and III spanned the period from 1960 to 1970, and NHANES I, II, and III were conducted in the 1970’s, 1980’s, and early 1990’s. Since 1999, NHANES has become a continuous survey.

Many reports in the literature use a statistically derived definition of overweight from NHANES II (1976–1980). This definition (based on the gender-specific 85th percentile values of BMI for 20 to 29 year olds) is a BMI greater than or equal to (>) 27.3 for women and 27.8 for men. NHANES II further defines “severe overweight” (based on 95th percentile values) as BMI > 31.1 for men and BMI > 32.2 for women.[4] Some studies round these numbers to a whole number, which affects the statistical prevalence. In 1995, the World Health Organization recommended a classification for three “grades” of overweight using BMI cutoff points of 25, 30, and 40.[5] The International Obesity Task Force suggested an additional cutoff point of 35 and slightly different terminology.[6]

The expert panel convened by NHLBI and NIDDK released a report in June 1998, that provided definitions for overweight and obesity similar to those used by the World Health Organization. The panel identified overweight as a BMI > 25 to less than (<)30, and obesity as a BMI > 30. These definitions, widely used by the Federal government and increasingly by the broader medical and scientific communities, are based on evidence that health risks increase more steeply in individuals with a BMI > 25.

BMI cutoff points are a guide for definitions of overweight and obesity and are useful for comparative purposes across populations and over time; however, the health risks associated with overweight and obesity are on a continuum and do not necessarily correspond to rigid cutoff points. For example, an overweight individual with a BMI of 29 does not acquire additional health consequences associated with obesity simply by crossing the BMI threshold > 30. However, health risks generally increase with increasing BMI.