History of Obesity Statistics
Over time, there have been several definitions and measurements for obesity. Most recently, the National Institutes of Health released a report that provided definitions for overweight and obesity. This report identified overweight as a BMI between 25 and 30, and obesity as a BMI greater than 30. These definitions are based on evidence that health risks increase significantly in people who have a BMI higher than 25.
The definitions or measurement characteristics for overweight and obesity have varied over time, from study to study, and from one part of the world to another. The various definitions affect obesity statistics, such as prevalence, and make it difficult to compare data from different studies. Prevalence refers to the total number of existing cases of a disease or condition in a given population at a given time. Some overweight- and obesity-related prevalence rates are presented as crude or unadjusted estimates, while others are age-adjusted estimates.
Unadjusted prevalence estimates are used to present cross-sectional data for population groups at a given point or time period. For age-adjusted rates, statistical procedures are used to remove the effect of age differences in populations that are being compared over different time periods. Unadjusted estimates and age-adjusted estimates will yield slightly different values.
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. More recently, many governmental 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, which spanned the period from 1960 to 1970. NHANES I, II, and III were conducted in the 1970s, 1980s, and early 1990s. Since 1999, NHANES has become a continuous survey.
Many reports in the literature use a statistically derived definition of overweight from NHANES II (1976 to 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 greater than 31.1 for men and BMI greater than 32.2 for women. Some studies round these numbers to a whole number, which affects the statistical prevalence.