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 varied definitions affect prevalence
statistics 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, without accounting for the effect of different age variations among
groups. For age-adjusted rates, statistical procedures are used to remove the
effect of age differences when comparing two or more populations at one point in
time, or one population at two or more points in time. 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
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 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 1970s,
1980s, and early 1990s. Since 1999, NHANES has become a continuous survey.
Many earlier reports 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 a BMI > 31.1 for men and a BMI > 32.2 for women. Some
studies round these numbers to a whole number, which affects the statistical
prevalence. In 1995, WHO recommended a classification for three “grades” of
overweight using BMI cutoff points of 25, 30, and 40. WHO suggested an
additional cutoff point of 35 and slightly different terminology in 1998.
The expert panel convened by NHLBI and NIDDK
released a report in September 1998 that provided definitions for overweight and
obesity similar to those used by WHO. 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 more frequently by the
broader medical and scientific communities, are based on evidence that health
risks increase 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 of > 30. However, health risks generally
increase with increasing BMI.