MANY MAY be questioning themselves why do statistics about overweight and obesity differ? 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. 

MANY EARLY 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.[5] 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.

OVERWEIGHT and obesity are found worldwide, and the prevalence of these conditions in the United States ranks high along with other developed nations.

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