The morbidity and mortality associated with being overweight or obese have been known to the medical profession for more than 2000 years . Overweight refers to a weight above the "normal" range. This is determined by calculating the body mass index (BMI, defined as the weight in kilograms divided by height in meters squared). Overweight is defined as a BMI of 25 to 29.9 kg/m2, obesity as a BMI of >30 kg/m2. Severe obesity is defined as a BMI >40 kg/m2 (or ≥35 kg/m2 in the presence of comorbidities).
The health hazards associated with obesity are reviewed here. The prevalence of and therapy for obesity, and the evaluation of the overweight patient are discussed elsewhere. (See "Overview of therapy for obesity in adults" and "Screening for and clinical evaluation of obesity in adults".)
A number of large epidemiologic studies have evaluated the relationship between obesity and mortality [2-11]. In general, greater body mass index (BMI) is associated with increased rate of death from all causes and from cardiovascular disease (CVD) (figure 1). This is particularly true for those with severe obesity . Estimates for the annual number of excess deaths attributable to obesity in the United States are variable and range from 111,909 to 365,000 [12,13].
Being overweight also appears to be associated with decreased survival in some [4,10,14], but not all , studies. As an example, a prospective study from the United States evaluated the relationship between BMI and risk of death over a maximum follow-up period of 10 years among over 500,000 men and women aged 50 to 71 years . Among the subset of individuals 50 years of age (when prevalence of chronic disease is low) who had never smoked, an increased risk of death was associated with being either overweight (20 to 50 percent increase in those between 26.5 to 29.9 kg/m2) or obese (two- to over threefold increase in those ≥30 kg/m2). The risk of all-cause mortality with increasing BMI of 25 kg/m2 or higher appears to be independent of gender and ethnicity [4,16].
Similar findings were noted in the Prospective Studies Collaboration analysis of 57 prospective studies (894,000 European and North American adults followed for a mean of eight years), in a pooled analysis of 19 prospective studies (1.4 million Caucasian adults with median follow-up of 10 years) from the National Cancer Institute Cohort Consortium (NCICC), and in a pooled analysis of 19 prospective cohort studies (1.1 million Asian adults with mean follow-up of nine years) from the Asia Cohort Consortium [10,14,17].