In 1976, about 15.1% of Americans were obese, which rose to 30.9% by 2000, Youfa Wang, Ph.D., M.D, and May A. Beydoun, M.A., of Johns Hopkins reported in Epidemiologic Reviews on the basis of a meta-analysis.

The epidemic reached the stage by 2004 that 37.2% of Americans were obese or extremely obese, they added. The increase in obesity climbed from 13% to 32% over the past three decades.

From the 1960s to 2004, the prevalence of obesity and overweight in the U.S. has increased at an average rate of 0.3 to 0.9 percentage points across different sociodemographic groups, according to the meta-analysis.

If the trend continues, 75% of adults will be overweight or obese by 2015, 41% will be obese, and nearly a quarter of children and adolescents are expected to be overweight or obese.

The authors searched studies published from 1990 to 2006.

Adult overweight and obesity were defined by using body mass index cutpoints of 25 kg/m2 and 30 kg/m2, respectively. Childhood ''at risk for overweight'' and overweight were defined as the 85th and 95th percentiles of BMI.

Among adults, obesity prevalence increased 2.5-fold (13% to 32%) from the 1960s to 2004. Currently, approximately two-thirds (66.3%) of adults are overweight or obese; 32.4% are obese, and 4.8% are extremely obese (BMI ≥ 40kg/m2), the researchers reported.

Of children and adolescents, 16% are currently overweight and 34% are at risk of overweight.

Minority and low-socioeconomic-status groups were disproportionately affected at all ages, Dr.Wang reported.
Non-Hispanic Blacks had the highest prevalence, while minority groups (non-Hispanic Blacks and Mexican Americans) had a higher combined prevalence than non-Hispanic Whites by almost 10 percentage points.

Among non-Hispanic Black women 40 years or older, more than 80% were overweight or obese, and more than 50% were obese.

In 1999-2002, the prevalence of extreme obesity among African-American women was more than twice that among white and Mexican-American women (13.5% versus 5.5% and 5.7%).

Prevalence also vaired across states. In general, states in the southeast have higher prevalence rates than states on the west coast, in the midwest, and in the northeast.

Epidemiologic Reviews 2007; Wang Y, et al. "The Obesity Epidemic in the United States -- Gender, Age, Socioeconomic, Racial/Ethnic, and Geographic Characteristics: A Systematic Review and Meta-Regression Analysis"