The Increasing Prevalence of Obesity

Obesity prevalence rates will rise to 40 percent for men and 43 percent for women by 2020 ... Of those in the highest socioeconomic status (SES), 1.9 percent are obese at age 18 and 19.6 percent are obese at age 40. Of those in the low SES group, 4.6 percent are obese at age 18 and 31.3 percent are obese at age 40.

NBER Research Associate Christopher Ruhm predicts that 33 percent of American men and 38 percent of American women will be obese (as defined by a body mass index, or BMI, above 30) in 2010. Obesity prevalence rates will rise to 40 percent for men and 43 percent for women by 2020, he predicts using data from the National Health Examination Survey (NHES 1) and various National Health and Nutrition Examination Surveys (NHANES).

As Ruhm explains in Current and Future Prevalence of Obesity and Severe Obesity in the United States (NBER Working Paper No. 13181), there was little change in the body weight distribution of the U.S. population between NHES 1 in 1960-2 and the first and second NHANES surveys in 1971-4 and 1976-80. After that, however, measured body mass index grew 50 percent faster and that growth was disproportionately strong among the most overweight. Ruhm notes that after 1980 the prevalence of class 2, 3, and 4 obesity (that is, BMI above 35, 40, and 45 respectively) "tripled, quadrupled, and quintupled."

The morbidly obese are defined as having a BMI above 40. To reach this level, a 5'5" woman must weigh more than 240 pounds. Ruhm's estimates suggest that the fraction of people who are morbidly obese will reach 6 percent of men and 13 percent of women by 2020. He concludes that population-wide health campaigns to reduce growth in overweight and mild obesity will be less effective in combating severe obesity, and that additional strategies focusing on the heaviest people will be necessary to reduce severe obesity.

What has caused this increase in massive overweight? In Age, Socioeconomic Status and Obesity Growth (NBER Working Paper No. 13289) Ruhm and co-author Charles Baum find that excessive body weight grows with age for both men and women, and that it is inversely related to socioeconomic status (SES). High SES individuals have higher incomes, are more physically active, smoke less, and are lighter. Of those in the highest SES group, 1.9 percent are obese at age 18 and 19.6 percent are obese at age 40. Of those in the low SES group, 4.6 percent are obese at age 18 and 31.3 percent are obese at age 40. The authors' preferred SES measure is maternal education, but they obtain similar results when using other SES measures.

The most important correlate with obesity is years of schooling, they find, followed by race/ethnicity. Including both factors in predicting body weight explains almost half of the overall disparity among SES groups observed at age 40. An individual's family income, marital status, number of children, his propensity to drink, smoke, or exercise, or whether his job is physically demanding explains relatively little of the SES weight difference.

The detailed data for this study come from the National Longitudinal Survey of Youth (NLSY). The information in the NLSY allows the authors to explain about half of the correlation between weight and SES.

-- Linda Gorman