"The estimated age-adjusted percentage of overweight U.S. adults between the ages of 20 and 74 increased from about 43 percent in 1960-2 to about 54 percent in 1988-94. The fraction of the population that is obese - that is, with a BMI greater than 30 - increased from about 14 percent in the mid-1970s to about 29 percent in 2000. "
Since the early 1970s, the U.S. federal government has used the National Health and Nutrition Examination Survey (NHANES) and various body mass index (BMI) categories to estimate the percentage of the U.S. population that is overweight. BMI is defined as weight in kilograms divided by the square of height in meters. In the 1980s, being overweight was defined as having a BMI greater than or equal to 27.8 for men and 27.3 for women. Applying those standards to survey data from 1988-94 results in an estimated 33 percent of American men and 36 percent of American women being overweight. In the late 1990s, though, the federal government redefined being overweight as having a BMI greater than or equal to 25. Thus, the estimated number of overweight adults increased from 61.7 million to 97.1 million.
Using the new definition, the estimated age-adjusted percentage of overweight U.S. adults between the ages of 20 and 74 increased from about 43 percent in 1960-2 to about 54 percent in 1988-94. Although the age-adjusted fraction of the population in the "pre-obesity" category, with a BMI between 25 and 29.9, has been fairly stable since 1970, the fraction of the population that is obese - that is, with a BMI greater than 30 - increased from about 14 percent in the mid-1970s to about 29 percent in 2000. Because obesity is correlated with a variety of health problems, the increase in markedly overweight individuals has generated substantial concern among public health officials.
In The Super Size of America: An Economics Estimation of Body Mass Index and Obesity in Adults (NBER Working Paper No. 11584), coauthors Inas Rashad, Michael Grossman, and Shin-Yi Chou use individual-level data from the First, Second, and Third NHANES surveys to explore whether the increase in the rate of obesity is attributable to economic changes that have caused changes in individual behavior. Controlling for ethnicity, age, gender, household income, marital status, and years of formal education, they consider the effects of state restaurant density, state gasoline taxes, and state controls on smoking - including laws against smoking in public places and the cigarette tax - on individual BMI and obesity in men and women.
The authors ask whether the relatively recent change in the proportion of severely overweight individuals suggests that environmental factors, not genetics, play a central role in the increase in overweight. They note that technological change has reduced the amount of physical effort that people expend in their jobs, and that "the ready availability of inexpensive restaurants has not only caused people to consume more, but has made them less active - less likely to prepare food at home or travel further distances to obtain a healthy meal." The cigarette tax and smoking prohibition laws are included to account for the possibility that the increase in U.S. BMI may be related to the success of public health efforts to decrease smoking. When people quit smoking they often gain weight.
The authors can account for 79 percent of the change in BMI for males and one percent of the change in BMI for females. Their results suggest that blacks, Hispanics, males, older people, and those who are married or widowed, have higher BMIs. People with higher incomes and those with a college education have lower BMIs. Men are "more likely to have a higher BMI but less likely to be obese, reflecting the fact that BMI tends to overestimate overweight and obesity in people with more muscular mass." As the number of restaurants per capita increases so does BMI. The average BMI will rise by 0.09 percent if the per capita number of restaurants increases by one percent. The authors note that the rapid increase in obesity in the 1980s is partly an "unintended consequence of the campaign to reduce smoking." On balance, however, they conclude that "the increase in the per capita number of restaurants makes the largest contribution to the BMI outcome, accounting for 54 percent of the growth" in a pooled sample of m en and women
-- Linda Gorman