Increased caloric intake accounted for 93 percent of the change in adult obesity from 1990 to 2001 (the remainder is attributable to reduced energy expenditure).
Aside from the physiological fact that the number of calories consumed must be larger than the number expended in order to gain weight, the causes of the dramatic increase in obesity over the last few decades are not well understood. Although some experts point to the decline in work-related physical activity, it has both been comparatively gradual and has largely predated the recent rise in obesity. Furthermore, obesity among children and the elderly, two groups that we would not expect to be affected by changes in work-related physical activity, has risen along with adult obesity. Finally, the obesity increase has been remarkably similar across countries, which suggests a worldwide phenomenon.
In Why is the Developed World Obese? (NBER Working Paper No. 12954), authors Sara Bleich, David Cutler, Christopher Murray, and Alyce Adams show that rising obesity in the developed world is primarily the result of consuming more calories. Specifically, they find that increased caloric intake accounted for 93 percent of the change in adult obesity from1990 to 2001 (the remainder is attributable to reduced energy expenditure). The increase in caloric intake appears to be driven by technological innovations, such as lower food prices and the ease with which businesses can enter the marketplace, as well as changing sociodemographic characteristics such as increased labor force participation and increased urbanization.
Across the developed world, average food prices fell by 12 percent from 1980 to 2002, which the authors associate with a corresponding higher caloric intake of approximately 38 calories. A 10 percent increase in female labor force participation was associated with an increase of approximately 70 calories. A 10 percent increase in urbanization was associated with an increase of approximately 113 calories.
The authors point out that a very small net increase in calories may lead to a large increase in obesity, and they predict expected changes in weight based on the associations they observe between caloric supply and the drivers of increased consumption. For example, they show that increasing food prices by 12 percent would be associated with a decrease of 1.5 kilograms (3.4 pounds) for the average 65-kilogram (143-pound) person. Similarly, they show that decreasing urbanization by 5 percent would be associated with a decrease of 2.2 kilograms (5 pounds) for the average 65-kilogram person.
The data used in the paper were constructed from a variety of sources including the food balance sheet from the Food and Agricultural Organization, obesity prevalence from the OECD, economic indicators from the World Development Indicators, and regulation indicators from the Economic Freedom of the World Index. The authors note that available data are likely subject to important measurement errors and that caution should be used in interpreting their results. They conclude that, while over consumption appears to be relatively more important to rising obesity than physical activity, energy expenditure is still important to weight management and overall health.
-- Linda Gorman