Why the Surge in Obesity?

The Weight of the Nation is a four-part series on obesity in America by HBO Films and the Institute of Medicine, with assistance from the Centers for Disease Control (CDC) and the National Institutes of Health (NIH). It’s been showing on HBO and can be viewed online. Each of the four parts is well done and informative.

Obesity is defined as having a body mass index (BMI) of 30 or more. For a person 6 feet tall, that means a weight of more than 220 pounds. For someone 5’6″, the threshold is 185 pounds. People who are obese tend to earn less and are more likely to be depressed. They are at greater risk of diabetes, heart disease, stroke, and some types of cancer, and they tend to die younger. The CDC estimates the direct and indirect medical care costs of obesity to be $150 billion a year, about 1% of our GDP.

The chart below, which appears several times in The Weight of the Nation, shows the trend in obesity among American adults since 1960, the first year for which we have good data. The data are from the National Health and Nutrition Examination Survey (NHANES). They are collected from actual measurements of people’s height and weight, rather than from phone interviews, so they’re quite reliable. After holding constant at about 15% in the 1960s and 1970s, the adult obesity rate shot up beginning in the 1980s, reaching 35% in the mid-2000s.

What caused the surge in obesity? The standard explanation is too much eating and too little physical activity, and The Weight of the Nation sticks with this story. But it shouldn’t, because the evidence suggests one of these two hypothesized culprits has been far more important than the other.

Here is the trend in eating, measured as average calories in the food supply (adjusted for loss and spoilage) according to data from the Department of Agriculture. This chart too is from The Weight of the Nation. The timing of change matches that for obesity; the level is flat through the 1970s and then rises sharply beginning in the 1980s. An alternative series, measuring energy consumption per capita, goes back to 1950 (see figure 6, chart F here); it too shows little or no change until 1980, and then a sharp jump. The rise in food consumption correlates closely with the rise in obesity.

That isn’t true of physical activity. We’re less active now than we were half a century ago, but the timing of the decline in activity doesn’t match up with the shift in obesity.

We don’t have good historical data for a comprehensive measure of activity, such as calories expended, so we have to look instead at individual components. We can begin with the most-often-cited culprit: television. Here too The Weight of the Nation presents data, shown below, with the suggestion that TV watching is a significant cause of rising obesity. But the trend doesn’t support that inference. Time spent watching television has increased steadily since 1950. There was no sudden rise in the 1980s.

What about video games, the internet, and smartphones? The internet and smartphones arrived on the scene too late to account for the rise in obesity in the 1980s and most of the 1990s. The timing doesn’t work for video games either; they’re played mostly by the young, beginning in the 1980s, but obesity rates rose sharply in the 1980s and 1990s among adults of all ages, even among the elderly (see table 2 here).

More Americans now have sedentary jobs and drive to work. Yet as David Cutler, Edward Glaeser, and Jesse Shapiro noted in a paper published nearly a decade ago, these shifts have been going on for a long time, with no acceleration in the 1980s.

“Between 1910 and 1970, the share of people employed in jobs that are highly active like farm workers and laborers fell from 68 to 49 percent. Since then, the change has been more modest. Between 1980 and 1990, the share of the population in highly active jobs declined by a mere 3 percentage points, from 45 to 42 percent. Occupation changes are not a major cause of the recent increase in obesity.

“Changes in transportation to work are another possible source of reduced energy expenditure — driving a car instead of walking or using public transportation. Over the longer time period, cars have replaced walking and public transportation as a means of commuting. But this change had largely run its course by 1980. In 1980, 84 percent of people drove to work, 6 percent walked, and 6 percent used public transportation. In 2000, 87 percent drove to work, 3 percent walked, and 5 percent used public transportation. Changes of this minor magnitude are much too small to explain the trend in obesity.”

Another reason to doubt the importance of declining physical activity is that the elderly probably have become more active over time, rather than less, and yet we observe a rise in obesity among the elderly too, similar in timing and magnitude to that of younger adults (again see table 2 here).

In short, the evidence suggests that reduced physical activity has not been a key cause of the surge in obesity in America (more here, here, here, here, here).

This doesn’t mean physical activity plays no role in determining which persons become obese. And it doesn’t mean an increase in activity won’t help reduce obesity’s prevalence. But it does suggest that a strategy focused on increasing activity — and The Weight of the Nation leans in this direction — may not get us as far as we’d like. To make serious progress in reducing obesity, we need to significantly reduce the number of calories many of us consume.