What’s Behind the Obesity Epidemic? Easily Accessible Food, and Lots of It

Among the American public and even some policymakers, it has become conventional wisdom that poverty, a dearth of supermarkets, reduced leisure time, and insufficient exercise are key forces behind the U.S. obesity epidemic.

Conventional wisdom is an unreliable guide, however, and in this case, much of it is wrong: The epidemic actually coincides with a falling share of income spent on food, wider availability of fruits and vegetables, increased leisure time, and more exercise among the general population.

Of course, there are differences between individuals, but we need to explain the change in obesity over time, not why people differ. Some differences in body mass index (BMI) are associated with genetic makeup. But genes haven’t changed in the past 50 years, so differences between individuals don’t explain trends.

Data from a new analysis of this issue indicates that the same argument applies to other characteristics, such as geography. Southern hospitality’s heavy food hasn’t caused the obesity epidemic any more than an active Colorado lifestyle has prevented it. There are differences at a given point in time, but the trend is the same, as shown in the figure below.

Percentage of Population with a BMI Over 25 in California, Colorado, and Mississippi


SOURCE: Calculations based on Behavioral Risk Factor Surveillance Survey; smooth trend adjusted for 2010 demographics.

Increases in obesity have also been surprisingly similar by level of education and by racial/ethnic group, as the following figures show.

Increase in Average BMI Nationwide, by Highest Education Level Achieved


SOURCE: Calculations based on Behavioral Risk Factor Surveillance Survey; smooth trend adjusted for 2010 demographics.

Increase in Average BMI Nationwide, by Racial/Ethnic Group (Men)


SOURCE: Calculations based on Behavioral Risk Factor Surveillance Survey; smooth trend adjusted for 2010 demographics.

Increase in Average BMI Nationwide, by Racial/Ethnic Group (Women)


SOURCE: Calculations based on Behavioral Risk Factor Surveillance Survey; smooth trend adjusted for 2010 demographics.

Across all these characteristics, the striking feature over time is not how different the trends are but rather how similar the weight gain has been, regardless of geography or social group. There are differences between groups—often sizable ones—at any point in time, but addressing such disparities is different from stopping the obesity epidemic.

That no group is immune to rising obesity rates suggests that universal environmental factors are driving the trend. The clearest change concerns food availability and cost. Since the 1970s, there has been a significant drop in the share of income spent on food—yet each food dollar buys a lot more, as shown below.

Food Expenditures as a Percentage of Disposable Income, Total and by Type of Food

Screen Shot 2014-07-16 at 5.22.34 PM

SOURCE: U.S. Department of Agriculture data.

Average Daily Per Capita Calories, Adjusted for Spoilage/Waste

Screen Shot 2014-07-16 at 5.22.43 PMSOURCE: U.S. Department of Agriculture data.

As the obesity epidemic has grown and food prices relative to income have dropped, Americans have been eating more of everything, including fruits and vegetables. In terms of macronutrients, most extra calories come from carbohydrates.

U.S. markets have succeeded in largely solving the age-old problem of food scarcity, so the answer isn’t to return to higher food prices across the board. But with the solution to food scarcity contributing to a new threat, Americans need market forces to shift them in a different direction and help stem the obesity epidemic.

Market forces happen on both sides: supply and demand. On the supply side, agricultural policy has historically tried to promote output and improve food security. Undesirable side effects weren’t uncommon. Europe, for example, often resorted to discount prices to eliminate its surplus “butter mountains” and “milk lakes.”

On the demand side, consumers do substantially alter their shopping and improve their diets if motivated by price changes. An encouraging example comes from South Africa, where the country’s largest health insurer implemented a nationwide rebate program for healthy foods. However, even substantial price incentives—in South Africa, a 25-percent rebate—can close only a small part of the gap between recommended and actual diets. In addition, price discounts may improve diet quality, but they may not reduce obesity. (A discount for healthy foods doesn’t mean that people will buy fewer calories overall.)

South Africa isn’t the only country taking action. In Mexico, where the obesity rate now exceeds that in the U.S., policymakers have enacted a tax on sugar-sweetened beverages and energy-dense snack items. Europe, perhaps spurred by swelling obesity rates of its own, is also taking action. Hungary has imposed special taxes on unhealthy foods, while Denmark went back and forth: It implemented a tax, only to repeal it a year later.

Changes in social norms that shift the demand curve could be just as important as policies that affect food prices. There was a time when it was polite to offer a guest a cigarette. Americans today might offer cookies or soda. When they begin to regard junk food as they do tobacco, curbing the obesity epidemic may become more attainable.

Under the influence of conventional wisdom, many policy interventions focus on “positive” messages: Eat more fruit and vegetables. Get more exercise. However, given that fruit and vegetable availability and physical activity have both increased while relative food prices have plummeted and obesity rates have soared, reducing discretionary calorie consumption may be a more promising lever to reduce overweight and obesity.

Roland Sturm is a senior economist at the nonprofit, nonpartisan RAND Corporation and a professor at the Pardee RAND Graduate School.

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Murray WalkerEXO KpopEmily CleathAl LewisVik Khanna Recent comment authors
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Murray Walker

Fatty foods have become so addicting to people that it’s hard for people to say no. It’s a process that needs to be drilled into the young generation early, mandate P.E. in every state and instill the importance of exercise as much as homework is.

EXO Kpop

“The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common. Wear heavy cloth or thick rubber gloves to protect hands, wrists and fingers when shooting in the field. Together with the Cassini Mission, HST has studied this aurora and the influence of solar winds on it.

Emily Cleath
Emily Cleath

There seems to be a gaping hole in this analysis, a hole roughly the size of, say, a food desert. The statement “U.S. markets have succeeded in largely solving the age-old problem of food scarcity” is a blithe denial of the ample research about nutritious food access in this country – or the lack thereof. You can make all kinds of generalizations with aggregate data. Preferably, one would break consumption and obesity trends down by income level – a key metric that Sturm left out. Low-income neighborhoods are often food-poor neighborhoods or “CLAs” (communities with low-supermarket access). Those residents are,… Read more »

Al Lewis
Al Lewis

Spot on as usual, Roland (just like when you take on the wellness ignorati). Let me add a little data. My numbers may be a bit off, but my point isn’t. When I was a kid the minimum wage was $1,35 and you could get an entire meal at McDonalds for $0.95. They even had a jingle (“change back in your pocket.”) Today the minimum wage is more like $8, but a burger, fries and a Coke would be about $5. So ajunk food is a little more affordable. But now throw in that each of those three items is… Read more »

Vik Khanna

This is a great essay that elucidates nicely just how complex the body composition issue is. It is ironic, however, that something so incredibly robust and successful — the American food industry, from farmers to retailers — might become the target of the ire of policy makers who use these data to “punish” them for succeeding at their businesses. I am skeptical about using tax policy to alter the equation. I think we manipulate the tax code excessively to try to achieve social policy ends, and I can envision now the battle over how to define a taxable food item… Read more »