Why are Americans so fat? A new study suggests we've been getting it all wrong.
It's not that we don't exercise enough or spend too much time at the office, or that good, healthy food is too expensive, researchers reported Thursday. And it's not about socioeconomic status, race or geography.
Rather, the main reason we're getting fatter — all of us — is because we are surrounded by tasty temptations that cost very little, from fast food menus to processed snack foods, said the study's lead author, Roland Sturm, a senior economist at the RAND Corporation and a professor of policy analysis at the Pardee RAND Graduate School.
Up till now, researchers have focused on group differences: between rich and poor, between those who live states like Colorado and Mississippi, or between blacks and whites, Sturm said.
"But what we've really got to look at is changes over time for everyone," he explained. "People aren't becoming obese because they don't have enough money or because Southern hospitality makes you fat. And they aren't staying thinner because they live in a mountain state like Colorado. This is something that cuts across the whole population."
Sturm reviewed the all the available research for a study published in CA: Cancer Journal for Clinicians and concluded that everyone, regardless of race, ethnicity or socio-economic status, is gaining weight at about the same rate. His graphs clearly show parallel tracks of increasing flab among all groups.
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"If we want to solve the obesity problem, we have to figure out what has changed for everybody," Sturm said. "And the thing that pops out is our food environment. That's where the action is. And it's not just that food is now cheap relative to income it's also that it's so much more convenient."
In the 1930s, Americans spent a quarter of their disposable income on food. The most recent data show that share is now under one-tenth.
And while cheap, convenient food is a good thing, our biology and our social norms haven't caught up with the abundance. Foods that can be mass-produced and aren't perishable like potato chips, candy and sugar-sweetened drinks are prime culprits.
"We have a survival instinct that drives us to eat constantly because in human history starvation was the problem," Sturm said. "Also our social norms are guided by that. We've been brought up to believe that it's nice to offer food to guests. It's like smoking. There was a time when it was perfectly good manners when someone visited to offer them a cigarette."
Some researchers have suggested that part of the problem is the inaccessibility of healthy foods. But the data show that fruits and vegetables, for example, have become increasingly available and affordable over the past decade, he said.
While Sturm allows that Americans could eat more fruits and vegetables, he argues that wouldn't make them any thinner. That's because fruits and veggies wouldn't replace other, less healthy, choices, they would simply be consumed in addition to everything else.
But cost doesn't paint a full picture, says Marion Nestle, a professor in the Department of Nutrition, Food Studies, and Public Health at New York University, who was not involved in the research.
"This study attributes obesity to only one cause: low food prices.It does not discuss relentless marketing of cheap 'junk' foods, nor does it discuss Consumer Price Index data on the relative cost of foods," she said, noting that the cost of fruits and vegetables has gone up more than the average food cost.
"The higher prevalence of obesity among people of lower income and education can be explained by greater consumption of low-cost, high-calorie foods."
Other factors that seem to play into the obesity epidemic are the rise of electronic entertainment, increased reliance on cars and a shift away from physical jobs to more desk-bound ones. Although Americans aren't spending more time at work and have more leisure time today, they do spend more of that time sedentary, the researchers found.
Sturm may have offered up a new target, but he doesn't have a silver bullet to aim at it. He suggests small tweaks to prod people to make changes. For example, taxes on unhealthy food choices.
"An economic incentive allows us to nudge people in the right direction," he said. "It might be enough to start changing social norms. In half a decade smoking changed dramatically. Though we can't say that's all because of taxes and indoor smoking limits, those tweaks probably helped."
Linda Carroll is a regular health contributor to NBC News and Reuters Health. She is coauthor of "The Concussion Crisis: Anatomy of a Silent Epidemic" and "Out of the Clouds: The Unlikely Horseman and the Unwanted Colt Who Conquered the Sport of Kings."
Madelyn Fernstrom, PhD
Madelyn Fernstrom, Ph.D., is the Diet and Nutrition editor for TODAY.
Fernstrom is a professor of psychiatry, epidemiology and surgery at the University of Pittsburgh Medical Center and a board certified nutrition specialist from the American College of Nutrition. She is the founding director of the UPMC Weight Management Center.