Study: Can a Soda Tax Help Curb Obesity?

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Amid the many battles the United States is fighting — in Libya, with Congress, in Afghanistan, to name a few — there’s another costly war going on at home: against Americans’ expanding waistlines.

As the U.S. population continues to balloon, health and local officials are findng new ways to help us slim down: reimagining the food pyramid, posting calorie-counts on menus, banning chocolate milk from schools, taking toys out of Happy Meals. Now Illinois is proposing an increase on the state’s soda tax, which would add a penny an ounce to the cost of most soft drinks with added sugar or sweeteners. If it passes, you’ll have to pay a quarter more for a 24-oz. Coke in the Prairie State — and so, presumably, be less likely to buy it, consume less sugar and lose weight.

But new research from Northwestern University suggests that soda taxes don’t actually help obese people lose weight, largely because people with weight problems already tend to drink diet soda rather than the sugary kind. So taxing full-calorie sodas may not help many Americans make better dietary choices.

“I was very surprised,” says Ketan Patel, the study’s lead author, who says he anticipated that his results would be driven by the price sensitivities of obese people. “But price sensitivity is not what’s driving the results I’m getting,” he said. “After doing the analysis, it really turns out to be the case that obese people like diet soda so much more than regular soda that you can do whatever you want to the price.”

MORE: Studies: Why Diet Sodas Are No Benefit to Dieters

Patel ran computer simulations designed to track how soda prices would affect obesity rates. The findings demonstrated that a sugar tax would cause a negligible drop in obesity, about 1.4%, and that obese people would not lose much weight. “For people going from [body mass indexes] of over 30 to below that…most people are not having massive swings,” Patel said.

For the study, Patel’s team collected data on people with “all ranges of BMI” from the Centers for Disease Control and Prevention’s Behavioral Risk Factor Surveillance System, which has tracked health conditions in the U.S. for nearly three decades. They also collected a data set of soda prices and sales to estimate consumer practices, which they used to predict what people would purchase before and after the implementation of a soda tax. Based on the resulting change in total calories consumed per day over a set time period, the team modeled long-term changes in weight using existing nutrition literature.

Kelly Brownell, the director of the Rudd Center for Food Policy and Obesity at Yale University, has doubts about the accuracy of studies such as Patel’s. Simulations of the potential impact of public health actions such as a soda tax are based on a huge number of assumptions — about consumption, spending behavior, weight change — that are, in reality, difficult to make accurately, he explains.

“All of those changes are unknown,” he said. “So it’s not hard to allow those assumptions to create the results you want.”

MORE: The Best and Worst Foods for Healthy Weight 

Patel counters that assumptions are inevitable in research, and that previous studies that have produced results in favor of soda taxes have also made assumptions, typically about consumer preferences. “I’m trying to see if there are any critical assumptions here that really change the results, but so far I haven’t had anything like that,” he said. “It’s a somewhat valid criticism, but the paper is still being fleshed out, and there are a variety of robustness checks.”

But Patel acknowledges that his study could not predict whether a soda tax would help prevent people from consuming sweetened drinks in the first place and becoming fat later on — another point raised by Brownell. “The question of whether a soda tax could prevent people from becoming obese in the future…that’s still kind of an open question because there are some issues on how you model weight change that to my knowledge haven’t been addressed,” he said. “It’s possible that a soda tax could prevent people from becoming obese in the future, but for people already obese it’s not really going to do anything.”

Patel’s study is far from conclusive, and it remains to be seen whether the soda tax hike will ultimately fizzle out in the Illinois state Senate. But for the overall health and economy of the country, hanging delicately in the balance, soda taxes may not be a bad thing.

MORE: Study: Diet Soda May Raise Risk of Diabetes After All

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