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New evidence that junk food taxes would lead to weight loss ... maybe

March 8, 2010 |  6:21 pm

Pizza Finally, we may have scientific evidence in a top-notch medical journal supporting the common-sense idea that if junk foods such as soda and pizza were subjected to a sin tax – like cigarettes and alcohol – people would consume them less and slim down as a result.

Researchers tracked the diet and weight of 5,115 people who participated in a 20-year study designed to identify risk factors for cardiovascular disease. That information was linked to data on food prices, which fluctuated over the two decades of the study.

The researchers zeroed in on four food items: soda, whole milk, pizza and hamburgers. They figured that when prices rose, demand would fall. And that’s exactly what they found for soda and pizza – a 10% price hike was associated with a 7% decrease in soda consumption and an 11.5% cutback on pizza.

Other studies have linked changes in price with changes in consumption, but they failed to close the loop by showing that people who ate less junk food actually slimmed down. This report did – in a fashion. The researchers were able to link price, calories and weight in a formula that predicted a $1 increase in the price of soda would cause people to consume 124 fewer calories per day and lose 2.34 pounds.

Using this model, they concluded that an 18% tax on soda would cause people to shed 5 pounds over the course of a year. The results were published Monday in Archives of Internal Medicine.

This has got to be the most persuasive study to date showing that a sin tax on soda or junk food would actually have the desired effect -- and not just aggravate people who care to indulge in an occasional root beer (or threaten their sense of personal freedom). Soda tax advocates may embrace it as proof that their policy goals are justified.

But keep in mind – this is only a model, and models don’t necessarily reflect the real world. For instance, it is often said that 3,500 calories equals one pound of fat. But endocrinologists and other scientists understand that if you permanently reduce your daily caloric intake, you won’t keep losing weight – your body will simply adjust to its new energy level. This is why it can be so difficult for dieters to keep pounds off. It’s also a good reason to question the conclusion that an 18% soda tax would make the average American 5 pounds lighter.

What’s more, models are only as good as the data that’s plugged into them. Not to impugn the CARDIA study (which tracked those 5,115 volunteers), but these folks consumed an average of 2,972 calories per day at the start of the project and reduced that to an average of 2,403 20 years later, according to lead researcher Kiyah Duffey of the University of North Carolina’s Gillings School of Global Public Health. Kudos to those study participants, but do they reflect what’s happening across the United States as a whole? A 2004 report from the U.S. Centers for Disease Control and Prevention found that between 1971 and 2000, daily caloric intake rose 22% for women and 7% for men.

Senior researcher Barry Popkin, a nutrition epidemiologist who heads UNC’s Interdisciplinary Obesity Center in Chapel Hill, said the results would be more persuasive if they were based on better data linking a wider array of food prices, dietary intake, body composition and other variables for specific individuals. That might take a good while – for the Archives study, the team spent five years matching price data to each person for each moment in time. In the meantime, the continuing debate on the merits of soda food taxes will have to proceed without hard scientific evidence.

-- Karen Kaplan

Photo credit: Robert Lachman / Los Angeles Times

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