A substantial number of nutrition studies link sugary soda and fast food to the rise in obesity in the United States.
Yet new research forthcoming in the journal Obesity Science & Practice concludes that there is no relationship between these foods and body mass index (BMI) for the majority of individuals, including those who are overweight. People whose weight is normal, the authors found, appear to consume as much junk food as people who are overweight or obese.
The study, though it has limitations, suggests that junk food isn’t the prime culprit for obesity. If that is true, narrowly targeting junk food as a way to reduce weight may be ineffective and distract from underlying causes.
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“There have been many people pushing policies and diet advice that narrowly focus on soda or fast food, as if it was going to be the way to solve the problem of obesity,” says author David Just, a professor of applied economics and management at Cornell University. “We felt like it was over-promising.”
He and co-author Brian Wansink, also at Cornell, analyzed data from 4,895 adults who participated in the Centers for Disease Control’s 2007- 08 National Health and Nutrition Examination Survey. Each participant completed two in-person surveys about their diets over the previous 24 hours, and their BMI was measured by trained individuals.
The researchers had expected to find BMI went up moderately as a person ate more junk food, so they were surprised to find that for 95 percent of those surveyed, BMI was not positively linked to soda, fast food, or even candy. “We were astounded,” says Just. There was a correlation, however, for 5 percent of individuals at the extremes: those who are morbidly obese or clinically underweight.
Though the finding conflicts with previous nutrition studies, it could be that past efforts included the two extreme groups and therefore saw an overall net positive correlation, Wansink and Just suggest. They also say the finding is in accordance with data from the United States Department of Agriculture, showing that the dramatic increase in daily average calories consumed by Americans over the last 40 years is led by grains and fats, not added sugars.
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The paper had several restrictions that could have influenced the outcome. Since the survey was based on self-reports, participants could have underreported how many times they consumed soda or fast food each day. The study also didn’t account for portion size, so it is possible that overweight people were eating larger servings of junk foods. Follow-up work suggests there is no relationship even when portion size is taken into account, says Just, but those results are yet to be published.
The findings do not imply that it’s fine to live on a diet of hamburgers, Coke, and chocolate, but they do suggest these foods may not be the central difference between fat and thin, says Just. “Limiting these foods is a part of a healthy diet, but it might not be the whole thing,” he says. “We need to think more broadly about our overall diet and the amount of exercise we get.”