Just over a year ago, Berkeley, California, became the first city in the nation to tax soft drinks. In theory, that was supposed to cut into America's obesity epidemic: Make the stuff more expensive, and people—especially kids—will drink less of it. But Berkeley's new regulations may be misguided: For most of us, new research suggests, obesity has little, if anything, to do with eating fast food and drinking soda pop.

"To lose weight, patients are commonly told to reduce or eliminate their intake of indulgent foods, such as fast food, soft drinks and candy," Cornell University Food and Brand Lab's David Just and Brian Wansink write in a paper forthcoming in Obesity Science & Practice. "Interestingly, for the majority of patients ... there was no relationship between their intake of these foods and their BMI [body mass index] in this sample."

The connection between junk food and obesity really only applies to those with BMIs of 44.9 and above.

It's easy to see why researchers would think otherwise. Past studies have linked fast food and obesity. "Fast-food consumption has strong positive associations with weight gain ... suggesting that fast food increases the risk of obesity," according to a 2005 study that tracked the health and eating habits of more than 3,000 young adults for 15 years. In fact, recent research has suggested other studies may have even underestimated the impact of fast food on obesity.

The problem, Just and Wansink argue, is that the link might be due only to the people at the extremes—in particular, very overweight people who eat at fast-food restaurants more often than others. Suppose, for example, that everyone with a BMI under 45 ate at burger joints twice a week, but people with BMIs over 45 did so three times a week. Even though BMI and fast food are unrelated for all but the most morbidly obese, researchers using standard methods would conclude otherwise. Unfortunately, the standard methods aren't sensitive to such details.

Just and Wansick tested their hypothesis with data from the National Household and Nutrition Examination Study, which recorded height, weight, and eating habits for nearly 5,000 people in 2007 and 2008. Just and Wansick's conclusion: "After excluding the clinically underweight and most morbidly obese, consumption of indulgent foods was not positively correlated with measures of BMI." The connection between junk food and obesity, they find, really only applies to those with BMIs of 44.9 and above, or roughly one in 40 Americans.

"We were hoping to see what the impact of policies that were narrowly focused on a single food (like soda) or a small group of foods (like fast foods) would be," Just writes in an email. It's not that junk food isn't bad for you, he explains, "just that they don’t seem to be a differentiator between healthy weight and overweight on average."

From lawmakers' point of view, Just writes, "I think the takeaway is that the situation is not that simple. We need to be thinking much more broadly about overall diet and exercise."

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