Will New York's super-size drink ban work?

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As the New York Board of Health moves to prohibit sales of super-sized sugary drinks, Daniel Engber examines the conflicting studies around the issue

"What's next?" asks the industry-funded Center for Consumer Freedom, in response to a New York City plan to forbid the sale of sugary drinks larger than 16 ounces in restaurants, sports venues and movie theatres. "Limits on the width of a pizza slice, size of a hamburger or amount of cream cheese on a bagel?"

Mayor Michael Bloomberg defended his idea, assuring Matt Lauer of NBC's The Today Show show that "we're not banning you from getting the stuff. It's just if you want 32 ounces, the restaurant has to serve it in two glasses. That's not exactly taking away your freedoms."

It didn't take long for the details of this putative public-health measure to dissolve into a super-sized debate between libertarians and liberals, soda jerks and health nuts. But before we all start arguing about the costs of obesity and an apartheid of pleasure, let's consider the scientific facts ? contested or otherwise ? that led Bloomberg to propose his soda-size control in the first place.

Would the pint-cup cut-off reduce obesity, or not?

There are two questions here, both of them well-studied by nutritionists and epidemiologists. First, would people eat or drink less if they were served in smaller portions? Second, would they lose weight if they consumed less sugary liquid?

The answers seem obvious to most people: yes and YES! That's part of what makes it so hard to study the questions. Since we already think that both super-sizing and soda make us fat, the people who avoid super-sized portions and non-diet soda tend to be the same ones who join the gym, eat fish twice per week, and natter about the joys of kale. These health-conscious people also tend to have money, and given that obesity and poverty are related in all kinds of ways, it's tricky to determine which parts of an obesogenic lifestyle might count the most.

Tricky, but not impossible. There are plenty of data, for example, to show that portion sizes have increased over the last 30 years while childhood obesity rates have tripled. For a 2011 paper in the Journal of Nutrition, Carmen Piernas and Barry Popkin used national surveys to study the diets of more than 30,000 kids and measure how their junk food habits changed from 1977 to 2006.

They found that soft drink portions had increased by almost one-third. (Once upon a time, a "king-size soft drink" was just 12 ounces.) But sodas and fruit drinks weren't the only foods that swelled: the energy content of cheeseburgers increased by almost one-quarter, pizza slices jumped by 35 per cent, and portions of Mexican food by even more than that.

We know that kids got their junk food in bigger portions in the 1980s and 1990s, and that they got fatter overall. But what's the rationale for singling out the jumbo soft drinks as opposed to the double-wide pizza and 10-inch burritos? That's where things get interesting ? provided you're interested in contentious science, slippery datasets, and the connivance of shady technical consultants.

The case against soda

The case against soda starts with the idea that the body's response to beverages differs from its response to solid food. People regulate their caloric intake without thinking. If we eat more at one meal, we'll go lighter on the next. But for unknown reasons, drinks throw this compensatory mechanism out of whack. A sugar-sweetened beverage might give you as much energy as a candy bar or a heap of mashed potatoes, but you'll be less inclined to account for it with tempered eating down the line.

Beverages may be less filling, but that doesn't mean they're not filling at all. Soda-tax advocates like to talk about the "added calories" from sugar-sweetened drinks, as if drinking a 16-ounce cup of Coke would have no effect whatsoever on your appetite.

In 2009, the New York City health department released a PSA that warned consumers, "Drinking one can of soda a day can make you 10 pounds fatter a year" ? a calculation that only works if people added an additional soft drink without adjusting their diets in any other way. A New York Times investigation later discovered that city officials knew they were fudging the facts on soda to make a point. Still, the evidence of reduced compensation for high-calorie drinks makes them look like a special case, and a good target for public-health research.

Quite a few epidemiological studies have been carried out to see if consumption patterns correlate with changes in body mass index. A 2004 paper in JAMA, for example, checked the dietary habits and BMI of more than 50,000 middle-aged women over a decade. After adjusting for smoking status, cereal fibre intake, levels of physical activity and several other factors, the researchers found that women who started drinking more sugary beverages midway through the study gained more weight than women who did the opposite. (Strangely, those whose consumption remained steady ? high or low ? tended to do just fine.)

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