More than a year after its ill-fated introduction, New York City mayor Michael Bloomberg's plan to limit soda sizes just won't go away. If anything, it's putting on weight. Both San Francisco and Mexico are considering their own soda restrictions — and though their plans might run into the same opposition that Bloomberg's did, it won't be coming from public health experts.

"In a way, we are what we drink in terms of obesity," said nutritionist Barry Popkin of the University of North Carolina. Soda and other high-sugar beverages aren't the only problem, but they're "the first and easiest target."

Bloomberg's proposal, which was approved by New York City's Board of Health in September of last year, would have banned the sale of sugary drinks in cups larger than 16 ounces by the city's fast food restaurants, movie theaters and street vendors. The plan became a national laughingstock, derided by both liberals and conservatives — critics included Jon Stewart and John Boehner — as technocratic, nanny-state overreach. A New York state court ultimately overturned the restriction, arguing that Bloomberg had exceeded his legal reach.

Bloomberg appealed the decision, and New York state's highest court agreed this month to hear the case. Even if they don't bring the ban back, though, poll-leading mayoral candidate Bill de Blasio has pledged to make it happen. The city of San Francisco is also considering a soda tax, and Mexico, which recently edged out the United States as the world's fattest country, is considering its own Bloomberg-inspired soda restriction.

Another round of public discussion over beverage regulation seems imminent. The first time around, though, most of the talk was political, emphasizing debates over the role of government and importance of consumer freedom. Rather less attention was paid to whether restrictions might actually make people healthier. "This is a very important public health area," said Frank Hu, an obesity epidemiologist at the Harvard School of Public health, but public discussion "has kind of been dominated by political considerations."

As for the scientific discussion, it's been dominated by the basic fact that Americans drink massively more sugar-sweetened beverages — the term of art for non-diet sodas, fruit juices, Frappucinos and Slurpees and so on — than just a few decades ago. Consumption has more than doubled since the late 1970s; the average child or adolescent gets 270 calories per day, or between 10 and 15 percent of all their calories, from sugary drinks.

Some scientific questions do remain as to whether sugar is uniquely problematic, triggering physiologic changes specific to sugar metabolism, or simply represent excess calories. Even if sugar isn't toxic, though, raw caloric volume is problem enough. Sugar drinks represent more than half of the approximately 500 extra daily calories Americans now consume in comparison to late-1970s baselines.

Until a few years ago, said Hu, it was theoretically possible to argue that the increased sugar consumption played a minor role in obesity and related disorders, or was even irrelevant. Correlation isn't causation; obese people with diabetes tended to drink more sugar drinks than healthy people, but maybe they just had a taste for it. But the bulk of evidence, including two gold-standard randomized clinical trials published in 2012 in the New England Journal of Medicine, in which cutting soda intake led to long-term body mass reductions, underscores the role of the drinks.

"People who argue against the scientific evidence still say that sugar-sweetened beverages are not the main problem, that other dietary factors are also important, that sedentary lifestyles are responsible. And those are all true: obesity is caused by many factors, not just one," Hu said. "But that doesn't negate the fact that sugar-sweetened beverages are one of the most important factors that increases caloric intake in our country — and it's relatively easy to cut back on them."

As for what effect restrictions would have in a country where 25 percent of adults are obese, 8 percent have diabetes, as many as 34 percent have some metabolic syndrome, and total obesity-related costs topping $150 billion per year, it's impossible to predict with absolute certainty, but rough numbers do exist.

A recent study in Health Affairs estimated that a nationwide one-percent tax on sugary drinks would over the next ten years prevent up to 240,00 cases of diabetes, 95,00 cases of heart disease, 8,000 strokes and 26,000 premature deaths. Some $17 billion could be saved in medical costs. For New York City, other researchers calculated that Bloomberg's ban would, on any given day, affect more than 7 percent of all New Yorkers, the number that presently drink more than 16 ounces of soda daily from businesses covered by the restrictions. They would consume nearly 60 fewer calories per day.

An useful rule of thumb, said epidemiologist Claire Wang of Columbia University, a co-author of both studies, is that each long-term daily reduction of 10 calories translates to to a one-pound weight loss. "For 100 calories, you'll probably be looking at a 10-pound weight difference," Wang said. "It's more complicated in kids, but that rule works well for adults." When McDonald's first started selling soda, noted Wang, the regular size was 7 ounces, less than half the size of the restaurant's current small soda.

Wang's models do rest on two critical assumptions: For the tax, that increased prices lead to reduced consumption; and for Bloomberg's ban, that people won't simply buy more than one 16-ounce beverage, or make up for lost soda in the comfort of their homes. Popkin said the first is true, with soda consumption being very sensitive to price changes, and research by psychologists does suggest that smaller portion sizes really does lead to reduced consumption.

Whether the potential health benefits of Bloomberg's plan for New York City, or the restrictions in San Francisco and Mexico, outweigh the political objections, remains to be seen. Yet even if many people have objected to the restrictions, said Hu, it's worth remembering that tobacco regulations were once criticized as violating cherished freedoms.

"Fifteen or twenty years ago, nobody would have imagined not being able to smoke in a bar, or in public places," Hu said. "Obesity is much more complicated than smoking, but it serves as a good example of how public policies can make a huge difference in changing social norms and improving public health."

Image: Daniel Oines/Flickr