The so-called soda tax is back on the table. This time proponents say that a levy of a penny per ounce of sugar-sweetened beverage would not only raise $13 billion a year but also save $17 billion in medical costs by reducing the incidence of heart disease and diabetes.

These figures, published recently in the journal Health Affairs, are based on facts and several assumptions.

First, the facts: Americans drank 13.8 billion gallons of soda, punch, sports drinks, sweet tea and other high-calorie, nutrient-free beverages in 2009, according to industry data. That works out to about 70,000 calories a person. The sugar in all this "liquid candy," as it is often called, is considered to be a major contributing factor to the obesity crisis, which in turn has fueled the rise of Type 2 diabetes and other diseases.

Now for the assumptions: Based on previous studies by economists, the authors of the Health Affairs study assumed that a penny-per-ounce tax would reduce soda consumption by 15 percent. They also assumed that 40 percent of the calories saved by drinking less soda would be replaced by drinking more milk and juice.

Putting it all together, the study authors -- from Columbia University, the University of California-San Francisco and Virginia Tech -- calculated that their tax would result in the average U.S. adult, age 25 to 64, consuming 9 fewer calories per day. Over time, that would result in enough weight loss to reduce the number of obese adults by 867,000 in 10 years.

The researchers also used models to predict that the reduced consumption of sugar-sweetened beverages would prevent 2.6 percent of new cases of diabetes. Over a 10-year period, those reductions in obesity and diabetes would translate into "95,000 fewer instances of coronary heart disease, 8,000 fewer strokes, 26,000 fewer premature deaths, and more than $17 billion in savings from medical expenditures averted across the U.S. population," according to the study.

These numbers could be on the low side, according to Roger Feldman, a professor at the University of Minnesota School of Public Health.

"They [researchers] took a conservative approach. The response could even be greater," Feldman said.

It's easy to see why doctors, public health experts and policymakers are so taken by the idea of a soda tax. But political support for such a measure is weak, at best. Libertarians, soda addicts and anti-tax crusaders recoil at the idea that they can't decide for themselves what to drink.

Arguably, however, they can't. Sure, a soda now and then might be a legitimate treat, especially if you're the type of person who otherwise follows a sensible diet and gets regular exercise. However, the numbers clearly show that Americans as a whole aren't just treating themselves now and then.

Brendan Halleron, a U sophomore who drinks soda occasionally, said people should choose to drink the beverage in moderation.

"Everyone tries to be healthy and everything, but you want to have a soda every once in a while," Halleron said. "There's nothing wrong with that."

More to the point, there are costs associated with these decisions that aren't reflected in the price paid for a soft drink. The penny-per-ounce tax would go a long way toward fixing that, the authors of the Health Affairs study say.

What's still missing is hard evidence that raising the price of sugar-sweetened beverages would prompt people to make healthier choices. The Minnesota Department of Health has not developed or advocated a soda-tax proposal, department spokesman John Stieger said.

Studies have found that higher taxes on soft drinks (usually in the form of sales taxes, not sin taxes) do reduce consumption, but not by enough to make substantial reductions in body-mass index. Perhaps the taxes in place now are simply too low to make a difference. (Tobacco taxes, credited with reducing smoking rates, are much higher.)

Perhaps someday a pioneering state will implement a soda tax that's big enough to put all these ideas to a real-world test.

Peter Funk, a University of Minnesota journalism student on assignment for the Star Tribune, contributed to this report.