In other words, an overweight person with a BMI of 27 would end up with a BMI of 26.997 -- still well short of the 20-25 range considered healthy.
Even a soft-drink tax increase of 20 percentage points wouldn't help much, because soda accounts for only 7% of calories in the American diet.
Taxes on snack foods are likely to be even less effective.
A team from the U.S. Department of Agriculture contemplated a tax on salty foods such as cheese puffs and pretzels. But unless the tax topped 10%, it would translate into much less than 1 pound of weight loss a year, the researchers reported in 2004.
A 2007 paper titled "Cheap Donuts and Expensive Broccoli" used federal data to predict the effect of a 100% tax on all high-fat and low-nutrient foods. It concluded that even this unrealistically high tax would ultimately reduce BMIs by less than 0.2 points.
The experience of Maine's 5.5% snack tax is particularly discouraging. Enacted to help close a budget gap, it lasted 10 years before it was repealed under pressure from angry voters. Despite the price hike on such items as ice cream, hot cocoa, cake, cookies and pudding, Maine's adult obesity rate doubled from 10% to 20% while the tax was in effect.
The flip side of taxing junk food is subsidizing healthful foods. The Department of Agriculture has calculated that cutting prices of fruits and vegetables by 10% would increase consumption by 6% to 7%.
In the Changing Individuals' Purchase of Snacks -- or CHIPS -- study, Minnesota researchers cut prices in half for low-fat snacks in 55 high school and office vending machines. Sales nearly doubled, the researchers reported in 2001.
In another experiment, cafeteria sales of fruit and salad tripled when prices were cut in half. In a third, a half-off sale on fruit boosted purchases by a factor of four.
California public schools began removing soda from vending machines in 2004, but it's unclear whether students are any thinner -- especially because the machines still sell sports drinks that contain almost as much sugar, said Gail Woodward-Lopez, associate director of the UC Berkeley Center for Weight and Health.
Still, experts caution that government attempts to influence diet could have unexpected results -- especially when taxes are involved.
A 2007 British study used government data on household diets and food expenditures to predict the effect of extending the country's 17.5% sales tax to various foods. When items high in saturated fat were slapped with the tax, the model estimated that deaths would rise by 1,800 to 4,000 a year because consumers would be prompted to switch to foods with more salt.
To fix that problem, the scientists ran a model in which the tax applied to all kinds of unhealthful foods. This time, deaths fell by as many as 2,500 a year, but cholesterol levels rose as people switched from salty foods to fatty dairy items.
And unfortunately, consumers in both scenarios did the last thing any anti-obesity crusader would want: Facing higher grocery bills, they bought fewer fruits and vegetables.