In the interest of addressing a growing obesity problem, the French government is considering a tax —something between 5 and 19 percent — on all foods the government deems “too rich, too sweet, too salty, and which are not strictly necessary.”

    
There, one in three people is overweight or obese; here in the United States, we are close to doubling that rate. And now U.S. Senate leaders are considering putting a federal tax on soda and other sugary, high-calorie drinks. Estimates say that a 3-cent tax (per 12-ounce soda) could generate $24 billion in four years.

If people react by drinking less soda, obviously they’ll reduce the revenue potential. But that’s also, paradoxically, part of the intention. (Of course, they might also choose to substitute less expensive sugary items in their diets, there-by reducing revenue and dietary benefit.) Taxes generally are meant to raise revenue, but some also aim to discourage certain be-haviors (e.g., taxes on alcohol and tobacco). The more they succeed in reducing the behavior, the less revenue they generate.

    
And they can change behavior, studies show. One from the University of Florida demonstrated that when the price of alcohol rises, people drink less. And the considerable reduction in smoking rates —from 42 percent in 1964 to somewhere below 20 percent today — is often attributed to the adoption of high tobacco taxes.

    
But even if soda was slapped with a tax rate of 58 percent (comparable to what cigarettes bear), authors of a study from Emory University estimate that our mean Body Mass Index would fall by just 0.16 points. To put that in context, consider that in the last 20 years or so our mean BMI rose 2.3 points.

    
About a dozen states already have some type of tax on sugary drinks. But late last year, when New York Gov. Paterson proposed an 18 percent sales tax on non-diet soda and sugary juice drinks, the idea met strong opposition from consumers and from the beverage lobby, whose spokespeople were quick to point out that there is no proven causal relationship between soda consumption and obesity. Demonstrating disease causation is difficult, of course; it took decades for scientists to close the evidentiary gap between smoking and lung disease, and obesity’s causes may arguably be even more complicated.

    
Still, several other states are floating the notion of a soda tax while the Senate does the same. And there have been some recent victories in the general category of government-instituted dietary regulations. New York City recently required chain restaurants to display nutritional and caloric information on their menus. Restaurant owners argued that it was a violation of their First Amendment rights, but the courts disagreed.

    
Other states have had less success trying to rein in their residents’ expanding waistlines through regulation. In Maine, the governor signed a law to institute a tax on soda and the corn syrup that goes into soda; voters overturned it. In Mississippi — a state ranked as the nation’s fattest for several years running — a bill prohibiting restaurants from serving obese customers died almost immediately after being introduced.

    
Whether or not government can — or should be trying to — tinker with our BMI can be debated. Meanwhile, between lost productivity and medical expenses, obesity costs U.S. businesses about $45 billion annually — and nobody knows what to do about it.