Why tax soda? Obesity is expensive and soda is cheap.

To be sure, a lot of factors contribute to obesity, including enormous restaurant meals and the couch-potato syndrome. But sugary soft drinks are the only beverage or food that has been shown to increase the risk of obesity. And obesity, in turn, promotes heart disease, diabetes, cancer and other expensive-to-treat diseases. All told, Americans spend about $100 billion a year in direct medical costs related to obesity, of which half is paid with Medicare and Medicaid taxpayer dollars.

In other words, Americans are already taxed by soda. Out of every paycheck, and on every April 15, we're each paying to treat diseases — our own or others' — promoted by the overconsumption of these uniquely worthless beverages.

What was once an occasional treat is now the default drink for many, particularly young people. In 2005, teenage boys who drank soft drinks consumed an average of three 12-ounce cans and girls an average of two 12-ounce cans per day. One in 10 boys who drank soft drinks consumed five-and-a-half 12-ounce cans a day, or about 800 calories worth. It's not the only reason, but the increase in soda consumption since the 1970s certainly helps explain why obesity rates have tripled in teens over the past 30 years.

We all need food, but no one needs soda. Soda is basically liquid candy, providing empty excess calories without needed nutrients. (The same is true for "fruit drinks" with little juice, "sports drinks" like Gatorade or gimmicky concoctions like VitaminWater.) Besides promoting obesity and disease, soft drinks displace real foods with redeeming health-promoting properties.

In fact, in the 1970s, teens drank about twice as much milk as soda. By the 1990s, though, teens drank twice as much soda as milk. That trend has been fuelled by pervasive advertising, ubiquitous availability and the ballooning sizes of containers, as well as economics: soda keeps getting cheaper compared to real food.

To some critics, soda taxes are a radical new idea. "We can't live anymore," bemoaned Fox News' Sean Hannity. I don't know if he lives in New York or whether he drinks soda, but if he does, he already pays soda taxes, perhaps without knowing. New York is one of two dozen states that levy small taxes — usually just special sales taxes — on soft drinks or snack foods. Arkansas, California, West Virginia and the city of Chicago are others that do. Those taxes are designed solely to raise money and the revenues aren't earmarked for health programs, but they have been around for decades in some cases.

Congress should do it differently. A federal excise tax on non-diet soft drinks of three cents per 12-ounce can would raise $5 billion per year, and might reduce consumption by a couple of percent. A much heftier tax of one cent per ounce — as suggested by Dr. Tom Frieden, named to head the Centers for Disease Control and Prevention — would raise about $16 billion a year. Such a tax would reduce soda consumption by more than 10 percent and help reverse the obesity epidemic.

Others will claim a soda tax is regressive. That's true as far it goes, but don't look for this argument from anti-poverty advocates.

"Low- and moderate-income people would benefit disproportionately from the universal health care that this tax would help fund," says the Center for Budget and Policy Priorities. That group notes that "to the extent that lower-income consumers responded to the tax by buying fewer high-calorie soft drinks, their health would benefit."

Members of Congress crafting health reform legislation have to raise $1 trillion over the next 10 years. A tax on soft drinks might get a tenth of the way there.

But unlike payroll taxes, income taxes or taxes on employer-sponsored health benefits, a soda tax itself would promote health and reduce health-care costs. True conservatives should join liberals in seeing the value in that, even if Hannity doesn't.

Michael F. Jacobson, Ph.D., is the executive director of the Center for Science in the Public Interest. This article was written for The Progressive Media Project.