A soda tax? How about a potato tax?

The time has come, America, for a tater tax. Now that a groundbreaking study published in the New England Journal of Medicine has demonstrated that potatoes may be a bigger culprit in weight gain than sugary soft drinks or red meat, it seems appropriate to exact a little spud money. You want chips with that? Ante up.

No, we’re not being serious. But politicians and health advocates nationwide are very serious about imposing taxes on the culinary villain du jour, soda pop, which is thought to be a key cause of the country’s obesity epidemic. Make people pay a few extra pennies for their Pepsis, the theory goes, and they’ll drink less, lose weight and save the health system money. It sounds good, but the new research out of Harvard Medical School and the Harvard School of Public Health shows why singling out a particular food may not be such a good idea.

Researchers tracked the eating habits of 120,000 health professionals from across the country over at least 12 years and found that the most fat-inducing food wasn’t soft drinks but potatoes — especially French fries. People who downed an extra portion of fries daily were super-sized by 3.4 pounds on average after four years, according to the study. Next on the fat list was potato chips, which added on average 1.7 pounds. Even in non-chip form, an extra helping of potatoes added 1.3 pounds, making them worse for one’s waistline than sugar-sweetened soft drinks, which added 1 pound over four years.

The science on food is evolving, with researchers constantly turning up new dietary heroes and villains and sometimes reversing themselves. Potatoes have been a staple food for centuries and are a rich source of vitamins and minerals, but the Harvard study will doubtless send their reputation down the tuber. That’s what happened to coconuts. In 1994, a study from the Center for the Public Interest found that movie-theater popcorn popped in coconut oil was a coronary in a cardboard cup, hurting sales at theaters and prompting many to switch to other oils. Today, health-food stores have whole shelves devoted to non-hydrogenated coconut oil, which is now considered relatively beneficial.

Soda taxes were part of the federal healthcare reform bill before being stripped out, and cities and states nationwide are considering them. We’ll grant that soda pop is nearly devoid of nutritional value and that phasing it out of our diets would do more good than harm, but a tax wouldn’t stop people from switching to other drinks, such as fruit juices, that are just as fattening. Tax French fries, and people would just switch to onion rings. Moreover, the next study might find that an order of fries and a Coke are better for you than previously believed.