Last week there was a lot of fanfare over a new study showing that overweight people have less risk of dying than people of normal weight. Lovers of butter cookies and Popeyes the world over surely rejoiced at the news. But at the annual meeting of the American Economic Association over the weekend in San Diego, health economists I spoke with expressed dismay over the study’s fiscal implications.

“The study looked at quantity of life, not quality of life, and that’s a very important distinction,” noted Dana Goldman, director of the University of Southern California’s Leonard D. Schaeffer Center for Health Policy and Economics.

A higher body mass index — a standard measure for determining whether people are overweight or obese — is associated with a number of chronic illnesses, including diabetes, heart disease and hypertension. These chronic illnesses are expensive to treat. If, as it now turns out, overweight people are living longer in addition to racking up more chronic illnesses, that means Medicare is on the hook for paying for more expensive people for more years. (I should note that Americans in the two highest obesity categories — B.M.I. of 35 and up — had a higher mortality rate than everyone else. Research suggests, though, that the obese still cost the government more over the course of their lifetimes because their annual expenses are so high.)

In other words, your spare tire might help you live longer, but it is also accelerating the country’s debt problems.

Professor Goldman said the study underscored the fiscal imperative to get more Americans to a normal weight, whether through financial nudges to eat better, like a soda tax, or some sort of biomedical intervention, like an F.D.A.-approved pill that would reduce obesity. Right now, he said, the incentive system for medical practitioners does not encourage treatment of obesity as a disease itself — only the complications that later result from obesity.

He gave as an example a doctor he knows who has an obese patient taking expensive drugs for various health problems related to being overweight. The doctor told Professor Goldman: “The best thing I could do for this patient would be to take him for a walk. But I don’t get paid for that.”