Every time a new obesity study comes out, pundits latch onto it as proof that the government either should or should not take an interest in what Americans eat and how much they exercise.

 When researchers at the U.S. Centers for Disease Control and Prevention claimed 400,000 Americans (later revised to 365,000) die each year due to poor diet and physical inactivity, the champions of intervention were quick to proclaim an obesity "epidemic," demanding policies aimed at getting us off the sweets and on the StairMaster. When another team of government statisticians said the true death toll, once the apparent health benefits of being slightly overweight were taken into account, was more like 26,000, opponents of such meddling rejoiced.

 Now it's the interventionists' turn to claim vindication. A new study published in the journal Health Affairs says obese people, on average, cost health insurers some $1,200 a year more than people of "normal" weight (which, since most of us are "overweight," is actually abnormal). The researchers calculate that obesity accounts for more than a quarter of the increase in medical spending between 1987 and 2002 -- not only because more people are obese, but also because more treatments are available for obesity and related conditions, which are treated more aggressively than used to be the case.

 New York Times columnist Paul Krugman took the occasion to proclaim that "fat is a fiscal issue" and that anyone who is skeptical of mobilizing the government against it is "pro-obesity," "a blind ideologue," or a food industry flack. Although saying so will no doubt convince Krugman that I fall into the second category, I'd like to suggest that the appropriate government response to our expanding guts does not hinge on exactly how many of us are fat, how serious the health consequences are or how much treating them costs.

 The more fundamental question is whether the extra helping of meatloaf you had last night is the sort of thing the government should be worrying about. If the government's duty to protect "the public health" includes discouraging any behavior that might lead to disease or injury, it is self-evident that the government should try to stop us from overeating. But if that duty pertains only to external threats such as communicable diseases and contaminated water -- risks imposed on us by others, rather than risks we voluntarily accept -- there is no public health justification for a government-led war on fat.

 Krugman, who bemoans "an ideological landscape tilted in the direction of doing nothing," seems to recognize that most Americans are not quite prepared to accept the idea that every health issue is a public health issue. "One answer," he writes, "is to focus on the financial costs of obesity."

 Krugman does not distinguish between the voluntary risk pooling of private health coverage and the compelled subsidies of Medicare and Medicaid. Yet private insurers are free to charge fat people more, and their customers are free to shop around for a better deal if they don't like the terms of their coverage.

 By contrast, government-supported health care imposes the costs of fat-related illnesses on people who have not agreed to the arrangement and cannot opt out. But even here, the net financial impact is unclear. If overweight people tend to die earlier and therefore draw less on Medicare and Social Security in old age, fatness may save taxpayers as much as or more than it costs, as seems to be the case with smoking.

 More important, the implications of the argument based on taxpayer-funded health care are just as alarmingly open-ended as the implications of the "public health" argument. Perhaps sensing this, Krugman assures his readers "nobody is proposing that adult Americans be prevented from eating whatever they want" -- which, given the aims of anti-fat activists who support "junk food" taxes, simply isn't true. Nor is it reassuring that in a subsequent column, Krugman says overeating is one of those "situations in which 'free to choose' is all wrong," since "even adults have clear problems with self-control."

 The one encouraging sign is that Krugman describes himself as overweight even as he urges his fellow Americans, most of whom are also overweight, to support a government campaign against gluttony and sloth.

 People will be forced to consider how that effort might affect their own lives -- and whether they want to live in a country where their unhealthy habits are everyone else's business.