You could call Tommy Thompson a model of personal responsibility. After he took over as secretary of health and human services and started grappling with the national weight problem, he put himself on a diet and lost 15 pounds in nine months. He loves to hand out pedometers to get others walking and losing weight.

But, as any failed dieter will tell you, virtue is not enough. The statistics reveal our guilty secret: About 30 percent of American adults were considered obese in 2000, according to the Centers for Disease Control and Prevention, and 64 percent were considered overweight. Americans have been steadily gaining weight for years, and there is no sign of this trend reversing.

With all this excess weight causing disease and costing money, perhaps it should have been no surprise that even during a Republican administration -- where personal responsibility is a central policy theme -- HHS would classify obesity as a disease, removing it from the realm of simple character weakness.

What seems little more than a change in nomenclature, and restricted to purposes of Medicare coverage, could reverberate far beyond that. What HHS chooses to call obesity, and why it has to make that choice, goes to the heart of the federal Medicare program's very identity: as currently structured, Medicare is a disease care program rather than a health care program.

The decision by HSS says simply that obesity will no longer be automatically excluded from the list of conditions whose treatments are eligible for reimbursement by Medicare, the largest insurance plan in the world.

And even though the change raises the specter of hucksters everywhere trying to get the latest LOSE WEIGHT NOW fantasy covered by Medicare, it is a welcome development.

Welcome not because it will suddenly reverse the course of a health challenge of epidemic proportions -- it won't -- but because it draws attention both to the importance of the problem and to the hoops through which HHS must jump to get preventive measures covered.

In 1965, when Congress passed the Medicare legislation, it provided for coverage of services deemed reasonable and necessary only "for the diagnosis or treatment of illness or injury." Longstanding interpretation of that statute in the governing regulations for Medicare has ruled out payment for preventive services without case-by-case Congressional approval, which has had a detrimental effect on the nation's health by delaying or preventing coverage of effective screening tests and counseling measures.

It took 15 years of annual legislative proposals just for Congress to approve, in 1989, Medicare coverage for Pap smears for cervical cancer. Coverage for colorectal cancer screenings began in 1998, five years after the definitive study.

After 40 years, the essence of the Medicare legislation remains unchanged, despite dramatic advances in our understanding of what causes diseases, and how to prevent them. Many of those insights relate to behavior -- fully 40 percent of premature deaths among Americans are attributable to factors that would be affected by a change in the patient's behavior.

Still, the preventive measures that are now covered are chiefly ones that can qualify as treatment, such as using costly drugs, at the expense of measures promoting behavioral changes that could potentially have enormous effect. For example, smoking-cessation counseling, proven effective in dozens of studies, including the government's own Surgeon General reports, remains uncovered for Medicare patients. (While most state Medicaid programs cover tobacco cessation, Medicare does not.)

The HHS decision to change its obesity policy involves two elements: treating obesity like other established physiological risk factors for chronic disease, such as high blood pressure or elevated cholesterol levels; and deferring, until further analysis, the specific treatments that might be covered. That is, the announcement was more a signal flare than a starting gun.

While the decision provoked a flurry of news reports -- and commentary both indignant and endorsing -- whether you call obesity a disease or not is largely a question of semantics. There is no question that obesity is a risk factor for several diseases, similar to high blood pressure and high blood cholesterol. Obesity is associated with diabetes, heart disease, stroke, osteoarthritis and certain cancers. It can be easily measured, and the heavier you are, the greater your risk for developing these other conditions. Reducing weight clearly reduces risk, most strikingly perhaps with respect to diabetes.

When it comes to organ system damage, can a "cause" be a "disease," or must the label of disease be reserved only for the effect? Ultimately, whether obesity is a disease or not is the wrong question; medicine's chief focus ought to be on the cause of disease, rather than its end results. The policy irony is, of course, that, while it makes more sense to pay for services that prevent disease than to pay for the costs of treating preventable illness, it literally takes an act of Congress to cover prevention.

A more rational and straightforward approach -- not to mention more economical -- would be to unfetter HHS from this arbitrary constraint and allow it the flexibility to cover preventive services that have been proven effective and efficient in improving health, and to relieve Congress from the inappropriate burden of playing doctor and having to make those case-by-case clinical determinations.

There is an existing vehicle for determining which services are effective. Medicare already has an outside advisory panel -- the Medicare Coverage Advisory Committee -- that could make recommendations on preventive care. The MCAC, in turn, could obtain evidence of effectiveness from the U.S. Preventive Services Task Force, a non-federal expert panel that has for two decades helped change medicine by reviewing and classifying studies on what preventive measures actually work.

This type of review has created a stronger base of evidence for effective preventive services than exists in most other areas of medicine. If Medicare were able to use this method instead of turning constantly to Congress, it would dramatically streamline the process and yield a sound policy of coverage for disease prevention.

Why is the recent policy change on obesity a hopeful sign for broader progress? Certainly not because of the prospect of coverage for stomach stapling or drug treatment for older obese people. Obesity takes its toll over decades, and there is scant data to support the effectiveness, not to mention efficiency, of various treatments for this population.

Rather, the important development here lies in other dimensions: 1) the triggering of studies and analyses by HHS that are likely to conclude that obesity and its related diseases cannot be effectively addressed in health care without sustained attention to individual behavior; 2) the realization that effective medical attention to behavior change has to be linked to supportive community-based health promotion efforts; and 3) serious attention to resolving the archaic, cumbersome and literally life-threatening contortions inherent in the current case-by-case legislative decision-making process for preventive services.

All three of these are important if Medicare is to emerge, in the vision of Mark McClellan, administrator for the Centers for Medicare and Medicaid Services, as a public health program drawing more effectively on all we know to ensure that taxpayers' dollars are leading to improved national health.

The obesity debate helps stimulate the discussion, but, just as the consequences of poor diets and sedentary lifestyles are not limited to the overweight and obese, the lessons for policy change go beyond that condition.

Although obesity treatments are not likely to be covered any time soon under Medicare, the discussions can focus attention on important preventive measures, policy refinements and program perspectives with the potential to improve the health of the Medicare population now and in the years to come.

Obesity itself is a pressing health challenge, of rapidly growing prevalence, that deserves stronger attention in medical care. Equally importantly, the attention garnered by this decision casts a spotlight on the shortcomings of the current approach to health care financing. We can now see a little more clearly why we need to move the system from a focus only on disease treatment to one focused on health improvement.

And if politicians worry about straying into the realm of behavior, perhaps they should take heart in this: A Time/ABC poll in June found that 87 percent of Americans blame themselves -- their choice of diet and lack of exercise -- for their weight problems. But 53 percent say the federal government is doing too little about it.

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J. Michael McGinnis, an assistant surgeon general through four administrations, from 1977 to 1995, is a senior official at the Robert Wood Johnson Foundation.