THE WORD “prevention” has a nice ring in any health-care discussion. Thus, many politicians argue that programs to stop smoking, improve diets and otherwise promote wholesome living save money in the long run. A healthier population at less cost. Sounds like a win-win situation.

Unfortunately, that formulation is a pleasant fantasy.

Medical economists agree that cancer screenings and gym classes can lead to physical well-being and longer lives. But in the interests of honest accounting, they add that prevention does not reduce overall health-care spending. On the contrary.

Let’s put it bluntly: Longer lives cost money. Those who make it to 90 thanks to exercise and six daily servings of vegetables are more likely to suffer the expensive ravages of old age. Everyone dies of something. So he who avoids a fatal heart attack at 70 is more at risk of cancer at 80. Those extra 10 years can mean extra CT scans, hip replacements and physical therapy, even for those in relative good health.

Despite this reality, the cost-saving virtues of wellness programs echo along the presidential campaign trail. Democrats Hillary Clinton and John Edwards talked them up, as did Republican Mike Huckabee. The presumptive major-party nominees, Democrat Barack Obama and Republican John McCain, both include “prevention” in their health-care proposals.

“Prevention is absolutely the right thing to do,” Arthur Garson Jr., the provost at the University of Virginia Medical School, told me. “The issue is, do not expect to get to the end of the day and have enough money from prevention savings to pay for the uninsured.”

Author of the book Health Care Half Truths, Garson cites a recently published study on the high cost of healthy people. Dutch researchers found that medical spending on smokers, who died at an average age of 77, was $100,000 less than on non-smokers who were thin and died at 84.

Companies that include wellness programs in their employee benefits may indeed see a savings in lower medical costs and absenteeism. But what they have done, really, is delay the onset of serious illness during the employees’ working years and pass it onto Medicare, when the taxpayers take over.

A study reported in The New England Journal of Medicine’s February issue finds that only 19 percent of preventive interventions save money. Their value depends on the particular intervention and the sort of people who get it. “For example, drugs used to treat high cholesterol yield much greater value for the money if the targeted population is at high risk for coronary heart disease,” write the authors, Joshua T. Cohen, Peter J. Neumann and Milton C. Weinstein.

Rapidly rising prices for health-care also add to the expense of moving big-ticket medical procedures into later years, explains Garson, a cardiologist. “In today’s world, where the rate of medical-care inflation is twice the rate of regular inflation, anything done 10 years from now is, in real dollars, 25 percent more expensive.”

There are two ways to deal with that problem, according to Garson. Get medical costs down, and “keep people as healthy as possible as long as possible so that they don’t spend as much money being sick.”

The best kind of aging, he says, is to “have early old age last as long as possible and late old age last 15 minutes.”

Yes, preventive care is a good thing because a long and healthy life is a good thing. We shouldn’t lose sight of that big picture. But let’s not deceive ourselves into believing that wellness programs can reduce medical spending over the long haul. That may come off as an attractive selling point for a health-care plan, but it’s not so.

Froma Harrop is a member of The Journal’s editorial board and a syndicated columnist.