THE promises are everywhere. Sure, you smoked. But you can erase all those years of abusing your lungs if you just throw away the cigarettes. Eating a lot of junk food? Change your diet, lose even 5 or 10 pounds and rid yourself of those extra risks of heart disease and diabetes. Stay out of the sun - who cares if you spent your youth in a state of bronzed bliss? If you protect yourself now, skin cancer will never get you.

Maybe it should be no surprise that America's popular and commercial cultures promote the idea of an inexhaustible capacity for self-rejuvenation and self-repair. After all, if America as an idea has meant anything, it has meant just that - the possibility of continual transformation - becoming wealthier, more spiritual, more beautiful, happier and feeling younger.

That optimism has helped create a society of unmatched vitality - a source of bewilderment, alarm and envy to the rest of the world. But Americans often forget, or aren't aware, of how unusual they are in this respect, notes Dr. Daniel Haber, director of the cancer center at Massachusetts General Hospital.

"I grew up in Europe and I travel in Europe," he said. "And there's an amazing contrast." Europeans are far more fatalistic about their lives, he said. They believe "you need to enjoy life," so they smoke, they bask in a sun, they take pleasure in a leisurely, indulgent meal and they don't feel compelled to go to a gym.

Americans, Dr. Haber says, believe in control - of their bodies, their mental faculties and their futures. So shedding some pounds or some unhealthy habits is not merely sensible. It suggests a new beginning, being born again.

Maybe that is why people may feel betrayed when Peter Jennings explains that he stopped smoking, at least for a while, and still got lung cancer. Or why, two decades after his death, people still talk about Jim Fixx, the running guru who lost weight, stopped smoking, ran every day and dropped dead of a heart attack.

In fact, science is pretty clear on all of this: There are real limits to what can be done to reverse the damage caused by a lifetime of unhealthy living. Other than lung cancer, which is mostly a disease of smokers, there are few diseases that are preventable by changing behavior in midlife.

But that is not what most people think, said Dr. Barnett Kramer, the associate director for disease prevention at the National Institutes of Health. Instead, they believe that if you reform you'll erase the damage, in part because public health messages often give that impression. "It is easy to overestimate based on the strength of the messages," Dr. Kramer said. "But we're not as confident as the messages state."

Eating five servings of vegetables and fruit has not been shown to prevent cancer. Melanoma, the deadly skin cancer, occurs whether or not you go out in the sun. Gobbling calcium pills has not been found to prevent osteoporosis. Switching to a low-fat diet in adulthood does not prevent breast cancer.

At most, Dr. Kramer said, the effect of changing one's diet or lifestyle might amount to "a matter of changing probabilities," slightly improving the odds. But health science is so at odds with the American ethos of self-renewal that it has a hard time being heard. Here, where people believe anything is possible if you really want it, even aging is viewed as a choice.

"It's hard to find an American who doesn't believe that, with enough will, he or she can achieve anything - we've been brought up to believe that," said Dr. Barry Glassner, a sociology professor at the University of Southern California. Health, he emphasized, is no exception: "It's the same whether you're 40, 50 or 80. It doesn't matter whether you are male or female, black or white. "

But in matters of health, the strongest willed person simply cannot wipe the slate of life clean and begin again. This is true even with lung cancer and smoking. Those who quit may greatly reduce their risk of lung cancer. But they cannot eliminate it.

"The best you can be is a former smoker - you can't be a 'never smoker,' " said Dr. Kramer. "It's not all or none. It's a matter of changing probabilities."

In fact, in every area of desired physical self-renewal, the probabilities make it hard to argue that life allows one to start over.