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.
At health clubs, pear-shaped people in their 40's and 50's obsessively lift weights, trying for those defined muscles that, even in youth, come only to those with a certain genetic predisposition. But by middle age, the overweight tend to stay that way, and the body has a harder time increasing muscle mass. So even the greatest personal trainer will not produce rippling abs.
At the cardiologist's office, middle-age men, learning that their arteries are starting to clog, swear they'll never eat chips and hamburgers again, and that they will take up jogging. Some do and a small percentage even stick with it. But no amount of exercise or diet change will make the plaque in their arteries disappear. Despite common public health recommendations, walking for half an hour a day, five days a week probably won't make most people lose weight. And while a regular regimen of walking or running will likely improve your stamina and cardiovascular fitness, there is no guarantee that it will reverse heart disease, prevent or forestall a heart attack or in any way extend your life.
The effects of other measures, like changing lifestyle or switching to a diet rich in raw vegetables, are even less clear when it comes to preventing cancer, said Dr. Kramer. "Even if they do affect the cancer," he said, " it may be that it's over an entire lifetime."
So what are Americans, with their faith in starting over, to do? When it comes to making oneself over, said Dr. Glassner, they have two options. One, he said, "is that you can consider yourself inadequate or inferior" for failing to force the years to melt away. The other is to shift the definition of rejuvenation from a arduous restructuring of the self to a paint job.
"Now, instead of losing the weight you're going to go for cosmetic surgery," Dr. Glassner said.
That is not really an answer. Collagen injections or surgery may give people more youthful looking faces, but only for a while. And liposuction won't help for long if the body restores the fat that was suctioned off the patient's arms or buttocks.
Then there is a third possibility for the resourceful, Dr. Glassner said. An overweight person can simply redefine himself as a "food adventurer."
There is one group, however, for whom a strong sense of control over the future may be an unalloyed good: the sick.
"Protective illusions," says Dr. Shelley Taylor, a psychology professor at the University of California at Los Angeles, can be a good thing. In her research, she found that among people with serious diseases, those who felt they still had control over their lives coped better with their illnesses.
The optimists fared better psychologically even when they became more ill - shattering the illusion of control. "What you often see is people use something like cancer as an opportunity to discover value in their lives, and meaning," Dr. Taylor said. "They reorder their priorities. They focus on relationships more. Control and optimism shift to things that can be dealt with."
For those in good health, there is still another option, though it is decidedly a minority position. This is simply to scale back on one's self-engineering and take more pleasure in simply getting from day to day.
The American essayist Joseph Epstein nicely expressed this view in an essay, written when he hit the age of 60, which he gave the mordant title, "Will You Still Feed Me?" In it, Mr. Epstein expresses the virtue of just enjoying the ride.
"At 60," he writes, "one probably does well not to expect wild changes, at least not for the better. Probably best not even to expect a lot in the way of self-improvement. Not a good idea, I think, at this point to attempt to build the body beautiful. Be happy-immensely happy, in fact-with the body still functional."
Of course, many in midlife will still decide to hit the gym, to eat better, drink less, relax more. And that's a good thing, if only because they will feel better for being fitter. But they shouldn't expect it to erase the effects of all those years that came before.