DEAR DOCTOR K:

I've heard it may one day be possible to slow or stop the aging process. Is there any truth to this?

DEAR READER:

In the past, most experts believed that aging was inevitable. Recent research indicates that may not be true.

I remember hearing in medical school the theory that the maximum life span of any species is roughly six times the stretch between birth and maturity. Using this formula, the maximum life span for humans would be 120 years. Indeed, very few human beings have lived longer than 120 years.

On the other hand, life expectancy in developed countries has risen continually for 165 years. At the turn of the 20th century in the United States, just 112 years ago, life expectancy was about 50 years. Today, it is approaching 80 years -- a 60 percent increase. Nothing like that has ever happened in human history. How far can it go? We don't know.

One reason that some people are changing their minds about aging being inevitable is the discovery that some animals don't seem to age. Many cold-water ocean fish, some amphibians and the American lobster continue to grow bigger, and are able to reproduce and live until something -- an accident, a predator or a disease -- kills them.

Clearly, though, this is not the case for humans. Why, then, are some scientists wondering if we might be able to overcome the biological cap on aging? Each of us is composed of cells, about 13 trillion of them. When we age, it's because our cells age. Very recently, we have begun to understand the forces that determine how rapidly cells age -- and to control those forces.

Research has shown that restricting calories slows aging. Animals forced to live on 30 percent to 40 percent fewer calories than they would normally consume live 30 percent to 50 percent longer. They also become resistant to age-related diseases such as cancer, heart disease and Type 2 diabetes.

We don't know whether this would be true for humans. And it might seem irrelevant: Few people would willingly curtail their diet that much. However, scientists have identified some of the natural substances in animals that seem to link calorie restriction to delayed aging. They are testing treatments based on that knowledge.

Genes have been discovered that profoundly affect aging in animals. For example, turning off a gene called "daf-2" causes worms to live up to five times longer. Mutations in another gene called "Indy" (inspired by the Monty Python line, "I'm not dead yet") can double the life span of certain flies. It's not just that these animals live longer as a result of these genetic changes: During the added time they remain vigorous, sexually active and fertile.

That last point is important. Few of us would wish to live longer if our added years were just prolonged decrepitude. But many of us might choose to live more vital, active years. Would people living longer and healthier be a good thing for a crowded world? That's a tougher question to answer.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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