If you have lived beyond the age of 100, countless people may ask you for your secret to long life. The truth is you probably haven't a clue, but you may theorize that it has been a result of your clean living, or even your occasional indulgences.

Researchers are asking the question in scientific studies of centenarians because they think you really do have a secret. And it may have to do with your genes. So if your answer was something like, "because my mother lived to a ripe old age," you may be on to something.

Do centenarians have "genetic booster rockets"?

The length of a person's life span is probably a combination of many factors, both environmental and genetic, says Dr. Thomas Perls, associate professor of medicine at Boston University School of Medicine.

"We used to think centenarians were rare because they had some vary rare genetic variations and maybe some rare environmental factors. A different picture is emerging. I think they are rare because the planets have to line up for them. So they can have some relatively common variations, but they have to have the right combination," Perls said.

Two recent studies have found genes that vary between centenarians and the general population. They also found that the children and siblings of the centenarians they studied were highly likely to have the same variations.

Dr. Nir Barzilai is the director of the Institute for Aging Research at the Albert Einstein College of Medicine of Yeshiva University. In the first study, he and his colleagues found a gene variation that affects the size of both "good" and "bad" cholesterol particles in the blood. It also affects cholesterol levels.

The study found that centenarians-and their family members-were more likely to have the gene variation, which resulted in the larger cholesterol particles. The larger particles are probably less likely to penetrate artery walls. As a result, the centenarians were less prone to high blood pressure, cardiovascular disease, and a pre-diabetic condition called metabolic syndrome, Barzilai said.

In the second study, performed by Dr. Perls and his colleagues, researchers found a gene that they associated with higher levels of cholesterol for subjects who had a particular variation. Centenarians lacked the "bad" variation.

It may be no coincidence that both of these studies have found links to cholesterol.

"Cardiovascular disease is extremely important," Perls said. "It is the number one killer by far and away. So it would make sense that, for centenarians to have a 20- to 25-year survival advantage over the rest of us, it would be by virtue of some genetic booster rockets. They cannot get the same level of heart disease the rest of us get and live to 100."

The promise of genetic research

While both scientists are enthusiastic about their findings, they are most excited about the promise it holds for future research. Such results confirm the idea that there is truly something different in the genetic makeup of people who live extremely long lives.

"The real revolution in this research is that most people have been looking at specific diseases," Barzilai said. "What we have done is to say, 'you know, one out of 10,000 people gets to 100. What's unique in them?'"

Research in animals has found that manipulating certain genes can make them live three to six times longer than normal, suggesting that the same might also be possible with humans. The specific genetic variations that would produce the same results in humans remain for scientists to discover, Barzilai said.

Some researchers remain hopeful that they will discover a gene mutation that protects centenarians against a whole host of diseases. Perls calls this sort of variation a "longevity-enabling gene." "I think they exist because I've come across individuals who have thrown everything equivalent to an atomic bomb at their bodies and they're still getting to 100," he said. "It's kind of my next Holy Grail."

The effects of useful gene variations can often be duplicated with drug therapies. Successful treatments may be far down the road as more genetic variations are discovered and pharmaceutical companies strive to develop drugs without damaging side effects. But scientists are confident that genetic research will lead to greater understanding of the processes underlying human aging, and that they will be able to develop ways to live healthier lives.