07/13/2011 04:21 pm ET Updated Sep 12, 2011

Searching for the Secrets to Longevity

My co-author and wonderful colleague Dr. Leslie Martin was on the Today Show (NBC) this week with the maker of the cool new documentary "How To Live Forever." As a health scientist, Leslie was there with filmmaker Mark Wexler to provide research-based insight into the real reasons why some people stay healthy and live long.

Wexler's intriguing film interviews a gaggle of healthy old people, including my favorite, Jack LaLanne. Jack lived in good health until age 96. I admire Jack because he was always willing to defy the conventional wisdom. Starting as a teenager, Jack set out to keep himself healthy, becoming a nutrition nut ("nut" is his term), a performer (with the first TV fitness show) and a body-builder. He founded one of the first work-out gyms, back in the 1930s! Later in life, Jack was famous for selling a juicer, so that everyone could drink lots of vegetable juice.

But what was really Jack's secret? He did many health-nut things, but which really mattered? From other studies, there is of course now good evidence that staying in shape with strong muscles is generally a healthy obsession -- despite the many doctors who at the time warned against risks of becoming muscle-bound, impotent, and worse. The juicer? Maybe, but there is really no good evidence that vegetable juice is the secret to longevity.

Plus, Jack was not like you and me. In his 40s, he swam the Golden Gate channel while towing a one-ton boat. At around age 60, he swam from Alcatraz Island to Fisherman's Wharf, handcuffed and shackled. I don't know about you, but even if I managed to spend every minute of my life lifting weights in Jack's gym and drinking his vegetable juice, I could never accomplish any of his amazing feats. In other words, Jack differed from the typical person in many ways. Even Jack did not know precisely what his key to good health was.

The good news is that there is a way to find out. The way that I would proceed is to follow a large number of promising, healthy children for their whole lives and see who stays healthy and lives longest. This is exactly what Leslie and I have been doing for the past 20 years.

Our findings (reported in The Longevity Project) reveal many, many other things that Jack did that bode well for good health. For example, after an early divorce, LaLanne married Elaine Doyle in 1959 and stayed happily married to her until his death earlier this year. That is over 50 years! In The Longevity Project (which started in 1921), we discovered that long, happy, supportive marriages are especially good for a man's health. (Not necessarily for women, but that's a different story.)

Our eight-decade research study also discovered that having a successful career, striving for a worthwhile cause and being in daily contact with lots of people are very good predictors of staying healthy and living long. Jack did all of these with a vengeance. And all are being confirmed in the few other life-long studies now ongoing.

On the "Today Show," Leslie pointed out that you cannot understand much about the causes of health and long life by studying only people who succeeded. They don't really know the answer. And they usually don't remember what they were doing 50 or 60 years prior. So why do we keep turning to old people and asking them about the secrets to health and longevity?

Well it makes for a provocative, entertaining film, but it is only a story. In long-term research like the Longevity Project, we can see more clearly how one set of healthy steps brings on other healthy strides, until one is on a healthy path. Health accumulates. This deeper, long-term approach is more difficult to master, but it is the road most taken by individuals destined to stay healthy -- and wind up in a film about how to live forever.