OUT ON the fringes of the biomedical establishment, a growing community of scientists and lay followers has decided that life is too short.

Sooner or later, humans will be living to 120, 140 and beyond--the Life Extenders are unanimous on that point. But they want it to happen sooner rather than later, and if their bodies will oblige, they want to play a direct personal part in the phenomenon.

"People are taking all kinds of things right now to expand their life spans," says Saul Kent, author of "The Life-Extension Revolution" and president of the Florida-based Life-Extension Foundation, which publishes a newsletter called "Anti-Aging News."

It will be a while before the world can pass final judgment on the value of this activity, but if by any chance the Life Extenders know what they're doing, they could be the first specimens of an elite new race of human beings--and they could be in a position, a few years down the road, to say one of the more resounding "I told you so's" in the history of the species.

For now, they base their faith mostly on inspiring examples of scientifically induced longevity in the animal kingdom. To wit:

* A black mouse will live an average of 653 days (nearly 20 percent longer than normal) if it is given large doses of pantothenic acid, a member of the Vitamin B family.

* A female octopus will survive up to 9 months after spawning (compared to the usual 10 days) if its optic glands are removed.

* Laboratory rats, when placed on an extreme low-calorie diet, have been known to reach the ripe old age of 1,800 days (the apparent equivalent of 200 years in a human).

Listen to some of the more enthusiastic of the Life Extrenders, and the word "flake" may come to mind. But their ranks also include people who could only be called "respectable members of the medical community." And their ideas have aroused the interest of at least one candidate for the presidency in 1984.

"We are on the brink of a major leap forward in medical science--nothing less than a profound redefinition of human life," says Sen. Alan Cranston (D-Calif.), the 68-year-old marathon runner and founder of the Fund for Biomedical Research (FIBER), which is devoted to "productive prolongevity." Cranston holds frequent meetings with Life Extension experts, and reports that "what I learn from these sessions is mind-boggling."

Cranston is a friend of UCLA gerontologist Roy L. Walford, and he has written a book-jacket blurb for Walford's "Maximum Life Span," which describes the near-starvation diet the author has adopted in hopes of living as long, in human terms, as underfed laboratory rats.

Walford's program, which he calls "undernutrition without malnutrition," is one of the two basic approaches to Life Extension that have emerged from the animal research. The other, spelled out in the best-selling book "Life Extension--A Practical Scientific Approach" by an upbeat pair of Southern Californians named Durk Pearson and Sandy Shaw, emphasizes mega-doses of "antioxidants," a diverse group of chemicals that includes Vitamins E and C, selenium, cysteine and the food preservative BHT.

Pearson and Shaw are not without their own celebrity connections. In their book, they cite a disciple they call "Mr. Smith," a "professional movie actor, well-known for his physically vigorous roles," who has been feeling unusually youthful of late, and who requested pseudonymity "because of his concern that irresponsible tabloids might take quotes out of context."

Mr. Smith's fears may have been confirmed when a reporter recently cornered him at a celebrity golf tournament, and wrote a story that showed up in the tabloid The Star under the headline "Bucket-a-Day Vitamin Regimen That Keeps Clint Eastwood Young at 52."

"This is the first time I have revealed that 'Mr. Smith' is really me," Eastwood was quoted as saying.

"I was sort of tricked into that," he says now. "I understood it was about golf."

But he has since come out of the closet to acknowledge a complex dietary formula largely derived from the ideas of Pearson and Shaw. "You can actually feel a difference and see a difference in yourself," he says, although he is careful to establish that "I'm not necessarily interested in extending life. To me, what seems most intriguing is just keeping the quality of your life up as long as fate decrees that you'll be here on the planet."

"I don't take as much stuff as they do," he adds. "I'm a bit more conservative, probably."

It is not hard to be more conservative than Durk Pearson and Sandy Shaw. They have laid out a scenario in which, incredible as it may seem, the Life Extenders of today (they themselves, for example, and conceivably Eastwood as well) could actually wind up living forever.

The "doubling time" for biological knowledge is somewhere between five and 10 years, Pearson and Shaw observe. This statistic leads them to expect such a rush of imminent scientific breakthroughs that people alive today will be able to hang around until the middle of the next century or so, when--thanks to cloning, cryogenics, bionics, computer-implants and the like--immortality itself should be in the bag.

Or so, at any rate, say Pearson and Shaw.

And austerity, they have decided, is no prerequisite to longevity.

"If you do not wish to alter your life style or your habits," they say, "this need not preclude your taking effective steps to extend your life span."

To prove the point, they consume large amounts of butter, whole milk, eggs and meat, "and we're able to control cholesterol far better than someone on the most rigorous Pritikin-type diet with a rigorous program of exercise," Pearson reports.

On the subject of exercise, they recommend about 30 minutes a week (you heard them right, 30 minutes), but don't always meet this tough standard themselves. And while they admit that smoking and drinking may be harmful, they say the harm can be substantially undone with the right mixture of powders and pills.

As for food additives, they take the unusual view that if a chemical helps preserve food, it probably has the same good effect on the human body. The preservative BHT, for example, is one of their favorite dietary supplements. They take two grams of it in powdered form a day.

"I would be scared to death of bacon that didn't have nitrites in it," says Pearson. "I wouldn't touch the stuff with a 10-foot pole."

But cigarettes and sloth are not central to the Pearson-Shaw program. These things are a matter of individual choice, they say. What really counts in their scheme of things is the daily ingestion of massive amounts of antioxidants, which are thought to counter the ill effects of another group of substances called "free radicals."

The free radicals were singled out in the 1950s as central players in the aging process. They have been described as "great white sharks in the biochemical sea," attacking cell membranes and other tissues, and rendering them feeble and brittle. (Life Extension is a field that abounds with easy metaphors for the lay person. Saul Kent's book tells us, for example, that lysosomes act as a "wrecking crew" inside the body, while catecholomines trigger a "cascade" of unfortunate internal events.)

Luckily for our tissues, however, the antioxidants (think of them as biochemical equivalents to Richard Dreyfuss in "Jaws") will hunt down the free radicals in our body and knock them right out of commission, if given the chance.

In the laboratory, antioxidants have increased average life span in mice, rats, fruit flies and worms, according to the University of Nebraska's Dr. Denham Harman, who formulated the free-radical theory of aging in the late 1950s and now heads the American Aging Association (AGE), a lay-scientific group devoted to aging research.

The increase in mice, he says, "is equivalent to raising the human life span from 73 to 95 years."

Harman's faith in antioxidants is not shared by all gerontologists, but within the Life Extension community his is the readiest available route--the "current fad," as Walford says skeptically.

Pearson and Shaw have been taking antioxidants for 14 years now, "and if our livers were going to fall out," says Shaw, "they would have fallen out a long time ago."

"How long do we expect to live?" they ask.

"We just don't know," they answer. But "we personally believe that our own life extension research applications of the last 12 years have extended our life expectancy by well over 12 years."

If that calculation is correct, it does not take a math genius to figure out that Pearson and Shaw are on course toward immortality. But if, by some quirk of biology, they should wind up living normal lives (lengthwise, that is), they will be carrying on an old tradition.

"No single subject is more obscured by vanity, deceit, falsehood and deliberate fraud than the extremes of human longevity," the Guinness Book of World Records declares, before adjudging Mrs. Delina Filkins, an upstate New York woman who died on Dec. 4, 1928, to have reached "the greatest authenticated age to which a human has ever lived"--113 years and 214 days.

The Guinness people scoff at the picture of whole communities of centenarians in the Vilcapamba valley of Ecuador or in Soviet Georgia (where, as the expatriate Russian geneticist Zhores A. Medvedev has pointed out, there is no positive documentation to prove that anyone has lived beyond 108).

"The most reliably pedigreed large group of people in the world, the British peerage, has, after ten centuries, produced only two peers who reached their 100th birthdays, but only one reached his 101st," says the Guinness book. "However, this is possibly not unconnected with the extreme draftiness of many of their residences."

Since 1951, the Romanian physician Dr. Ana Aslan has been treating youth-hungry clients (Charles de Gaulle, W. Somerset Maugham and Nikita Khrushchev, among them) with a variant of Novocain called Gerovital. But the case for Gerovital is based on "the least acceptable kind of scientific evidence, that of testimonials," says Walford. And the same is true, he says, of the various spas where celebrities like Charlie Chaplin, Pope Pius XII, the Duke of Windsor and William Butler Yeats have gone to be injected with extracts of unborn lamb cells and ape or goat testicles.

The serious Life Extenders would rather not be confused with the perpetrators of these wild schemes, or with the Immortalists, a Los Angeles-based group with an elaborate nutritional-spiritual program, or with the Breatharians, a San Francisco outfit that claims to have discovered how people can live indefinitely on nothing but air.

But this hasn't rendered their own claims invulnerable to criticism. Many gerontologists and biologists agree with Dr. Edward Schneider, the National Institute on Aging's associate director for biomedical research and clinical medicine.

Research into the nature of the aging process is "a very exciting and a very promising area," says Schneider. "But it's very new. We don't have the answers. It's tantamount to somebody 40 years ago saying 'I have the cure for cancer--all you have to do is stay away from cancer viuses.'

"All the evidence points to aging being caused by many, many factors," Schneider continues. "So to think that a single magic bullet, or a single antioxidant, can cure aging is extraordinarily naive."

Specifically, Schneider says that antioxidants (along with most of the other substances recommended by the Pearson-Shaw school) tend to cause weight loss. So it may be the weight loss rather than anything else that yields the benefit, he suggests.

And even weight loss may not be everything it's cracked up to be, Schneider says.

The interest in undernutrition dates back to the mid-1930s, when Clive McCay of Cornell University succeeded in apparently doubling both average and maximum life spans in rats. McKay began depriving his rats early in life, and more recent research, according to Walford, suggests that a similar result can be achieved with a later start.

"Everybody in gerontology agrees that this kind of program will work if one cares to do it," says Walford. And undernutrition, he says, is the only practical way to increase maximum life span, while antioxidants may, at best, increase average life span.

But Schneider counters that the "underfed" laboratory rats could actually be eating at a pace equivalent to that of normal wild rats, which can't always get food when they feel like it. No one knows what the maximum life span of wild rats would be in a world without predators, says Schneider, so no one knows if any rats are exceeding that maximum. The truly aberrant rats, he theorizes, may be those on normal laboratory diets, eating virtually on demand and perhaps dying sooner as a result.

Just the same, Schneider describes Walford as an "eminent scientist" and says, "I don't think his program could cause harm to anyone."

By contrast, he says, megadoses of antioxidants "could get you in an awful lot of trouble with other parts of your body." In general, he dismisses Pearson and Shaw's book--an 850-page volume liberally illustrated with graphs, cartoons and pictures of the authors flexing their muscles in gymnastic garb--as an "uncritical review" tainted by "pervasive prejudice" against the elderly.

"Their book leaves people with the idea that it's a curse to be old," he says. (He might have gotten this impression from an early chapter titled "Aging Isn't Beautiful," in which we learn that "The best that can be said for aging is that it is better than being dead.")

Schneider and his NIA associates advocate a more cautious approach to aging research, beginning with the designation of certain "biomarkers of aging"--specific physical traits to measure the effects of proposed Life-Extension techniques on individual test subjects.

To a true Life Extender, however, a little criticism from a federal bureaucrat is hardly daunting. It's an article of faith in the Life Extension movement that the government is part of the problem--and a big part at that.

They are particularly scornful of the Food and Drug Administration and the standards by which it sets the recomended daily allowances (RDAs) of vitamins and minerals, and decides what substances to approve and not approve. To get around some FDA restrictions, the Life Extension Foundation is assembling a network of sympathetic physicians who will supply Life Extenders with prescription medicines (such as L-Dopa) for their own unorthodox purposes.

More broadly, the Life Extenders want the government and the medical profession to move away from "pathology management"--the search for ever-more-sophisticated and ever-more-expensive methods of treating the ailments of age--and to instead invest far more time and money in trying to modify the aging process itself.

At first glance, though, it is not clear how the country will profit by trying to increase the length of its citizens' lives. Won't that just mean more retirees, more illnesses and more pressure on an already overburdened social safety net?

None of the above, say the Life Extenders. The reason is their concept of "squaring the curve." As maximum life span increases, they foresee average life span increasing even faster, and with it the number of years of healthy, productive existence. Then, bypassing all the painful, expensive, protracted ailments that accompany aging today, the organism will just give out.

"This is not extending the part of life span where you're in bed with a lot of tubes and so forth," says Pearson.

"What we're talking about is extending the period of youth and middle age," says Walford. "The 90-year-old man of the future will have the physical vigor of a 50-year-old man of today."

Walford offers no predictions about his own life span, and he insists he isn't pushing his longevity program on anyone. But at 58, this lean, smooth-skinned man with a bald head and a Fu-Manchu moustache may be his own best advertisement for eating about 40 percent less than a normal human diet.

In addition to his own example, he would have us consider that of Luigi Cornaro, a minor Venetian nobleman and author of a popular autobiography, "The Art of Living Long," who is said to have died at age 103 in 1567. According to his memoirs, Cornaro's early habits were so gluttonous and decadent that he nearly died at 37. Then he adopted a dietary program Walford finds strangely similar to his, and (as Walford interprets it) had a faster's "high" for the last 66 years of his life.

Pearson and Shaw report similarly splendid results for their procedures. They are feeling not only fitter but smarter, they say, thanks to daily doses of choline and other brainpower-enhancing substances.

"Not only have the number of ideas increased but also the quality of ideas," says Shaw. "The ideas are more valuable."

And how does she know they're more valuable?

"We've been able to sell more of them," she says.