Could People Live to 120?

A life span of about 120 years is the maximum that humans can attain under current medical and environmental conditions. But 120 might become the average life expectancy if medical advances could duplicate the mechanisms at work in the only life-extension technique now known to be effective in lab rodents -- extremely low-calorie diets -- say two University of Southern California scientists.

The Finch-Pike study uses the Gompertz model -- named after Benjamin Gompertz, the British actuary who first described it in 1825 -- to predict maximum life expectancies for various species.

The Gompertz rule predicts that the risk of dying accelerates at a constant rate over time for all organisms, be they fruit flies or humans. After human beings reach puberty, for example, their chances of dying increase by approximately 9 percent each year -- thus doubling approximately every eight years.

This steady acceleration of death risk, called the alpha factor, is particularly striking in that it seems to hold for humans (and certain other animals) living under all kinds of environmental conditions, whether harsh or benign.

A 1990 Science paper by Drs. Finch and Pike and Matthew Witten (of the University of Texas at Austin) cited the example of Australian soldiers in Japanese prisoner-of-war camps during World War II. All age groups suffered higher death rates than their counterparts back home in Australia. Indeed, both 24-year-old and 48-year-old prisoners were approximately 30 times more likely to die in a given year than their same-age counterparts in Australia. But the acceleration of death rates between the two age groups remained constant. In other words, the 24-year-olds died in the same ratio to 48-year-olds in jungle death camps as in Australian cities and towns.

"The Gompertz model," Finch explains, "allows scientists to express life expectancy as a product of two distinct factors.

"One is environment, either stressful (as in the prison camps) or benign.

"The other, designated by the alpha factor, is a measure of a built-in species clock, something that seems to be part of our design as animals, a burden of aging that keeps accumulating, like interest compounding on a debt. It includes spontaneous diseases of all kinds, which are largely responsible for the alpha factor."

While the Gompertz model successfully accounts for a large volume of aging data for a number of species, seeming anomalies have been reported in some species, especially at the extreme old age end of the life-span spectrum.

In some populations -- notably those of laboratory mice -- the maximum life spans observed are substantially greater than those predicted by the model. Life expectancies in certain other species -- invertebrates and some reptiles -- seem to reach an old-age plateau. After approximately 90 percent of a population has died, the mortality rate becomes constant, rather than continuing to accelerate.

The Journal of Gerontology paper attempts to deal with these anomalies, examining the best mortality data from a wide variety of species to see what useful conclusions about life span can be drawn from Gompertz modeling -- with specific reference to possible extension of the human life span.

Finch and Pike analyzed vital statistics from bird and mammal species, including numerous lines of purebred mice and rats; various breeds of pedigree dogs, gerbils, hamsters, turkeys, peafowl, finches, pheasants and Japanese quail. All of the animal data, they found, fit Gompertz curves closely, with mortality rates showing no sign of leveling off at extreme old ages.

Finch and Pike also analyzed human mortality rates in different areas of the globe and found high disparities in risk for death by disease or environmental hazards. Mortality rates varied by as much as a factor of 10.

"Despite this," the authors write, "it is striking how little [the compound-interest rate of generalized aging] differs between human populations that differ widely in genetic poly-morphism and in environmental conditions."

The second possibility is that manipulation of the mysterious species-specific alpha factor will reduce vascular diseases, cancer, diabetes and other chronic killers. Reduction of the alpha factor by 45 percent would raise average life expectancy to the 120-year threshold.

Only one treatment is currently known to affect the rate of aging. In numerous species, including rats, long-term maintenance on extremely low-calorie diets retards aging in general. Indeed, it changes the aging rate of a population by as much as 45 percent.

"While such ... plasticity in human life span is completely plausible from evolutionary perspectives," the Finch-Pike study notes, "we do not know the generalizability of diet restrictions from short-lived rodents to the much longer-lived humans."

Finch, USC's University Professor of Gerontology and Biological Sciences, holds the ARCO/William F. Kieschnick Chair in the Neurobiology of Aging at USC's Ethel Percy Andrus Gerontology Center and directs that center's neuro-gerontology program.

Pike, professor and chairman of preventive medicine and holder of the Flora L. Thornton Chair in Preventive Medicine at the USC School of Medicine, is a member of the Institute of Medicine of the National Academy of Sciences.

The Finch-Pike research was supported by the John D. and Catherine T. McArthur Foundation Network in Successful Aging. Dr. Finch is a resident of Altadena, Calif., and Dr. Pike is a resident of Long Beach, Calif.