Why do we die? The ultimate existential question is also theultimate biological one. And it sends scientists into fits oftheorizing.
This is because so many issues around aging are murky. Forinstance, what exactly do we mean by biological 'aging'? When doesit begin? Why can't we be immortal? What causes us to growdecrepit?
'The major problem is that we don't know what aging is, andit's very difficult to measure what you don't understand,' saysEdward Masoro, professor of physiology at the University of TexasHealth Science Center in San Antonio.
Ideally, researchers would like to find 'biomarkers' of aging,measurable changes in cells that would tell them how biologicallyfresh or feeble an organism is and also predict its chances ofdying.
'But there are no agreed-upon biomarkers,' says Huber Warner, abiochemist at the National Institute on Aging.
Gray hair crops up with age -- but not in everyone. Even thediseases associated with aging and mortality, such asarteriosclerosis or cancer, don't exclusively strike elderlypeople. Experts don't even know if the processes that cause agingare the same ones that cause age-related disease.
Then there's the muddy line between aging and neglect of ourbodies. 'It's the $64,000 question,' says William Evans, directorof the Noll Physiological Research Center at Pennsylvania StateUniversity in University Park, Pa. 'The more we look at it, themore unclear it is: What is aging, and what is the effect ofatrophy?'
All of us suffer some degree of irreversible failure -- boneloss, reduced lung capacity and lower maximum heart rates. But thecultural hallmarks of aging, such as weight gain and sapped musclestrength, actually accrue from a sedentary life.
Recent studies at Tufts University in Boston and elsewhere showthat people of any age, men and women in their 80s and 90s, can,with progressive-resistance training, double or triple theirstrength and alter their ration of fat and lean tissue. Age, inother words, need not be yoked to infirmity.
When does aging begin? Some say at birth or even before, sincedamaging biochemicals such as free radicals are already makingmischief in our bodies. Others counter that since infants andchildren grow stronger and more neurologically complex, they can'treally be said to age.
'To me, what senescence means is that you're deteriorating. Andyou can't tell me a baby is deteriorating,' says Masoro.
Steven Austad, associate professor of zoology at the Universityof Idaho, in Moscow, Ida., agrees. One way to discern aging, hesays, is to determine when the probability of dying in a stableenvironment goes up. In humans, mortality rates steadily drop frombirth to the age of 12, the onset of puberty. After that, deathrates go up.
'When we can dictate the conditions,' says Austad, 'speciesafter species seem to be at their physical peak just prior tosexual maturity. The sex hormones have an enormous impact on thebody.' Austad calculates that if humans could somehow maintain thepristine health they enjoy at the age of 11, our life expectancywould soar to 1,200 years.
But Nature doesn't care how long we live. It cares only that westick around long enough to launch the next generation.
Scientists speculate that since nature selects only for traitsthat ensure survival up through the age of reproduction, we may beleft with a grab bag of genes that help us when young but ill serveus later.
Among these, says Austad, are genes that code for synthesis ofthe female sex hormones estrogen. Estrogen makes women fertile andstimulates the immune system when a woman is not pregnant, but overmany years it may leave her more susceptible to breast cancer.Likewise, in men, testosterone suppresses immune function andboosts the proliferation of cells in arterial walls -- potentiallysetting the stage for prostate cancer and heart disease.
The hows of aging are just as elusive -- and multitudinous.Several years ago, in a British science journal, a researcher withan archivist's heart tallied more than 300 separate theories of howour bodies go awry.
Today, two theories predominate. One says that senescence is inour breeding, that our genes code for proteins that turn offcertain body processes and turn on others, just as genes set atimetable, for growth and sexual maturity. The other school ofthought says that our bodies falter because a corrosive tide ofbiochemicals assaults our cells.
Among such deleterious chemicals are free radicals,highly-reactive oxygen molecules that are a byproduct of energymetabolism. Another is the blood sugar glucose, which fuels ourcells but wreaks havoc on proteins. The same cellular processesthat keep us alive do us in.
So what's the real secret of aging, or of aging well? The bestthing we can do for ourselves is to follow the exercise andnutrition advice we hear endlessly. Studies suggest that if we workout regularly and eat high-antioxidant foods (especially those richin vitamins C, E and beta carotene), we actually can rein in theforces that undermine us.
'There's no way to prevent aging. What you can do is slow downthe rate,' says Jeffrey Blumberg, chief of the antioxidantsresearch laboratory at the U.S. Department of Agriculture HumanNutrition Research Center on Aging at Tufts University.Theoretically, if we could slow the rate of aging and preventchronic disease, we would enjoy a longer span of vigor andindependence, followed by a brief, rapid period of decline.
The old saying about choosing your parents wisely is just partof the story. Lifestyle actually may be more crucial than heredityin determining the rate at which we age. And then there's a thirdfactor, rarely discussed in the literature.
As Steven Austad observes, in standardized lab conditions inwhich all the animals are genetically identical, there's still awide variation in longevity. His conclusion? 'There's got to be anawful lot of luck involved.'
