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COPYRIGHT 2007 All rights reserved. Reproduced by permission of The Condé Nast Publications Inc.
The hardest substance in the human body is the white enamel of the teeth. With age, it wears away nonetheless, allowing the softer, darker layers underneath to show through. Meanwhile, the blood supply to the pulp and the roots of the teeth atrophies, and the flow of saliva diminishes; the gums tend to become inflamed and pull away from the teeth, exposing the base, making them unstable and elongating their appearance, especially the lower ones. Experts say they can gauge a person's age to within five years from the examination of a single tooth--if the person has any teeth left to examine.
Scrupulous dental care can help avert tooth loss, but growing old gets in the way. Arthritis, tremors, and small strokes, for example, make it difficult to brush and floss, and, because nerves become less sensitive with age, people may not realize that they have cavity and gum problems until it's too late. In the course of a normal lifetime, the muscles of the jaw lose about forty per cent of their mass and the bones of the mandible lose about twenty per cent, becoming porous and weak. The ability to chew declines, and people shift to softer foods, which are generally higher in fermentable carbohydrates and more likely to cause cavities. By the age of sixty, Americans have lost, on average, a third of their teeth. After eighty-five, almost forty per cent have no teeth at all.
Even as our bones and teeth soften, the rest of our body hardens. Blood vessels, joints, the muscle and valves of the heart, and even the lungs pick up substantial deposits of calcium and turn stiff. Under a microscope, the vessels and soft tissues display the same form of calcium that you find in bone. When you reach inside an elderly patient during surgery, the aorta and other major vessels often feel crunchy under your fingers. A recent study has found that loss of bone density may be an even better predictor of death from atherosclerotic disease than cholesterol levels. As we age, it's as if the calcium flows out of our skeletons and into our tissues.
To maintain the same volume of blood flow through narrowed and stiffened blood vessels, the heart has to generate increased pressure. As a result, more than half of us develop hypertension by the age of sixty-five. The heart becomes thicker-walled from having to pump against the pressure, and less able to respond to the demands of exertion. The peak output of the heart decreases steadily from the age of thirty. People become gradually less able to run as far or as fast as they used to, or to climb a flight of stairs without becoming short of breath.
Why we age is the subject of vigorous debate. The classical view is that aging happens because of random wear and tear. A newer view holds that aging is more orderly and genetically driven. Proponents of this view point out that animals of similar species and exposure to wear and tear have markedly different life spans. The Canada goose has a longevity of 23.5 years; the emperor goose only 6.3 years. Perhaps animals are like plants, with lives that are, to a large extent, internally governed. Certain species of bamboo, for instance, form a dense stand that grows and flourishes for a hundred years, flowers all at once, and then dies.
The idea that living things shut down and not just wear down has received substantial support in the past decade. Researchers working with the now famous worm C. elegans (two of the last five Nobel Prizes in medicine went to scientists doing work on the little nematode) were able to produce worms that live more than twice as long and age more slowly by altering a single gene. Scientists have since come up with single-gene alterations that increase the life spans of Drosophila fruit flies, mice, and yeast.
These findings notwithstanding, scientists do not believe that our life spans are actually programmed into us. After all, for most of our hundred-thousand-year existence--all but the past couple of hundred years--the average life span of human beings has been thirty years or less. (Research suggests that subjects of the Roman Empire had an average life expectancy of twenty-eight years.) Today, the average life span in developed countries is almost eighty years. If human life spans depend on our genetics, then medicine has got the upper hand. We are, in a way, freaks living well beyond our appointed time. So when we study aging what we are trying to understand is not so much a natural process as an unnatural one. Inheritance has surprisingly little influence on longevity. James Vaupel, of the Max Planck Institute for Demographic Research, in Rostock, Germany, notes that only six per cent of how long you'll live, compared with the average, is explained by your parents' longevity; by contrast, up to ninety per cent of how tall you are, compared with the average, is explained by your parents' height. Even genetically identical twins vary widely in life span: the typical gap is more than fifteen years.
If our genes explain less than we imagined, the wear-and-tear model may explain more than we knew. Leonid Gavrilov, a researcher at the University of Chicago, argues that human beings fail the way all complex systems fail: randomly and gradually. As engineers have long recognized, many simple devices do not age. They function reliably until a critical component fails, and the whole thing dies instantly. A windup toy works smoothly until a gear rusts or a spring breaks, and then it doesn't work at all. But complex systems--power plants, say--have to survive and function despite having thousands of critical components. Engineers therefore design these machines with multiple layers of redundancy: with backup systems, and backup systems for the backup systems. The backups may not be as efficient as the first-line components, but they allow the machine to keep going even as damage accumulates. Gavrilov argues that, within the parameters established by our genes, that's exactly how human beings appear to work. We have an extra kidney, an extra lung, an extra gonad, extra teeth. The DNA in our cells is frequently damaged under routine conditions, but our cells have a number of DNA repair systems. If a key gene is permanently damaged, there are usually extra copies of the gene nearby....
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