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Book review

Good news on aging - if you're a fruit fly

By TOM VALEO
Published November 1, 2005

Every living thing ages, but no one really knows what causes the relentless decline.

Moreover, no one really knows how to slow it down.

Michael Rose has spent his career trying to figure out what causes aging, and he has found a way to achieve longer life - for fruit flies, at least.

He did this through a remarkably simple and ingenious experiment. He raised colonies of drosophila melanogaster, as fruit flies are known to biologists, and let them all die except for a few hardy hangers-on, which he used to breed the next generation. After a few generations, he had a colony he called Methuselah flies, because they lived 10 percent longer than normal flies. After repeating this experiment for more than two decades, he has bred fruit flies that live more than twice as long as normal.

The secret to their longevity, obviously, lies in their genes, but how do their genes produce such long lives?

In The Long Tomorrow, Rose tries to explain. Longevity, he says, is related to reproduction. Organisms that breed later in life, such as elephants and humans, tend to live longer than animals that breed early and often, such as rabbits and mice. This is a function of natural selection, Rose says. In general, evolution favors animals that breed early and often, because they increase their chances of producing offspring that will survive to maturity. Animals with few predators, however, don't face as much evolutionary pressure to reproduce early.

Rose created his Methuselah flies by subjecting them to a form of artificial natural selection. He removed the eggs every day until all but a few of the fruit flies had died. Therefore, the longest-living flies were the ones to reproduce. After a few generations, he had a colony of fruit flies that began to lay eggs later in life. They also lived longer than normal.

But what enabled them to live longer?

One possibility is that Methuselah flies possess greater resistance to stress. When an assistant forgot to feed the flies one weekend, all the normal flies were dead by Monday morning, while the Methuselah flies appeared vigorous and robust. The Methuselah flies, Rose discovered, had larger stores of fat in their abdomen, apparently because they reproduced later, which allowed them to conserve calories, thereby enabling them to endure a period of starvation. This inspired Rose to study the biological basis of longevity, which appears to be closely related to nutrition and metabolism.

Animals fed a nourishing diet containing 20 to 40 percent fewer calories than they normally consume will live about 20 to 40 percent longer. Apparently, flies that consumed less food diverted calories from egg production to fat storage, which kept them healthier longer.

So fat promotes longevity? No, but caloric restriction certainly improves health and extends longevity in many species, including humans. When humans consume fewer calories, their blood pressure drops, their bad cholesterol declines, their good cholesterol increases, their immune function improves and the levels of insulin and glucose in their blood remain enviously low. But how does caloric restriction produce these benefits?

No one knows, not even Rose, who ultimately fails to deliver on the promise made by his book's subtitle: How Advances in Evolutionary Biology Can Help Us Postpone Aging.

Tom Valeo writes Body of Information, a column on aging, for the Seniority section of the Times.

"The Long Tomorrow: How Advances in Evolutionary Biology Can Help Us Postpone Aging," by Michael R. Rose, Oxford University Press, $26, 140 pages.

[Last modified November 1, 2005, 08:57:31]