After more than 50 years of experiments in which laboratoryanimals were able to live longer when given only enough food to stayhealthy, researchers are beginning to give more thought to whetherhumans also can extend their lives by cutting back on how much theyeat.
The 10 percent of Americans that live the longest average 96years. A 10 percent extension of maximum life-span - less than hasbeen achieved in laboratory animals - would make living to age 105 acommon experience.
"The hard evidence in humans isn't there, but major healthstudies keep pointing in that direction," said Dr. Richard Weindruchof the National Institute on Aging in Bethesda, Md.
Restricted diets have succeeded in prolonging the lives ofanimals from single-celled protozoa to fruit flies, spiders, guppies,mice and rats, and researchers are trying to determine if low-caloriediets keep monkeys alive longer.
It was Weindruch who produced the longest living mice on record- 53 months, about 35 percent longer than the mice would typicallylive. He did it with the kind of restricted diet typical inlongevity experiments - his animals got less than half the totalcalories they would consume if allowed to eat as much as they wanted.
In more typical experiments, animals' calorie intake is reducedby 30 percent to 50 percent. The resulting life-span is lessdramatic than Weindruch's mice, but consistently the animals livelonger than their free-eating peers.
The life-extending restricted diets are based solely on totalcalories, regardless of whether those calories come in the form offat, protein or carbohydrates.
George Roth of the National Institute on Aging's Baltimorelaboratories, has been studying 90 rhesus and squirrel monkeys forabout three years. The monkeys on restricted diets get about 30percent less food than their unrestricted eating partners - "That'sless severe than what you'd get if you went to Weight Watchers or oneof those diet programs," he said.
Most current dietary recommendations for Americans emphasizebalancing calorie intake with exercise and restricting fat to 30percent of total calories. Dietary fat has been linked to heartdisease and several forms of cancer, according to recent reports fromthe U.S. surgeon general and the National Academy of Sciences.
Experiments with restricted-calorie diets in different animalspecies have consistently produced good effects: They develop fewer cancers and develop them later in life than theirfree-eating peers. Researchers have found that mice and rats on restricted diets madeless of a natural body chemical called a prostaglandin thatsuppresses the immune system. By allowing the immune system to staymore active, the animals were better able to fight off infections andnip cancers before they got established. In rats and mice, there was less cross-linking of collagenmolecules, a process that causes wrinkled skin. In one study, mice on restricted diets showed fewer age-relateddeclines in coordination, learning and ability to stay active. The life-extending benefits are greatest if the restricted diet isbegun in infancy, but even starting later in life produces some gainsin longevity.
Other researchers are trying to determine just how a lifetime ofeating fewer calories adds years to the normal life-span.
The theory that intrigues many scientists is that it lessens thedamage done by the toxic waste that builds up in cells as a result ofnormal energy production.
Arlen Richardson, a chemist at Illinois State University,accidentally came upon another process that may be involved inkeeping animals on restricted diets alive longer.
When the air conditioning in his laboratory failed one weekend,the rising temperatures killed three-fourths of his free-eatinganimals, but only one fourth of the animals on restricted diets.
Richardson said this may be a sign that animals on restricteddiets are better able to cope with many kinds of stress, includinginjuries, which become increasingly deadly with advanced age.
Like many of the other researchers studying the effects oflow-calorie diets on aging, Richardson said he tries to restrict hisown calorie intake on the assumption that dietary restriction worksin humans as well as rats and mice.
"In the scientific community, there is a gut feeling that itwill work in all organisms," he said.
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