WASHINGTON -- Although diets that merely restrict calories are losing favor among some weight control specialists, other scientists are accumulating evidence of different benefits from holding down food intake.
Laboratory animals that are kept on restricted diets consistently outlive others that are allowed to eat all they want.
Scientists say there is evidence that the body's ability to repair everyday damage to the genetic code contained in DNA is improved when food is restricted.
Also, enzymes that sweep cancer-causing free radicals out of cells are more active and immune functions are enhanced under reduced food intake.
In a new experiment, scientists at the University of South Florida removed sections of the livers of rats and found that animals whose diets were restricted replaced the lost liver tissue faster than they would have if they had been allowed to eat all they wanted.
"What you might expect to find is that when these animals are kept on a diet [that] we call 'undernutrition without malnutrition,' then the overall food energy available to them is less and, therefore, the rate at which they regenerate liver tissue is reduced," said Dr. Robert Good, one of the University of South Florida researchers.
"In fact, however, just the opposite happens. Lowering their food energy increased the speed at which the liver cells multiplied," he said.
Ordinarily, a rat that has had two-thirds of its liver surgically removed will grow back the lost portion in 14 to 17 days, Dr. Good said.
To measure the impact of calories on this regenerative process, he and his associates removed liver tissue from 80 rats, then divided the rats into two groups.
One group was fed "ad libitum" diets -- all they wanted. The other group was restricted to diets that contained 40 percent fewer calories, all they needed but less than they wanted. A roughly equivalent diet for a 175-pound man would be about 1,800 to 1,900 calories a day, Dr. Good said.
In an account of the experiment released yesterday in the Proceedings of the National Academy of Sciences, Dr. Good said that liver cell regeneration among animals on the "undernutrition without malnutrition" diet was about 25 percent faster than among the heavy eaters.
In addition, the restricted diet animals experienced stronger, speedier and healthier recoveries than did those on the unlimited diets, he said.
"We're very excited about this," Dr. Good said in a telephone interview.
"The overall reaction to reduced caloric intake is reduced cell growth, but in some of these adaptive processes, like tissue regeneration, the effect appears to be reversed: Reduced energy leads to increased cell multiplication," he said. "We think this may have tremendous survival advantages."
Dr. Good said that further experiments will be aimed at understanding the underlying mechanism that seems to make a little hunger a good thing.