It may never be too late to change your diet, according to a study led by a UC Riverside professor.

The research found that a low-calorie diet extended the life span of older mice by about 40 percent, or up to six months. In addition to delaying aging, researchers saw fewer tumors in the dieting mice.

Scientists have known for years that low-calorie diets reduce aging, the onset of cancer and heart disease.

But this may provide more insight into the benefits of changing your diet later in life, said Stephen Spindler, professor of biochemistry at UCR and the project's leader.

Even very late in life, you can start a low-calorie diet and it acts quickly, Spindler said. "We're hopeful that it can be provided to humans. We know that low-calorie diets improve the health of humans."

The UCR study appears this week in the Proceedings of the National Academy of Sciences, a scientific journal.

But other experts warn against assuming that what occurs in mice also applies to humans.

"It's but one piece of the aging puzzle when you're talking about a human being," said Dr. Peter Pressman, assistant professor of clinical medicine at USC's Keck School of Medicine. "The leap from a rodent to a human being is an exponential one."

Pressman and other experts urged dieters to be aware of the dangers associated with limiting your calories without consulting a physician.

"In a nutshell, extremes of caloric restriction will kill you. It's that simple," Pressman said.

Researchers separated 19-month-old mice. One group ate as much as they wanted while the rest had their calories restricted. Scientists studied the mice until death. The dieting mice lived longer.

The all-you-can-eat mice consumed 93 kilocalories per week, while the calorie-restricted mice were given 52.2 kilocalories per week.

"It's very close to the lowest-calorie diet that you can give the mice and have them remain healthy," Spindler said.

"It's not a starvation diet."

Spindler added that the diets of mice fed on the low-calorie diet would be comparable to the most dedicated members of caloric restriction societies, groups of people who seek the benefits of ultra-low-calorie diets.

While the older mice whose food was limited lived longer than those who didn't, they still didn't live as long as mice whose calories were restricted earlier in life, the study found.

The incidence of tumors among the mice on restricted-calorie diets also was slowed.

"I think it's an important finding," said George Roth, senior guest scientist of the National Institute on Aging, one of the National Institutes of Health.

But Roth also said it's too easy to assume the same is true for humans. For example, humans already see a loss in appetite and body mass as they age. Other possible side effects to practicing caloric restrictions are malnutrition, sluggishness and hyperactivity, he said.

"We should not assume that it's necessarily true in humans until there's a way to test this in humans," Roth said.

Physicians advise that as with anything, moderation is the best approach to dieting.

"Malnutrition doesn't do anybody any good," said Pressman, the USC professor.