NEW ORLEANS (Reuters Health) - Want to stay buff well past retirement age? Studies in some very old--but surprisingly toned--mice suggest the key to lifelong muscle health is to eat less, and better.

"With age there is quite a significant loss of muscle function," explained Anthony Payne, a graduate student researcher in exercise physiology at the University of Florida, Gainesville. "But dietary caloric restriction basically prevented that loss, so the function of these very old animals is the same as the function of the young animals."

He presented the findings here this week at the annual Experimental Biology 2002 conference.

Most people view a gradual decline in muscle responsiveness, strength and tone as an inevitable part of aging. And, under typical dietary and exercise conditions, that's probably the case.

"With aging, muscle loses fiber number," Payne said, especially type 2 fibers, needed for quick, strength-dependent responses. "It's thought that nerve fibers that enervate those type 2 fibers die or recede with age," leading to fiber death and a lowering of muscle tone.

Oxidative stress--the build-up of "free radicals" that can damage cells--is a main culprit in this process, and the Florida researcher wondered if antioxidant improvements in diet might help prevent muscle decline. "Calorie restriction not only reduces the production of free radicals--the damaging molecules--but it also seems to have a beneficial effect on antioxidant enzymes that help prevent the damage," he said.

In their study, Payne and his colleagues tested the muscle responsiveness of rats fed either an unrestricted diet, or a diet with 40% fewer daily calories.

As expected, rats free to eat at will had relatively flabby muscles by the time they reached the ripe old age of 26 months--about the equivalent of a 75- to 80-year-old human.

In contrast, geriatric rats accustomed to the restricted-calorie diet were smaller, leaner, and decidedly buffer. "The muscles were quite strong," Payne said, especially muscles dominated by type 2 fibers. In fact, he told Reuters Health that he observed no difference in terms of muscle responsiveness and strength between the calorie-restricted, elderly mice and other mice half their age.

"Basically you end up with a thinner animal whose muscles function better and the muscle is a greater percentage of the total body weight. That's going to translate to better life function," he explained.

But what about humans? According to Payne, it's probably more practical to embark on a calorie-restricted diet in middle age than in youth.

"In young life when you're wanting to go out and run or play basketball, calorie restriction is probably not very feasible," the Florida researcher noted. "It's probably a lot more feasible in humans once you get to the age where you're in mid-life and sitting behind a desk."

And lowering calories doesn't mean going without delicious, nutritious foods. "It's a healthier diet," he said, full of "very nutrient-dense foods, lots of fruits and vegetables. The Western diet is very meat-oriented. If it's shifted to more of a fruit and vegetable-based diet you'll be taking in less calories anyway."

Payne stressed that exercise--especially resistance training--remains key to muscle health. "There's so much evidence out there that you can increase muscle mass and strength in the elderly--even 90 years old--with strength training," he said.

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