Severe calorie restriction prevents certain aging-related changes in the brain, including the accumulation of free radicals and impairments in coordination and strength, according to a mouse study at Washington University School of Medicine in St. Louis.
However, the dietary changes did not seem to prevent mice from developing some cognitive deficits associated with age, such as declines in memory.
"Our findings help us understand the processes underlying both normal aging and calorie restriction benefits," says principal investigator Laura L. Dugan, M.D., associate professor of neurology, of medicine and of anatomy and neurobiology. "If some aspects of aging are influenced by free radical damage, we may be able to prevent or reverse these impairments."
Though numerous studies have shown severe calorie restriction helps animals live longer and resist some effects of aging, scientists still do not know why. One theory suggests a restrictive diet decreases the effect of free radical damage.
Free radicals are chemically reactive molecules produced either as byproducts of the body's natural processes or as a result of stress from the environment, like smog or sunlight. It's normal to have some free radicals, but scientists think accumulating too many may cause cell damage and contribute to a variety of diseases ranging from stroke to cancer. Antioxidants like vitamins C and E help prevent free radicals from wreaking too much havoc.
Since there is evidence that both antioxidants and calorie restriction increase lifespan and reduce aging-related diseases, Dugan and her colleagues hypothesized that calorie restriction, like antioxidants, helps protect the brain against free radical damage.
To test their theory, the team compared young and old mice fed normal diets with old mice fed 35 percent fewer calories starting at about one year old. One year for mice is roughly the physiological equivalent of 40 years in humans.
The animals were injected with a fluorescent dye that changes color when it interacts with a free radical called superoxide. The researchers studied brain slices from the three groups to measure levels of superoxide in specific areas of the brain.
Old mice fed normal diets had significantly more superoxide in several regions of the brain than their young counterparts, particularly in one region implicated in Parkinson's disease, called the substantia nigra. But calorie-restricted old mice did not.
"For the last 20 years there have been studies that suggest free radicals, particularly superoxide, are involved in cumulative damage with aging and that the nervous system may be one of the most vulnerable targets," Dugan says. "Most of that evidence has been indirect, though. By using sensitive, state-of-the-art methods, we were actually able to see which cells are producing excess levels of free radicals."
Dugan and her colleagues then evaluated animals that retained low levels of superoxide to see if they maintained their ability to do a range of behavioral tasks. They found that old, calorie-restricted animals were just as good as young animals at tests of grip strength, coordination and flexibility, like quickly climbing down a pole and hanging upside down from a screen. Old animals fed normal diets were significantly worse at these tasks.
But calorie restriction had almost no effect on several drills used to measure pure cognitive performance. In one test of spatial learning and memory called the Morris water maze, mice are placed in a pool of opaque water and have to learn to find a submerged platform in order to climb out of the pool. Old mice, regardless of their diet, performed much worse than young mice on this task when they were required to learn different consecutive platform locations.
Calorie restriction also did not result in improved performance on a fear-conditioning task. When a tone is matched with an electric shock, young mice eventually learn to "freeze" when they hear the tone. Old mice, regardless of diet, were much worse at this. In fact, the researchers noticed a trend that suggests mice on calorie restriction diets were even worse than old mice on normal diets. The researchers are not sure, though, whether this poor performance is a sign of learning deficits or of hearing problems that often develop in older mice.
"It's interesting to me that calorie restriction does not seem to reverse age-related cognitive impairments," says David Wozniak, Ph.D., research associate professor of psychiatry, who supervised the behavioral studies. "We need to do bigger, more extensive studies to fully understand these findings, but the bottom line is that you don't get uniformly positive results from calorie restriction. I don't think anyone has really stressed this point before, particularly with regard to the lack of effects on cognition."
In addition to validating these findings in larger groups of mice, the team also is exploring the possibility that adjusting other dietary factors may enhance and add to the calorie restriction diet's benefits. The researchers also have begun testing the protective effects of potent antioxidants on aging mice fed normal diets to see whether they too can prevent or reverse some of the effects of aging.
"We believe sensitive signaling pathways that are particularly important in the brain are disrupted by high levels of free radicals and that these disruptions may explain why, under normal circumstances, brain function declines over time," Dugan says. "Fortunately, it would be much easier to reverse a misregulation in signaling than it would to reverse cell damage."