Want to live longer? Just eat less -- a lot less. Some say that cutting calories may well be the key to longer life that researchers have been seeking for generations.

Caloric Restriction, or "CR," as it's known, is by no means a new theory. For some 60 years, scientists have proven repeatedly that feeding lab animals about one-third fewer calories than normal was a sure-fire way of extending their lifespan. The technique worked consistently, whether the animals were worms or fruit flies, mice or monkeys.

In monkeys, for instance, caloric restriction has enabled some animals to live as much as 40 percent longer than their well-fed peers, and they seem to maintain their youthful vitality, as well. The diet restrictions also seem to inhibit the growth of cancerous tumors and many other age-related diseases.

Willpower: the Ultimate Human Weakness!

But can it really be that simple? Just eat less and live longer? Well, here's the catch: We humans aren't lab animals, and our diets aren't strictly controlled and monitored by researchers. Instead, most of us have unlimited access to food, as our bulging national waistlines can attest. And despite our best dietary intentions, when that aroma of pizza or cheeseburger wafts our way, we are apt to yield to temptation.

Don't despair, though. If you want to live a longer, healthier life, you may not have to live on a diet of spinach and bean sprouts, after all. Once researchers understand the mechanisms behind caloric restriction, they may be able to achieve the same life-extending results without depriving us of our favorite foods.

How Does it Work?

Here's how scientists think caloric restriction works. The food we eat helps maintain the body's metabolism and body temperature. Restricting the number of calories results in a lower body temperature and, in turn, a slower metabolism. Scientists speculate that the slower metabolic rate may retard production of cell-damaging free radicals, which are byproducts of metabolism. As a result, cells appear to sustain less genetic damage at lower body temperatures.

Limiting caloric intake also seems to preserve cells' capacity to reproduce, ensuring a ready supply of healthy new cells. Finally, caloric restriction somehow limits the age-related decline in a key growth hormone, which helps the immune system continue to function at youthful levels, thereby delaying the onset of many diseases.

Caloric restriction should not to be confused with starvation, however. Scientists are careful to include all necessary nutrients for animals on these restricted diets. In other words, think of caloric restriction as under-nutrition, not malnutrition or starvation.

Still, for humans, maintaining caloric restriction poses a daunting challenge to one's willpower. Until recently, only a handful of health nuts were interested in such spartan diet regimens. But as the world's population ages, the issue of caloric restriction in humans is becoming the focus of growing scientific interest.

New Studies

The National Institutes of Health has conducted a number of caloric restriction studies involving primates, whose genetic makeup is quite similar to those of humans. Then, in August 2002, the National Institute on Aging (NIA) announced the results of a 25-year caloric restriction study of more than 700 men, ages 19 to 95.

It is important to point out that the men in this long-term study were not placed on restricted diets. Instead, researchers examined three key "biomarkers," which are physiological indicators of a particular biological function. Researchers examined the men's (1) body temperatures, (2) blood insulin levels, and (3) blood levels of DHEAS, a growth hormone that declines as part of the normal aging process.

The researchers then compared the men's biomarkers with similar indicators from a group of 60 rhesus monkeys, half of which were on calorically restricted diets. For the first time, researchers found a relationship between well-established biomarkers found in calorically restricted animals, and longevity in humans who do not severely limit their calorie intake.

Specifically, they found that men who had naturally lower body temperatures, lower blood insulin levels, and higher levels of DHEAS, are more apt to live longer than their peers. These same three characteristics are present in monkeys on calorically-restricted diets. Researchers noted that the diet-restricted monkeys in this study had just half the death rate of the monkeys in the group that were allowed to eat freely.

George Roth, PhD, lead author of the study, says these findings suggest that caloric restriction does indeed cause metabolic shifts that can affect the rate at which individuals age. What's more, the fact that the men were not practicing caloric restriction is also significant.

"It means there may be other ways to achieve biological hallmarks without having to undergo drastic dietary changes," Roth says. "Although we don't yet know what these pathways are, this finding suggests it may be possible to develop compounds that offer the benefits of caloric restriction without having to resort to it."

Good News For Us?

That's good news for those of us who find it tough to cut back on the meat and potatoes. But it will likely be years before scientists fully understand how caloric restriction affects the aging process, and longer still before these mechanisms can be implemented without drastic reductions in food intake. As always, be sure to consult your physician before making any radical changes in your diet. So for now, why not just try the salad bar instead of the steak? You might just help your cells - and yourself - to a longer life.