In the 1930's, researchers discovered that they could extend the life of rats by 33% if they limited them to a very low-calorie diet. Not only did the animals live longer, they suffered fewer late-life diseases, appeared more youthful, and their bodies' biological aging processes were slowed. Since that time, scientists have produced similar life-extending results with many other creatures, ranging from fruit flies to fish.

Is caloric restriction per se responsible for the observed benefits, or some other factor that is reduced when calories are cut? Studies show that limiting either fat, protein or carbohydrate, without accompanying caloric reduction, does not seem to increase maximum life span. Nor does supplementation with extra antioxidants and multivitamins. Varying the types of fats, carbohydrates and proteins ingested also had no effect. In fact, no other intervention except caloric restriction has yet been shown to slow aging.

Can caloric reduction work in humans? Probably, but there is no definite proof. Human life spans are long compared to many other creatures, and longevity studies can . . . well, take a lifetime. Then, there is the added difficulty of finding human volunteers willing to stay a little bit hungry their whole lives. Studies with primates, our close genetic cousins, provide some clues. Investigations on monkeys have been underway since 1987, and preliminary results suggest that caloric restriction increases both health and life span in primates. Biomarkers of aging, such as insulin levels, glucose levels and blood pressure, lead researchers to conclude that monkeys eating less age more slowly.

In most studies of this nature, calories are restricted by 30 to 50% of what the animal would normally eat. Care is taken to see that enough vitamins, minerals, protein and fat are ingested for the proper functioning of tissues. On this regime, monkeys seem healthy and happy, albeit anxious for their meals.

Here in the U.S., the participants of the Biosphere 2 experiment were forced to eat a low-calorie diet for two years because their food production was less than projected. They experienced the same anti-aging trends in biomarkers as were found in the monkey experiments.

There are many hypotheses to explain the life-extension power of a low calorie diet. The one that has the most support posits that low calorie intake reduces the amount of free radical damage to cellular mitochondria, although no one knows the mechanism of how this might occur.

While it seems probable that caloric restriction is an effective way to prolong life, researchers warn of some pitfalls for those attempting such a regime. Care should be taken that the diet is adequate in vitamins, minerals, protein and other nutrients. In addition to suffering hunger pangs, if the diet is too severe, it also is possible that the ability to handle stresses, such as cold temperatures or infection, could be compromised. Women may become less fertile or stop ovulating, and this might increase the risk of osteoporosis and loss of muscle mass later in life. Anyone under 20 years old should not engage in caloric restriction.

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