Of all the potential interventions to increase longevity studied by scientists and attempted by the rest of us, the only one that has worked reliably under laboratory conditions is caloric restriction, or a diet with about two-thirds the normal number of calories. Called "undernutrition without malnutrition," caloric restriction has been shown to extend both the maximal and average life spans of worms, insects, and mice.

Recognizing that the benefits demonstrated in mice may or may not translate to humans, scientists at the National Institute on Aging began studying caloric restriction in nonhuman primates (monkey species) in the 1980s. Researchers at the University of Wisconsin began a similar study at the University of Wisconsin in the 1990s. Because primates are so closely related to us (chimpanzees, for example, have DNA that matches humans 98% of the time), what happens in them with caloric restriction could happen in us. But because primates have a relatively long natural life span, only today, after over a decade of research, are results coming in.

Not surprisingly, monkeys placed for years on calorie restricted diets weigh less and have less total body fat than their peers with normal diets. But the calorie-restricted monkeys also have lower body temperatures, lower fasting blood sugar and insulin levels, and their blood cholesterol and triglyceride levels are lower. Interestingly, they have higher levels of a form of HDL, the so-called "good" cholesterol. In humans, all of these traits are associated with a decreased risk of heart disease, so these results in our close relatives are provocative.

Still, before any of us jump on the calorie restriction bandwagon, and find ourselves weak, cold, tired, and constantly hungry, we need to wait until we know that these monkeys actually live longer and healthier lives. And then we need to wait and see if calorie restricted people actually live longer and healthier, as well.