In 1935, two Cornell University food scientists, Clive McCay and Leonard Maynard, published the first in a long series of papers describing the effects on laboratory animals of a particular kind of semi-starvation. The scientists fed lab rats on a diet that was adequate in vitamins, minerals, and protein, but deficient in calories—it provided less energy than previous studies had shown the rats needed to sustain normal activity and growth. Surprisingly, the low-cal rats lived rather longer than similar rats that ate the same food but with additional calories.
A great deal of subsequent research has confirmed and extended this observed link between low-cal diets and longevity, finding, for instance, that it applies to creatures as diverse as small worms, fruit flies, fish, and dogs. Many of the questions this observation raises, however, remain to be answered. Is the life-lengthening effect caused purely by energy deprivation (as McCay and Maynard thought), or do other specific nutritional deprivations—in the case of the fruit flies *, yeast and sugar reduction—play the key role? What's the biological mechanism for the increased longevity, and is it at work in humans?
It is obviously difficult (actually, it is impossible) to determine whether low-cal diets increase human longevity. Who would volunteer to participate in the kind of lifetime study needed to answer the question? But recent research by a Louisiana State University team headed by Drs. Leonie Heilbronn (of the Garvan Institute for Medical Research in Australia) and Eric Ravussin is beginning to shed some light on what happens in the body when people eat energy-deficient diets over an extended period of time. Their most interesting finding is that calorie restriction might slow the breakdown of DNA that occurs as animals show the effects of aging.
The study worked like this: About 50 subjects, all of them healthy, slightly overweight men and women between the ages of 26 and 49, were randomly assigned to one of four groups. For six months, the groups ate 1) a nutritionally adequate diet designed to maintain a constant weight; 2) a similar diet, but with 25 percent fewer calories; 3) a similar diet, but with a 12.5 percent reduction in calories, plus they exercised to increase energy expenditure by another 12.5 percent; or 4) an extremely low-cal diet to drop 15 percent in body weight quickly, and then ate a diet designed to maintain the lower weight.
Older animal and human studies have suggested that mammals that live a long time share 1) higher-than-average levels of dehydroepiandrosterone sulfate (or DHEAS), a material produced by the adrenal gland; 2) lower-than-average levels of insulin; and 3) lower-than-average body temperature. So, the first question that Heilbronn and Ravussin's study addressed was whether six months of calorie restriction changed any of these values. It did—the low-cal diet was associated with lower-than-average insulin levels and with decreased body temperature. But it had no effect on the DHEAS level. We have no idea what these changes really mean for longevity. But they suggest that whatever role elevated DHEAS levels play, it probably doesn't relate much to calorie restriction.
The study found another effect of extended calorie reduction: a subtle turning-down of the metabolism, so that after six months of deprivation, the low-cal diet eaters expended less energy to maintain the same level of physical activity. It isn't clear how—or if—this finding relates to increased longevity, but it suggests that when energy-starved, the body makes unusually efficient use of the calories it has.
Finally, this study produced the striking finding about DNA that may bear on the question of increased longevity. There is considerable evidence that people age as their DNA accumulates damage with the passage of time. During the six months of the study, the DNA of the subjects who ate plenty of calories showed a slow but constant rate of damage, exactly as would be expected in any of us. But the three groups of subjects who consumed low-cal diets showed markedly less damage to their DNA.
The study as a whole doesn't tell us whether calorie restriction will actually increase longevity in humans. But if that turns out to be the case, I'm betting that the mysterious mechanism that staves off DNA damage will turn out to be the long-sought fountain of youth. For the moment, I'm not a bit sure this deprived diet would make my life longer. But I'm pretty sure it would feel longer.