Two experts on aging, Jan Vijg of the Albert Einstein College of Medicine and Judith Campisi of the Lawrence Berkeley National Laboratory, argued recently in Nature that the whole phenomenon of caloric restriction may be a misleading result unwittingly produced in laboratory mice. The mice are selected for quick breeding and fed on rich diets. A low-calorie diet could be much closer to the diet that mice are adapted to in the wild, and therefore it could extend life simply because it is much healthier for them.

“Life extension in model organisms may be an artifact to some extent,” they wrote. To the extent caloric restriction works at all, it may have a bigger impact in short-lived organisms that do not have to worry about cancer than in humans. Thus the hope of mimicking caloric restriction with drugs “may be an illusion,” they write.

To decide whether life extension by caloric restriction is an artifact of mice in captivity, why not try it on wild mice? Just such an experiment has been done by Steven N. Austad of the University of Texas Health Science Center. Dr. Austad reported that caloric restriction did not extend the average life span of wild mice, suggesting the diet’s benefits are indeed an artifact of mice in captivity. But others interpret his results differently. Richard A. Miller of the University of Michigan, says the maximum life span of the wild mice was extended, and so the experiment was a success for caloric restriction.

Laboratory mice are very inbred, and researchers can get different results depending on the breed they use. To put the mouse data on a firmer footing, the National Institute on Aging has set up a program to test substances in three labs simultaneously. Its first round of candidate agents for reversing aging include green tea extract and two doses of resveratrol.

The resveratrol tests are still under way, but last month the results with another substance, the antifungal drug rapamycin, were published. Rapamycin was found to extend mice’s lives significantly even though by accident the mice were already the equivalent of 60 years old when the experiment started.

Rapamycin has nothing to do with caloric restriction, so far as is known, but the study provided striking proof that a chemical can extend life span.

Another result, directly related to the caloric restriction approach, emerged last month from a long-awaited study of rhesus monkeys kept on such a diet. The research was led by Richard Weindruch of the University of Wisconsin. As fellow primates, the monkeys are the best possible guide to whether the mouse results will apply in people. And the answer they gave was ambiguous.