How could the two studies arrive at such different conclusions?

At the beginning of the Wisconsin experiment, its designers decided the control monkeys should eat like the American population at large — as much as they wanted of a not particularly healthy diet, loaded with sucrose, and in which all foods were laboratory purified. At the National Institute on Aging, by contrast, the control monkeys were fed whole foods and were given fixed portions, based on what they naturally ate before the study started.

The upshot is that even their control animals had a diet that was restricted in calories, at least to a moderate extent, Rozalyn M. Anderson and colleagues from Dr. Weindruch’s team say in the new report. So no wonder the Baltimore test monkeys fared little better than the controls — both were benefiting from caloric restriction, Dr. Anderson says. Moreover, the Baltimore study indicates that the minor restriction of calories may be just as effective as significant restriction. If so, “this would be an extremely important discovery,” Dr. Anderson writes.

Dr. de Cabo said in an interview that it was appropriate for the Weindruch team to write such a paper, but that it confused the issue. The two teams are working on a joint report that he hopes will better explain the differing results.

The best way to design a feeding study is to have healthy controls, Dr. de Cabo said, and his animals were much healthier than the Wisconsin control monkeys. Caloric restriction does not work all the time, in his view, but is context dependent, meaning it works with some individuals and some diets, so the task ahead is to find out who will benefit. There is no doubt that with an overweight population, a 10 percent reduction in body weight would have tremendous health benefits. “But will that improve longevity?” Dr. de Cabo asked. “That’s a question that remains to be seen.”

The varying conclusions drawn by the two teams put emphasis on a difference in their studies’ designs. Is it better to have healthy monkeys as controls, or ones that are allowed to get obese and prone to diabetes?

Dr. Anderson said that “in hindsight, it’s great that the studies differed in design,” because more can be learned from them.

But Steven Austad, an expert on aging at the University of Alabama at Birmingham, said it would have been better if both studies had used the same kind of controls, whether healthy monkeys or those allowed to feed until obesity.