Dr. Weindruch said the study data offered “very encouraging” signs that resveratrol could duplicate in people some of the effects of caloric restriction.

Critics, however, are not yet ready to accept that the rhesus study proves caloric restriction works in primates.

If caloric restriction can delay aging, then there should have been significantly fewer deaths in the dieting group of monkeys than in the normally fed comparison group. But this is not the case. Though a smaller number of dieting monkeys have died, the difference is not statistically significant, the Wisconsin team reports.

The Wisconsin researchers say that some of the monkey deaths were not related to age and can properly be excluded. Some monkeys died under the anesthesia given while taking blood samples. Some died from gastric bloat, a disease that can strike at any age, others from endometriosis. When the deaths judged not due to aging are excluded, the dieting monkeys lived significantly longer.

Some biologists think it is reasonable to exclude these deaths, but others do not. Steven Austad, an expert on aging at the University of Texas Health Science Center, said some deaths could have been due to caloric restriction, even if they did not seem to be related to aging. “Ultimately the results seem pretty inconclusive at this point,” Dr. Austad said. “I don’t know why they didn’t wait longer to publish.”

Leonard Guarente, a biologist who studies aging at the Massachusetts Institute of Technology, also had reservations about the findings. “The survival data needs to be fleshed out a little bit more before we can say that caloric restriction extends life in primates,” Dr. Guarente said. In mouse studies, people just count the number of dead animals without asking which deaths might be unrelated to aging, he said.

The second rhesus monkey study, being conducted by the National Institute on Aging, is not as advanced as the Wisconsin study. The researchers have not yet reported on the number of deaths in the dieting and normal monkey groups. But there are signs that the immune system is holding up better in the dieting group, said Julie Mattison, the leader of the institute’s study.