Rodents, yeast, and roundworms all have something in common: They live longer when they consume less. Now a primate has joined the calorie-restriction club. After 20 long years of waiting, scientists have concluded that rhesus monkeys that eat nearly a third less food than normal monkeys age more slowly. The results come as close as any can to proving that calorie restriction could significantly slow aging in humans--even if such a lean diet would not appeal to most of us.
Researchers first discovered the connection between lean diets and extended life spans in a 1935 study of calorie-restricted rats. In the past decade, studies in yeast and worms have pinpointed some genes that may be responsible. Scientists believe the genes somehow ramp up systems to protect an organism from environmental stress and may have evolved to help organisms survive in environments where food was scarce. In rodent studies, calorie restriction can extend life span by 20% to 80%. Whether calorie restriction also slows aging in primates wasn't known, however.
Two decades ago, three different research groups in the United States decided to fill this gap. The groups have previously published updates on their monkeys' health, but in tomorrow's issue of Science, one of them reports survival data from their colony of 76 rhesus monkeys. The team, led by gerontologist Richard Weindruch of the University of Wisconsin, Madison, began monitoring the animals when they hit 7 to 14 years old--monkey adulthood. Researchers allowed half of the monkeys to eat as much as they wanted during the day, while restricting the other half to a diet with 30% fewer calories. The scientists gave the restricted monkeys vitamin and mineral supplements to ensure they did not suffer malnutrition and treated any animals that fell sick, says Weindruch.
Studying aging in monkeys takes patience. Mice and rats only live for a couple of years, while these monkeys can live to 40, and the average life span is 27 years. Now that the surviving monkeys have reached their mid- to late 20s, the Wisconsin group could glean how calorie restriction was affecting their life span. Sixty-three percent of the calorie-restricted animals are still alive compared to only 45% of their free-feeding counterparts. For age-related deaths caused by illnesses such as cardiovascular disease and cancer, the voracious eaters died at three times the rate of restricted monkeys: 14 versus five monkeys, respectively. Another seven control and nine lean monkeys died from causes not related to aging such as complications from anesthesia or injuries. Leaner diets also reduced muscle and brain gray matter deterioration, two conditions associated with aging. (The team has not yet studied cognitive differences between the two groups.)
Researchers who study aging are split on how much stock to put in the study. Leonard Guarente, a molecular biologist at the Massachusetts Institute of Technology in Cambridge who has studied aging in yeast, believes that not enough monkeys have died yet to make definitive comparisons between the two groups. As of March, when Weindruch's group submitted the paper, about half of the colony was still alive. "The gap [in survival rates] may separate more, but it's still too early to tell," Guarente says. On the other hand, molecular biologist Matthew Kaeberlein of the University of Washington, Seattle, thinks the gap as it stands now is still compelling. He points to the difference in age-related deaths between the two groups as the more relevant statistic. "The fact that they see a significant effect at this point suggests there will be a robust effect when they finish the study," he says.
Weindruch and his collaborators plan to continue monitoring the remaining monkeys, which could stretch the study's length past 3 decades. "If we reach the 40-year-old life span, the study could continue for another 15 years," Weindruch said. "That would probably round out my career."