Calorie consumption: The rhesus monkey on the left, Canto, eats a calorie-restricted diet and is 27 years old. The monkey on the right, Owen, consumes a normal diet and is 29.
Of course, as Colman points out, “it’s not a realistic goal for humans to practice caloric restriction.” The ultimate goal of the study, she says, is to better understand the underlying mechanisms of aging in order to learn how people can live healthier, longer lives: “It’s something we use to understand the aging process better.”
There is some evidence that caloric restriction has metabolic and cardiovascular benefits in humans, but data from monkeys are important because these studies are difficult to perform in people, especially over the long term. “Human data are still sketchy–it’s difficult to get controlled experiments in humans,” says Leonard Guarente, a professor of biology at MIT. Even in monkeys, he says, “these are very difficult and long-, long-term studies to do.”
Two big questions remain, the researchers say. First, does caloric restriction extend life span in the primates? “Meaningful maximum-life-span data are probably 15 years away,” says Weindruch. The monkeys in the Wisconsin study fall into two age groups; the average age of the oldest group is 29, which is very old considering that these animals, on average, live to about 25 in captivity. However, the longest a rhesus monkey has been known to live is 40 years. “If the last of the monkeys on caloric restriction die at the same time as the last control monkeys, it means there is only a delay of the onset of disease, but not an extension of life span,” says Luigi Fontana, a research professor at the Washington University School of Medicine in St. Louis who is leading a study of the diet in people.
The other unresolved question is how caloric restriction actually works in the monkeys. Now that they have strong evidence of the diet’s benefits, Weindruch says, his group will establish another group of animals to study the underlying mechanisms.