But what does all this really mean for humans? Is it really “healthier” to starve oneself, as some people believe? Or will this latest monkey study finally let us off the hook?
Calorie-restriction data in humans has been pretty spotty, for good reason. You try cutting back 30 percent of your food intake, and see how it goes. (Slate’s Emily Yoffe attempted it a few years ago; read her account here.) Most studies have been short-term, and none have measured longevity, for obvious reasons. One of the few true long-term trials in humans came out of the drama-ridden Biosphere project from the late 1990s.
Funded by billionaire Ed Bass, Biosphere was a 3-acre sealed environment built in the Arizona desert that was supposed to simulate life in a space station. The facility would be completely self-sufficient, with greenhouses producing all the crew’s food, as well as the oxygen they’d need to breathe. The expedition scientist, Roy Walford, happened to be a strong proponent and key early researcher of caloric restriction. (He also bore a striking resemblance to Mr. Clean.)
When it became clear early in the Biosphere project that the earthbound “space station” could not grow enough food to feed the crew of eight, Walford took the opportunity to place his fellow crew members on a restricted diet—30 percent lower in calories, but dense in nutrients. In his study based on the two-year experience, Walford reported that the main effect of caloric restriction was to drastically lower his fellow crew members’ cholesterol levels, to 140 and below—well below the average for people in the industrialized world. Walford concluded that a calorie-restricted diet would have the same beneficial effects that he and other scientists had observed in mice.
Walford would never get to test his hypothesis fully; he died of Lou Gehrig’s disease in 2004, and in his later years he blamed his poor health on the Biosphere experience. Other, larger trials of calorie restriction in humans have pointed to similar results: In the short term, it’s been shown to benefit overweight or obese people, which is no surprise. And a longer-term study of voluntary calorie-restrictors has pointed to improved arterial health, as well as better blood glucose management and other markers of aging.
But so far, there’s no evidence that humans gain any longevity benefit from calorie restriction. “That data will not emerge until about 2040,” says Brian Delaney, president of the Calorie Restriction Society.
And when it does, chances are any effect of calorie restriction may vary from person to person, depending on genetics. “It’s complicated,” says Nir Barzilai of Albert Einstein College of Medicine in New York.* “To some of us it might work, and for some of us it might be dangerous.”
Several studies have shown that excessive leanness—seen often in calorie-restricting humans—can be as risky as obesity. Taken together, these studies suggest that the optimal body-mass index is about 25, which is on the verge of being overweight.
But if it’s OK to be almost overweight, it might not pay to go beyond that. Another key difference between the two monkey studies has to do with the definition of “ad libitum.” While the Wisconsin control-group monkeys were allowed to stuff themselves, with the equivalent of an all-you-can-eat buffet for several hours at feeding times, the NIA monkeys were given a fixed amount of food. “You could view it as the Wisconsin monkeys were overindulging, like the rest of the American population,” says Rozalyn Anderson, a member of the Wisconsin team. Compared with their Wisconsin brothers, then, the NIA monkeys in the non-calorie-restricted control group were arguably practicing a mild form of calorie restriction—and that, Anderson suggests, might have made a difference.
For decades, ever since McCay, the holy grail of aging research has been to extend maximum lifespan—to push out the frontiers of human longevity, past 100, 120, or more. But while in theory those limits may be malleable, a careful look at these major primate studies shows that they might not be, in practice. Even so, calorie restriction does seem to reduce—drastically in some cases—one’s risk of developing age-related diseases like cancer and diabetes. So while calorie restriction may or may not make you live longer, overeating and obesity will certainly make you die sooner. And if eating less doesn’t always increase lifespan, it does improve “healthspan,” our allotment of healthy years.
In the next few years, we’re going to learn a lot about how different genetic types respond to medicine, diet, and other things. And while we might not (yet) know how to live forever, more of us will be able to avoid a long, sad decline and will live longer, healthy lives. If we can get there simply by eating the right foods, but not too much, and avoid becoming obese—then just knowing that is a pretty good start.
Correction, Sept. 4, 2012: This article originally misstated the name of Albert Einstein College of Medicine. (Return to the corrected sentence.)