Rhesus monkeys in India. Credit: Thomas Schoch.

Caloric restriction is a diet in which humans or animals eat fewer calories than they normally would–anywhere from 20% to 50% less. It has become a fad diet for some, not just for losing weight (which is the obvious result), but also for improving overall health and extending one’s lifespan because of experiments that have been carried out particularly with rodents since the 1930s.

Caloric restriction (or CR) produces results in many species of mammals, but perhaps the most striking is in mice, where we see a very surprising result: if you feed mice half as much as usual, they live twice as long.

Now, proponents of the CR diet swear that by eating less–if you get your vitamins and maintain a healthy weight–you can live longer and healthier. But while their may be some health benefits when the diet is maintained correctly, you don’t see any Tibetan monks or members of other ascetic religious movements across the centuries living to 150. Obvious, caloric restriction doesn’t work quite as well in humans as it does in mice. So what gives?

Well, a recent study by the National Academy of Sciences this week presents an intriguing new answer. According to their analysis, it turns out that humans already eat half as much as other mammals. Apparently, we and other primates have very slow and efficient metabolisms that allow us to live longer lives.

This is already in line with something else we’ve noticed about mammals. Small mammals live very short lives, while large mammals live long lives, but most mammals live for about 1 billion heartbeats.

Consider this chart. A mouse’s heart beats 450-750 times per minute. Mice can live for about 3 years in captivity. That works out to between 700 million and 1.2 billion heartbeats. An elephant’s heart beats 25-35 times per minute. Elephants live 60-70 years. That’s between 700 million and 1.2 billion heartbeats.

But the average human can live for 3 billion heartbeats when well-cared for. And other primates are no slouches either. Our chimpanzee cousins can live for 2-2.5 billion heartbeats. It would appear that our slower primate metabolisms have already taken care of the easy caloric restriction solutions for longer lives for us. That’s bad news for anti-aging research, but it’s good news for human civilization. Having at least some people live to see their grandchildren grow up it very valuable for passing knowledge down from generation to generation. It’s a big part of what let us advance to the point of being able to do useful anti-aging research in the first place.

So don’t give up yet. Caloric restriction may be (mostly) a bust, but I’d say we have a decent chance of fixing some key diseases of aging in this century.

And then maybe we can get to 4 billion heartbeats.