Want to live a long life? Then eat your calories

But other researchers still think that it does, and one of the authors of the new study, Julie A Mattison, says there is still a bit of hope. The study is continuing until the youngest monkeys are 22 years old. While the data almost certainly rule out any notion that the low-calorie diet will increase average lifespans, there still is a chance that the study might find that the diet increases the animals’ maximum lifespan, she says.

Meanwhile, some others say that the Wisconsin study makes them reluctant to dismiss the idea that low-calorie diets result in longer life. “I wouldn’t discard the whole thing on the basis of one study when another study in the same species showed an increase in life span,” says Eric Ravussin, director of the nutritional obesity research centre at the Pennington Biomedical Research Center in Louisiana. “I would still bet on an extension of life.”

The idea that a low-calorie diet would extend life originated in the 1930s with a study of lab rats. But it was not until the 1980s that the theory took off. Scientists reported that in species ranging from yeast to flies to worms to mice, eating less meant living longer. And, in mice at least, a low-calorie diet also meant less cancer. It was not known whether the same thing would hold true in humans, and no one expected such a study would ever be done. It would take decades to get an answer, to say nothing of the expense and difficulty of getting people to be randomly assigned to starve themselves or not.

Researchers concluded the best way to test the hypothesis would be through the monkey studies at the University of Wisconsin and the National Institute on Aging, although the animals would have to be followed for decades. It was a major endeavour. The National Institute on Aging study involved 121 monkeys, 49 of which are still alive, housed at a facility in Poolesville, Maryland.

Those that got the low-calorie diet did not act famished, de Cabo says. They did not gobble their food, for example, but ate at the same speed as the control animals, even though their calories had been cut by 30 per cent.

As the studies were under way, some human enthusiasts decided to start eating a lot less, too. In those same years, though, studies in mice began indicating there might not be a predictable response to a low-calorie diet. Mice that came from the wild, instead of being born and raised in the lab, did not live longer on low-calorie diets. And in 2009, a study of 41 inbred strains of laboratory mice found that about a third had no response to the diets. Of those that responded, more strains had shorter life spans than had longer ones when they were given less food.