While monkeys in a control group eat as much as they want, those in the experimental group are fed 30 per cent fewer calories. Scientists say they are more active, weigh 20 per cent less, and seem to have greater resistance to diseases, especially those of ageing such as diabetes.
The next step would be to try the same principle on humans, using a diet that contains all the necessary vitamins and nutrients but one- third fewer calories.
That is the bad news: the good news is that you would no longer have to bother about ratios of fats, proteins and carbohydrates."The variable that seems to be most important is the total amount of calories," said George Roth, of the National Institute on Ageing in Baltimore.
If these findings are confirmed, they could lead to a change in feeding habits. "There are some non-Western cultures where the idea is that you should get up from your meal feeling 80 per cent full," said Dr Roth. "In developed countries such as the US, people tend to eat until they don't feel hungry. Maybe we need to rethink that," he told the American Association for the Advancement of Science, the world's biggest science gathering.
However, developing countries, many of which are prone to famine, do not provide a good example of how a restricted diet could help: "Most don't have good health care against disease, and their diets don't contain the necessary vitamins and nutrients," Dr Roth told the Association meeting.
Nobody knows yet why the method works. The monkeys' body temperature is between 0.5C and 1C lower than normal, leading scientists to speculate that their metabolism has been"reset", possibly at the cellular level, so that they use the available energy more efficiently.
So that could be the key to living longer - or is it just that it would seem longer? In order to benefit from the reduced-calorie method, you would have to stick to it all your life.
Some early tests when the experiments were carried out with mice suggested that they were depressed, compared with their satiated companions. Dr Roth's team is now beginning a further series of experiments to test their monkeys' alertness and mental agility. But he does know that they show plenty of activity and alertness at least twice a day: "At mealtimes."