Many people may be able to extend their lives by eating smaller amounts of food - although that food should be rich in vitamins and other nutrients.

These are the preliminary findings of Professor Roy Walford of the University of California, Los Angeles, who spent two years in a giant, hermetically sealed greenhouse - known as Biosphere Two - in the Arizona desert.

His evidence was presented to the annual meeting of the American Association for the Advancement of Science in Anaheim, USA.

Meagre rations

The study data covers eight individuals, Professor Walford included. They were all forced to exist on limited, albeit highly nutritious, rations.

Professor Walford monitored their physiology both before and during the experiment and said changes in cholesterol levels, body weight and blood pressure - among other factors - mimicked those seen in long-lived rodents on severely restricted diets.

Almost a century of research has shown that diets very low in calories can extend the healthy lives of laboratory rats and mice by a startling degree. Rats given half the food a normal rat would chose to eat live up to 70% longer than usual. They are also physically and mentally more active for much longer.

Hunger pangs

This knowledge, combined with his Biosphere work, has now led Professor Walford to argue that a similar postponement of ageing could be possible in humans.

Professor Walford conceded his results were preliminary but claimed a clearer picture will emerge from on-going experiments in monkeys in about five year's time. Asked to explain the shorter life expectancies of less well-fed populations in developing countries, he said they were partly the result of nutritionally poor diets, not small diets.

At 74 years old, Professor Walford is still on his well balanced, calorie-restricted diet. He said any hunger pangs are more than compensated by the greater physical and mental energy he experienced in his daily life.