Less is more if you want to live to a ripe old age, according to research into the ageing process. Several new studies show that reducing food intake also reduces many of the risk factors that lead to heart disease and shorten life. Advanced biochemical studies and genetic research are providing new insights into how the body and its cells change as we grow old.

The hope is that this process can be slowed, or even reversed, holding back the years or at least ensuring a better quality of life for our increasingly elderly population.

This research isn't simply a pursuit of a fanciful fountain of youth. Severe restrictions in the amount of food eaten by lab animals can add almost 50 per cent to their lifespan, according to studies of rats and monkeys.

Limited research over several years into the eating habits of humans also suggests that eating less can lower a range of factors which have a negative impact on longevity, including blood pressure, blood glucose, cholesterol and excessive hormone levels.

"The more you restrict food intake, the more you extend lifespan," said Prof Roy Walford, of the University of California, Los Angeles, who presented his studies of human subjects with other colleagues at the annual meeting of the American Association for the Advancement of Science, now under way in California.

Prof Walford monitored the men and women who participated in the Biosphere II project during which they were enclosed in a massive bubble, a unique habitat which was cut off from the outside world and which was self-sufficient in food.

Participants were kept on a limited diet, Prof Walford explained, about 1,800 calories per day and well below a typical food intake of about 2,400 calories for most developed countries. He monitored a range of factors during the two years inside the bubble and for two years afterwards.

All the participants experienced a lowering of cholesterol, weight loss of 18 per cent in men and 10 per cent in women, lower blood pressure and reduced glucose levels. All of these are risk factors in cardiovascular disease, he said.

"The results showed that humans on a high-quality low calorie diet did the same as rodents in similar studies," Prof Walford said. Reducing calorie intake in certain species of long-lived rats increased longevity from an average of 38 months to 56 months, an almost 50 per cent improvement. The rats had food intake reduced by anything from 10 to 50 per cent. Studies in monkeys, which provided similar results, involved food intake reductions of up to 30 per cent, he said.

These reductions did not cause any impact on either the physical or cognitive powers of the animals, according to Assistant Prof James Nelson, of the University of Texas health science centre in San Antonio. Laboratory rats given the reduced diet were just as able to traverse a maze as rats fed on demand, he said.

Nor were they physically less able. The rats were given access to a running wheel and young animals would run up to two kilometres a night. This dropped to just half a kilometre in the more mature demand-fed rats but the calorie-restricted rats ran between four and five kilometres per night, Dr Nelson said, and they continued to do so throughout their lives.

The reduced diet seemed to slow the ageing process, he said, although how this worked was not yet clear. The research, however, pointed to an improved ability to withstand the regular damage that took place within cells. This in turn slowed the ageing process and extended lifespan.

Dr Sudhir Gupta, of the University of California, Irvine, discussed his studies into apoptosis, the process by which ageing cells commit pre-programmed suicide as a way to keep the body healthy. He had studied how the effectiveness of the immune system declines with age. Apoptosis, he said, was a "very important physiological process" that was essential for health. "It is very tightly regulated." He examined the proteins and the genes responsible for controlling cell death and survival. "We discovered that a number of those genes and proteins that promoted cell death were up-regulated and those promoting cell survival were down-regulated with age," he said. Knowing the proteins involved meant that it may be possible to replace the missing proteins and so prolong life.

These studies have a long way to go before it is established that cutting back on the amount we eat will enable us to live longer. However, the human studies showed that many of the risk factors that contributed to cardiovascular disease were diminished during the two year-period when the Biosphere subjects were on a tightly controlled diet.

Since that time, Prof Walford has remained on his own reduced diet and Dr Gupta has also adopted a reduced calorie diet. Dr Nelson admitted that he was not on a calorie-restricted diet for the same reason that most people weren't - it is just too difficult to stay away from the foods you love, even if they are bad for you in the long run.