Scientists have found that the ageing process is critically determined by when an animal is able to reproduce, and believe that this might also apply to humans, especially women, who only have a limited number of years in which they can have children.
James Carey, professor of entomology at the University of California at Davis, said that experiments on fruit flies and rats show that reproduction is probably the ``key pacemaker'' of ageing for all animals.
In research on fruit flies, Professor Carey found that flies fed on a diet of only sugar with no protein failed to reproduce and lived much longer than those fed on both sugar and protein, which reproduced and then died while relatively young.
When the sugar-fed flies - equivalent in age to people of 90 - were then fed protein, their life expectancy increased by several weeks, which in human terms would be equivalent to a 90-year-old suddenly given an extra 20 or 30 years of life.
``It was like a rejuvenation. A small environmental manipulation had a huge impact on life expectancy. This is a very general finding and not just applicable to insects. If you shut down reproduction in rodents they also live longer,'' Professor Carey said.
The research, published today in the journal Science, suggests the dietary restriction prevents the animals from reproducing and it is the shutting down of reproduction that was the key pacemaker of ageing, Professor Carey said.
Only women who suffer a severe dietary deficiency that prevents ovulation, and who then return to a normal diet when they have children, are expected to live longer if the results with the fruit fly also apply to humans, Professor Carey said.
``Nuns don't live that much longer than women who have a lot of children. The key here is not the number of children but the rate at which the ovaries are depleted. In principle, if you can subject people to a dietary restriction that shuts down their reproduction, then that is the key,'' he said.
One way of testing the idea on people would be to study women who had suffered a temporary near-starvation diet in their early adolescence and who had then gone on later to have children, Professor Carey said.
``The key here is that this finding is obviously very general. I don't see any reason at all why a general concept cannot apply to humans.''