IT IS simple. We age with time, grow old and finally die. Right? Wrong, say researchers. It is not so simple. Contrary to popular perception, ageing can vary from species to species. And latest research conducted on Mediterranean fruit flies (Ceratitis capitata) show that depending on the kind of nutrition they get, these flies can age in two different ways.

And good food, the researchers found, is not always good for health. Researchers J R Carey and his colleagues of the University of California, USA, reported their findings in Science (vol 281, No 1456).

If these flies are fed only on sugar, they seem to get into a 'stand-by' mode where there are reduced chances of death and reproduction, say the researchers. On the other hand, on a rich diet of nutrients and proteins, they start living a 'fast' life, where mortality is low initially, but picks up once reproduction begins. Interestingly, the flies can switch from one 'mode' to the other, and those that switch from 'stand-by' to 'fast' mode seem to live longer.

The details are fascinating, even to the layperson. When fed on sugar, the flies' average life expectancy goes up to 10 days. On nutrient-rich diet, life expectancy is lower and the flies last some 33-odd days. Likewise, the maximum ages also depend on the diet. Those fed on a sugar-only diet can live up to 92 days, while those who eat well can expect to live for 60 days only. Switching flies to the nutrient-rich diet after a month of sparse sugar-only food gives them longevity and 30 days more to live, the researchers discovered.

They also observed that female flies raised on sugar-only diets tended to be poor egglayers. Many were even infertile and only a handful laid eggs. But if raised on a sparse diet for the first few months and then switched to better diet, they could lay eggs even when they were four to five months old - ages to which they would not have lived had they been given good, nutrient-rich diet all along.

Why does this happen? It seems that a poor diet in the initial stages of lifeguards the system from devoting the resources needed for the development of reproductive tissues. Instead of producing poor eggs, the mother goes into her 'stand-by' mode in which the ageing process is slowed down and eggs are not produced. The moment the mother starts getting 'good food', her life expectancy goes up and the reproductive processes begin. However, this is followed by high mortality and low fertility.

Therefore, says Carey, while levels of nutrition and reproductive activity are positively linked, a negative link exists between nutrition and life expectancy. While those in well-fed populations would tend to reproduce sooner but live short lives, those in impoverished situations would outlive their well-fed counterparts and reproduce at an older age. In the longer run, however, the second - impoverished group - would have fewer offspring than the first.