Even a slight decrease in calories may lead to longer lifespans.


Most people would not object to living a few years longer than normal, as long as it meant they could live those years in good health. Sadly, the only proven way to extend the lifespan of an animal in this way is to reduce its calorie intake. Studies going back to the 1930s have shown that a considerable reduction in consumption (about 50%) can extend the lifespan of everything from dogs to nematode worms by between 30% and 70%. Although humans are neither dogs nor worms, a few people are willing to give the calorie-restricted diet a try in the hope that it might work for them, too. But not many—as the old joke has it, give up the things you enjoy and you may not live longer, but it will sure seem as if you did.



Now, though, work done by Marc Hellerstein and his colleagues at the University of California, Berkeley, suggests that it may be possible to have, as it were, your cake and eat it too. Or, at least, to eat 95% of it. Their study, to be published in the American Journal of Physiology—Endocrinology and Metabolism, suggests that significant gains in longevity might be made by a mere 5% reduction in calorie intake. The study was done on mice rather than people. But the ubiquity of previous calorie-restriction results suggests the same outcome might well occur in other species, possibly including humans. However, you would have to fast on alternate days.




Why caloric restriction extends the lifespan of any animal is unclear, but much of the smart money backs the idea that it slows down cell division by denying cells the resources they need to grow and proliferate. One consequence of that slow-down would be to stymie the development of cancerous tumours.



Cancer is the uncontrolled growth of cells. For a cancer to develop efficiently, it needs multiple mutations to accumulate in the DNA of the cell that becomes the tumour’s ancestor. To stop this happening, cells have DNA-repair mechanisms. But if a cell divides before the damage is repaired, the chance of a successful repair is significantly reduced. A slower rate of cell division thus results in a slower accumulation of cancer-causing mutations.



At least, that is the theory. Until now, though, no one has tested whether reduced calorie intake actually does result in slower cell division. Dr Hellerstein and his team were able to do so using heavy water as a chemical “marker” of the process.



Heavy water is heavy because the hydrogen in it weighs twice as much as ordinary hydrogen (it has a proton and a neutron in its nucleus, instead of just a proton). Chemically, however, it behaves like its lighter relative. This means, among other things, that it gets incorporated into DNA as that molecule doubles in quantity during cell division. So, by putting heavy water in the diets of their mice, the researchers were able to measure how much DNA in the tissues of those animals had been made since the start of the experiment (and by inference how much cell division had taken place), by the simple expedient of extracting the DNA and weighing it.



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