For centuries mankind has sought the secret of a long and healthy life.

And for centuries it seems we were looking in the wrong place. Forget exotic pills and potions, the key to prolonged life could be as simple as a glass of water. Scientists believe 'heavy water' enriched with a rare form of hydrogen could add as much as ten years to life.

And by also modifying foods, such as steak and eggs, with the hydrogen the way could be cleared to allowing us to eat and drink our way to a healthy old age.

The idea is the brainchild of Mikhail Shchepinov, a former Oxford University scientist.

It centres on fortifying the body's tissues and cells against attack and decay caused by free radicals, dangerous chemicals produced when food is turned into energy. Such 'attacks' on proteins are particularly damaging and have been linked to cancer, Alzheimer's and Parkinson's.

Dr Shchepinov's theory is based on deuterium, a naturally-occurring isotope, or form of hydrogen, that strengthens the bonds in between and around the body's cells, making them less vulnerable to attack.

He found that water enriched with deuterium, which is twice as heavy as normal hydrogen, extends the lifespan of worms by 10 per cent. And fruitflies fed the 'water of life' lived up to 30 per cent longer.

He now believes people could also benefit from the sweet-tasting water, or from deuterium-enriched 'heavy foods'.

Shchepinov, however, is the first researcher to link the effect with ageing. It dawned on him that if ageing is caused by free radicals trashing covalent bonds, and if those same bonds can be strengthened using the isotope effect, why not use it to make vulnerable biomolecules more resistant to attack? All you would have to do is judiciously place deuterium or carbon-13 in the bonds that are most vulnerable to attack, and chemistry should take care of the rest.

In early 2007 Shchepinov wrote up his idea and submitted it to a journal called Rejuvenation Research. Unbeknown to him, the journal's editor is controversial gerontologist Aubrey de Grey of the Methuselah Foundation in Lorton, Virginia, who is well known for supporting ideas other gerontologists consider outlandish. De Grey sent the paper out for review and eventually accepted it (Rejuvenation Research, vol 10, p 47).

In the paper, Shchepinov points out that there is masses of existing science backing up his ideas. Dozens of experiments have proved that proteins, fatty acids and DNA can be helped to resist oxidative damage using the isotope effect.

Shchepinov's paper brought the idea to a wider audience, including successful biotechnology entrepreneurs Charles Cantor and Robert Molinari. Impressed, they teamed up with Shchepinov to set up a company called Retrotope, with de Grey as a scientific advisor.

It was around this time that I first got in touch with Shchepinov. I'd never heard of the isotope effect, and de Grey's involvement made me cautious. But there was something in the idea that intrigued me, and I kept on coming back to it.

There were obvious objections to the idea. For one, how do you get the isotopes to exactly the sites where you want them? After all, the human body contains trillions upon trillions of chemical bonds, but relatively few are vulnerable to free-radical damage. And what about safety - swallowing mouthfuls of heavy isotopes surely can't be good for you, can it? That, of course, is how I ended up sharing a teaspoon of heavy water with Shchepinov.

Neither, it turns out, is a big problem. Some heavy isotopes are radioactive so are obviously ruled out on safety grounds - hydrogen-3 (tritium) and carbon-14, for example. Others, notably deuterium and carbon-13, are just as stable as hydrogen and carbon-12. Both occur in small amounts in nature and are a natural component of some biomolecules in our bodies (see "Heavy babies").

Deuterium and carbon-13 also appear to be essentially non-toxic. Baby mice weaned on a highly enriched carbon-13 diet are completely normal, even when 60 per cent of the carbon atoms in their body are carbon-13. Deuterium also has a clean bill of health as long as you don't go overboard. Decades of experiments in which animals were fed heavy water suggest that up to a fifth of the water in your body can be replaced with heavy water with no ill effects.

Similar experiments have been done on humans, albeit with lower levels of deuterium. One recent experiment kept humans on a low-level heavy-water diet for 10 weeks, during which their heavy-water levels were raised to around 2.5 per cent of body water, with no adverse effects (Biochimica et Biophysica Acta, vol 1760, p 730). The researchers also found that some deuterium became incorporated into proteins.

Heavy water, however, isn't completely safe. In mammals, toxic effects start to kick in around the 20 per cent mark, and at 35 per cent it is lethal. This is largely down to the isotope effect itself: any protein in your body has the potential to take up deuterium atoms from heavy water, and eventually this radically alters your entire biochemistry. You'd have to drink a vast amount to suffer any ill effects - my 5 millilitres did me no harm whatsoever - but even so, Retrotope is not advocating heavy water as an elixir of youth.

Instead, it wants to package up heavy isotopes in what Shchepinov calls "iFood". This method has huge advantages, not least because it allows the heavy isotopes to be targeted to the most vulnerable carbon-hydrogen bonds. Of the 20 amino acids used by humans, 10 cannot be made by the body and must be present in the diet. That means if you supplement your diet with essential amino acids that have already had their vulnerable bonds strengthened, your body's proteins will have these reinforced amino acids incorporated into them. Some of the building blocks of fats and DNA can also only be acquired via your diet, which means they too can be targeted using the iFood approach. (read more)