(Daily Mail) -- In the genetic lottery of life expectancy, you might think 100 is a pretty lucky number.

Now it's just got luckier.

Scientists have discovered that a gene already known to treble your odds of living to 100 may also ward off Alzheimer's disease.

One in five of us is dealt this genetic hand that promises to extend our lives without the loss of mental agility.

The gene is the first to be identified that actually cuts the odds of Alzheimer's disease rather than raising them.

Its discovery could pave the way for new drug treatments to combat the devastating illness.

More than 700,000 Britons have Alzheimer's disease and other forms of dementia and the number of cases is expected to double within a generation. There is no cure and existing drugs, which raise levels of key brain chemicals, do not work for everyone and their effects wear off over time.

Researchers from the Albert Einstein College of Medicine in New York tracked the health of more than 500 elderly men and women for four-and-a-half years.

All were free of dementia at the start of the study but 40 had developed it by the end.

Blood samples showed that the CETP gene, already known to treble the odds of living to 100, also cut the odds of dementia by 70 per cent.

Genes come in pairs - and it was those with two copies of the 'centenarian' version who benefited, both from the added longevity and the Alzheimer's protection.

They also suffered less agerelated memory decline, the Journal of the American Medical Association reported. Researcher Dr Amy Sanders said: 'We found that people with two copies of the longevity variant had slower memory decline and lower risk for developing dementia and Alzheimer's disease. More specifically, those who carried two copies of the favourable variant had a 70 per cent reduction in their risk for developing Alzheimer's disease compared with those who didn't.' Around one in five of those studied had the required pair of 'centenarian' genes.

Researcher Dr Richard Lipton said: 'Most work on the genetics of Alzheimer's disease has focused on factors that increase the danger.

'We reversed this approach and instead focused on a genetic factor that protects against age-related illness, including both memory decline and Alzheimer's disease.' The CETP gene affects levels of artery-clogging cholesterol in the body.

The 'centenarian' version raises levels of 'good' cholesterol, cutting the risk of heart disease and, it appears, dementia. A pill that acts in same way could help people live longer, healthier lives.

Separate research has suggested that drugs widely used to treat high blood pressure may also be of use in dementia.

Those taking ACE inhibitor and angiotensin receptor blocker pills had half the risk of the disease, according to a report in the British Medical Journal.

The global population of centenarians is expected to rise from 340,000 today to almost 6million by the middle of the century. The UK share will rise from just over 10,000 today to 280,000.

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