A daily dose of caffeine blocks the harmful effects of high cholesterol that scientists have linked to the most common form of dementia.

The study found that caffeine can also defend the blood-brain barrier from the damage that occurs with a high-fat diet.

The barrier protects the brain from the blood that flows around the rest of the body.

A leak in this barrier occurs in disorders such as Alzheimer’s.

There are around 700,000 people in Britain with dementia, with two-thirds having Alzheimer’s. But it is predicted that the toll of sufferers will double within a generation and the cost of caring for dementia victims is set to treble to £16billion in the next 25 years.

In the new study, researchers from North Dakota university in America gave rabbits three milligrammes of caffeine each day, the equivalent of a cup of coffee for an average person.

The animals were also fed a cholesterol-enriched diet.

The scientists also studied a group of rabbits that were not given any caffeine. Tests after 12 weeks showed that the blood-brain barrier was significantly more intact with the animals that had the caffeine. Professor Jonathan Geiger, one of the study’s authors, said: “Caffeine appears to block several of the disruptive effects of cholesterol that make the blood-brain barrier leaky.

“High levels of cholesterol are a risk factor for Alzheimer’s, perhaps by compromising the protective nature of the blood-brain barrier.

“For the first time we have shown that chronic ingestion of caffeine protects the blood-brain barrier from cholesterol-induced leakage.”

He added: “Caffeine is a safe and readily available drug and its ability to stabilise the blood-brain barrier means it could have an important part to play in therapies against neurological disorders.”

Coffee appears to protect against a breakdown of the barrier by maintaining levels of vital proteins.

These bind the cells of the barrier tightly to each other to stop toxins and infections in the blood crossing into the brain.

Dr Susanne Sorensen, head of research at the Alzheimer’s Society, said: “This is the best evidence yet that the caffeine equivalent to one cup of coffee a day can help to protect the brain against cholesterol.

“In addition to its effect on the vascular system, elevated cholesterol levels cause problems with the blood-brain barrier. This barrier, which protects the brain from toxins and infections, is less efficient prior to brain damage caused by Alzheimer’s or strokes.

“This research in animals shows that caffeine can help keep the blood-brain barrier in good order, even with a diet rich in cholesterol.”

But she said more research was now needed to find out if the results would be the same in humans.

Last year, a study of 600 elderly men found that drinking three cups of  coffee a day could significantly reduce the risk of Alzheimer’s.

Despite the rising number of dementia sufferers in Britain, Labour has halved the funding of research into the condition in the past five years. While £289 per cancer sufferer is spent each year on tumour research, only £11 per victim goes on studying dementia.