A DIET that allows scientists to teach old dogs new tricks has been developed in America, raising the prospect that healthy eating could protect the human brain against Alzheimer’s disease and other mental signs of ageing.

When nine-year-old beagles — they are in late canine middle age — were fed a cocktail of dietary supplements, they showed significant improvement in a mental agility test in which performance usually dips sharply with age.

The findings, from a study led by Carl Cotman of the University of California at Irvine, suggest that extra antioxidant compounds included in the diet protect brain cells against damage from the ageing process of a sort known as oxidative stress. Cells that would normally die, making the dogs slower and more easily confused, survive and thrive, keeping the brain alert and active.

If the cocktail has similar effects on human beings it could potentially be used to help to stave off Alzheimer’s and other forms of dementia and age-related cognitive decline, Dr Cotman told the American Association.

The beagles in the experiment were given supplements of vitamins C and E, along with compounds called alpha lipoic acid and acetyl carnitine, and fruit and vegetables.

Oranges, kiwi fruit and broccoli are good sources of Vitamin C; Vitamin E can be found in chick peas, sweet potato and avocados; lipoic acid can be found in spinach, liver and brewer's yeast, while acetyl carnitine is in red meat, chicken, white fish and milk.

The same combination, the ingredients of which are generally available in health food shops, was used two years ago by Bruce Ames, of the University of California at Berkeley, to rejuvenate ageing rats.

In Dr Cotman’s test, about 70 dogs were trained to pick the odd one out of a collection of objects, and were rewarded with food if they did so correctly. Animals aged three normally perform much better than animals aged 9, but the dietary regime almost completely eliminated any difference between the two groups.

“What we found is that we can basically improve learning and memory in these ageing animals so that they can do much more complicated tasks and make many fewer mistakes,” Dr Cotman said.

“The data really startled us. The control (animals) continued to get worse and the other animals (on the diet) actually got better than even the beginning baseline performance.”

While dogs do not suffer from Alzheimer’s, there were encouraging indications from the study that the diet might protect against the brain condition in human beings. The brains of dogs on the diet had lower accumulations of a protein called beta-amyloid, which builds up into sticky “plaques” in Alzheimer’s patients.

A separate study, presented by Mark Mattson, of the US National Institute on Ageing, offers evidence that diet plays an important part in the onset of Alzheimer’s. Dr Mattson’s team has discovered that cholesterol and a fat called ceramide seem to trigger a “neurodegenerative cascade” that destroys the brain cells of Alzheimer’s patients. The results suggest that stopping the accumulation of these fats in the bloodstream could help to protect against Alzheimer’s.

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