Eating a high-fat diet when young could severely hamper a child's mental development, say scientists.

Their experiments on laboratory rats suggest that fatty foods, eaten to excess, cause memory problems.

It has long been thought that older people whose coronary arteries have been hardened by years of poor diet and smoking could be at risk of cognitive decline as the blood vessels supplying the brain also become furred up.

Gordon Winocur and Carol Greenwood, of the Baycrest Centre for Geriatric Care in Toronto, Canada, carried out experiments on young rats.

The one-month-old creatures were either fed a standard laboratory rat diet - with 10% fat included - or a diet rich in either vegetable or animal fat. This derived 40% of its calories from fat, although it had no more calories in total than the standard lab diet.

After a few months of this, at the rat equivalent of human adolescence, the rodents were given mental learning tests.

'Remarkably impaired'

The tests involved learning that pressing a lever every second time it was presented to them produced a food pellet. The challenge was remembering whether or not they had pressed the lever the last time it had appeared, up to 80 seconds earlier.

Some of the rats swiftly learned that pressing the lever every time it appeared was a pointless exercise, and these tended to be those reared on the normal-fat diet.

Winocur told New Scientist magazine: "High-fat diets impair performance on virtually all our measures. It's remarkable how impaired these animals are."

Although the results may not translate directly into the development of the more complex human brain, they may spark further concern among nutritionists. Many young children derive 40% of their calories from fat, most coming from junk food such as pizzas, crisps and hamburgers.

Winocur believes that the fat stops the brain taking up all the glucose energy it needs to develop properly, and that the developing brain may be more susceptible than the adult brain.

High-fat diets in childhood are also blamed, alongside a couch-potato lifestyle, for producing an epidemic of childhood obesity in the UK. The number of adults who are obese has tripled over the last 20 years, and is still rising.