Foods laden with sugar and fat appear to reduce levels of a natural brain chemical crucial for learning, say scientists. People whose diets are high in fat and sugar could unwittingly be harming their minds.
The study, carried out at the University of California's Brain Injury Research Centre, is published in the journal Neuroscience. A team led by neurosurgeon Fernando Gomez-Pinilla of UCLA carried out a series of experiments on rats.
One group of rats was fed a high-fat, high-sugar diet and allowed no exercise for two months. The second had the same diet but could exercise on a wheel. The third had a healthy diet and the fourth a healthy diet plus exercise.
The rats fed fatty, sugary foods fared significantly worse than those given healthy foods. However, exercise appeared to counteract the harm caused by too much junk food.
The rats in the first group were found to have a reduced level of a brain chemical called BDNF, which protects the adult brain from damage and allows it to respond to stimuli.
In order to learn or remember, the brain converts electrical impulses into chemical impulses, in a process called a synapse. A lack of BDNF affects the responsiveness of the brain during synapse - and this is associated with cognitive decline.
In memory tests involving a water maze, the rats on the high-fat diet were less able to remember to swim to a platform.
When the platform was removed, the rats on the healthy diet plus exercise spent 70 per cent of their time swimming where the platform had been - as if they were looking for it.
Those on the healthy diet alone spent half their time swimming where the platform should be, as did the rats on a high-fat diet plus exercise.
But the junk food-only group seemed to swim randomly around the pool, demonstrating their poor memory.