Exercising may help boost memory because it triggers a protein which boosts brain cell growth, scientists believe.

For several years, researchers have noticed that aerobic exercise, of the kind which gets the heart pumping, also appears to improve memory and learning. But nobody knew how.

Now researchers at the National Institute on Ageing in the US have discovered that when muscles exercise they produce a protein called cathepsin B which travels to the brain and triggers neuron growth.

The team has also shown that the levels of the protein soars when humans exercise.

"Overall, the message is that a consistently healthy lifestyle pays off,” said senior author Dr Henriette van Praag, a neuroscientist at the National Institute on Aging in the United States.

“We did a screen for proteins that could be secreted by muscle tissue and transported to the brain, and among the most interesting candidates was cathepsin B.

“Moreover, in humans who exercise consistently for four months, better performance on complex recall tasks, such as drawing from memory, is correlated with increased cathepsin B levels.”