A new study suggests aerobic exercise can increase the size of a region of the brain that plays an important role in consolidating memories, among other functions.

The results of the study, published in the journal NeuroImage, showed that aerobic exercise can improve memory function and maintain brain health as we age. It is thought that after the age of 40, our brains shrink by about 5% every decade.

Previous studies in mice and rats have shown that exercise increases hippocampus size, but until now, the evidence in humans has been inconsistent.

An international collaboration of researchers from Western Sydney University and the University of Manchester reviewed 14 clinical trials that examined the brain scans of 737 people before and after an aerobic exercise program, as well as a control group.

The participants ranged from 24 to 76 years old with an average of 66 and included a mix of healthy adults, people with mild cognitive impairments (such as Alzheimer's) and people with clinical diagnoses of mental illnesses, including schizophrenia and depression.

The researchers looked at the effects on the participants of various exercises including cycling on an exercise bike, walking and running on a treadmill for between three and 24 months, with sessions taking place between 2 and 5 times a week.

They found that exercise increased the size of the left region of the hippocampus over time by reducing the effects of ageing.

"When you exercise you produce a chemical called brain-derived neurotrophic factor (BDNF), which may help to prevent age-related decline by reducing the deterioration of the brain," said Joseph Firth, lead author of the study.

"Our data showed that, rather than actually increasing the size of the hippocampus per se, the main 'brain benefits' are due to aerobic exercise slowing down the deterioration in brain size. In other words, exercise can be seen as a maintenance program for the brain."

Firth says that the study could have implications for the prevention of age-related neurodegenerative disorders.