Regularly lifting weights could make you more intelligent, a new study of people with the early signs of dementia suggests.
A group of people aged 55 to 86 with 'mild cognitive impairment' - a precursor of Alzheimer's disease - were asked to carry out a mix of weight lifting and brain training.
While it is unclear whether this would apply more widely, the researchers said they had found a causal link between an increase in strength and better functioning of the participants' brains.
And, on that basis, they recommended people should lift more weights so that the world would have a "healthier ageing population". Some 135 million people are currently expected to have dementia by 2050.
The study, published in the Journal of the American Geriatrics Society, is the latest in a growing body of research which links the health of the body to the health of the brain.
The same team behind it published a paper in 2014 which revealed the participants' global cognition had improved significantly after the weight training, whereas cognitive training did not do this.
Researcher Dr Yorgi Mavros, of Sydney University, said: "What we found in this follow-up study is that the improvement in cognition function was related to their muscle strength gains.
"The stronger people became, the greater the benefit for their brain."
The weight training programme involved lifting weights that were 80 per cent as heavy as the maximum the participants were able to lift twice a week for six months. This is similar to training regimes used by athletes.
As they got stronger, the amount of weight they lifted was increased to keep to the 80 per cent level.
MRI scans revealed that specific areas of the brain increased in size among those who took part.
Dr Mavros said the beneficial effects were enough to recommend weight training for all.
"The more we can get people doing resistance training like weight lifting, the more likely we are to have a healthier ageing population," he said.
"The key however is to make sure you are doing it frequently, at least twice a week, and at a high intensity so that you are maximising your strength gains. This will give you the maximum benefit for your brain."
Professor Maria Fiatarone Singh, a geriatrician at Sydney University who took part in the research, said the "next step" was to work out if the increase in muscle strength was directly related to the increase in brain size.
"In addition, we want to find the underlying messenger that links muscle strength, brain growth, and cognitive performance, and determine the optimal way to prescribe exercise to maximise these effects," she said.
Dr James Pickett, head of research at the Alzheimer's Society, said: "New research is beginning to unravel how physical exercise may have benefits for the brain as people get older. This study suggests that people with minor memory and thinking problems, known as mild cognitive impairment, may benefit from weight training to improve their brain health.
"Not everyone with mild cognitive impairment will go on develop dementia - and it is not yet clear whether weight training could prevent dementia or help those who already have the condition.
"However, we do know that the best way to reduce your risk of developing dementia is a combination of taking regular exercise, not smoking and eating a healthy, balanced diet."