An Australian study into dementia is showing cutting down calories helps with brain health.

Co-director of the Centre of Healthy Brain Ageing at the University of NSW, Perminder Sachdev, said the study has been following people aged 70 to 90 for a number of years.

"We know that one thing, that obesity is not good for you," Prof. Sachdev said in regards to brain health.

"Especially if you are obese in your middle life, then that is not good for you late in life," he said.

He said reducing overall caloric intake was very important and certainly increased life expectancy.

"We know that what is good for your general health, for longevity, what is good for your heart is also good for your brain," Prof Sachdev said.

His study is bad news for people who develop diabetes late in life or don't control diabetes properly.

"People who started with normal levels and then had abnormal levels were most likely to have cognitive problems, problems with memory, executive functioning etcetera," Prof Sachdev said.