The average person is eating a lot more calories than they did 50 years ago and it is having devastating results on their brains and waistlines, a new study warns.

People are eating an extra 650 kilocalories a day - the equivalent of a burger, fries and soft drink - compared to what they were eating in the 1970s.

But it's the impact on people's brains that is the really scary news.

Australian National University professor Nicolas Cherbuin, the lead author of new research published in Frontiers in Neuroendocrinology, said there was a clear link between brain deterioration and unhealthy lifestyle choices.

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Even more worrying is that the damage starts a lot sooner than people think.

"We've found strong evidence that people's unhealthy eating habits and lack of exercise for sustained periods of time puts them at serious risk of developing type 2 diabetes and significant declines in brain function, such as dementia and brain shrinkage," Prof Cherbuin said.

"People are eating away at their brain with a really bad fast-food diet and little-to-no exercise."

Prof Cherbuin, who is from the ANU Centre for Research on Ageing, Health and Wellbeing, also found that brain health can decline much earlier in life than previously thought due, in large part, to a society that promotes unhealthy lifestyle choices.

"The damage done is pretty much irreversible once a person reaches midlife, so we urge everyone to eat healthy and get in shape as early as possible — preferably in childhood but certainly by early adulthood," he said.

"Many people who have dementia and other signs of cognitive dysfunction, including shrinking brains, have increased their risk throughout life by eating too much bad food and not exercising enough."

He said people were usually only advised to reduce their risk of brain problems like dementia once they were already aged in their 60s and it was often a case of "too little, too late".

It is people's diets in childhood and as young adults that are crucial.

"One of the best chances people have of avoiding preventable brain problems down the track is to eat well and exercise from a young age."

The ANU research reviewed results from about 200 international studies, including The Personality & Total Health (PATH) Through Life project in the Australian Capital Territory and Queanbeyan that has followed the brain health and ageing of more than 7000 people.

The research reports about 30 per cent of the world's adult population is either overweight or obese, and more than 10 per cent of all adults will suffer from type 2 diabetes by 2030.

"The link between type 2 diabetes and the rapid deterioration of brain function is already well established," Prof Cherbuin said.

"But our work shows that neurodegeneration, or the loss and function of neurons, sets in much, much earlier - we've found a clear association between this brain deterioration and unhealthy lifestyle choices."

He said the extra amount of energy that people were consuming daily, compared to 50 years ago, means that people have an unhealthy diet.

Another problem is that people are eating too much of the wrong type of food, such as fast food, which is low in nutrients and high in calories.

"As a society, we need to stop asking, 'do you want fries with that?'," he said.

"If we don't, then expect to see more overweight and obese people suffering from serious diseases.

"The message is simple, but bringing about positive change will be a big challenge. Individuals, parents, medical professionals and governments all have an important role to play."