The average person worldwide has added the equivalent of a burger, fries and soft drink in calories to their daily diet in the last 50 years, a trend that researchers have found is eating away at the brain.
An Australian National University study has found a link between irreversible decline in brain function and unhealthy lifestyles that start as soon as early childhood.
The link between rapid brain degeneration and type 2 diabetes is well-known already.
New research led by ANU professor Nicolas Cherbuin has discovered strong evidence that unhealthy eating habits and lack of exercise for long periods also put people at serious risk of developing dementia and brain shrinkage.
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.
Professor Cherbuin said fast food diets and little to no exercise was permanently reducing brain function.
"Unless we do something about it early, we accumulate this risk, and when the disease becomes detectable then much of the damage is done," he said.
"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."
The research, published in the journal Frontiers in Neuroendocrinology, found results of an ACT-based study on brain health shared common patterns with more than 200 international studies.
Professor Cherbuin said it showed Australia was not isolated from the health problems posed by obesity, despite popular thinking that the issue was worse in other countries such as the United States.
His team's message isn't to blame people for their health decisions. Solutions weren't simple and wouldn't be found only by telling people to look after themselves better, Professor Cherbuin said.
The causes of the problem included the environment obese and overweight people lived in, which made it harder to make healthy choices.
Cheap and widely available fast food, and unclear dietary information on packaging, were some barriers to making better lifestyle decisions.
"We should develop a social and economic environment and also an education environment that gives us the tools to do this as we should," Professor Cherbuin said.