Several labs are reporting similar results.
But if you cut sugar from your diet, how long does it take for your memory to return to normal?
About seven weeks, according to new, intriguing – but very early stage – research.
Dr Rooney has led a trial at the University of Sydney, although the first results have not yet been published.
His latest publication with Professor Robert Boakes, in Physiology and Behaviour, looks at the effect quitting sugary drinks has on memory in rats.
The team gave rats unlimited sugar-water (equivalent to soft drink) for eight weeks.
In a test of their ability to recall the location of things in their cages, the sugar-laden rodents performed significantly worse than those who did not have the soft-drink substitute.
Then the animals were switched onto water for seven weeks.
By the seventh week, their memory had returned to normal.
“Our data in rats seems to show we have the capacity to recover. But it seems to depend on how long you’ve been consuming the sugar for,” says Dr Rooney.
At Macquarie University, Professor Richard Stevenson has done some research on the effect on humans.
In a small study published in 2017, his team found that just four days of eating a toasted cheese sandwich plus a bottle of chocolate milk for breakfast significantly reduced memory and learning performance.
It’s “probable” that sugar has an effect on human brains, he told Fairfax in December, “but far from confirmed”.
If there is an effect on memory, there are lots of theories for what sugar might be doing to the brain.
“At the moment, we don’t know why sugar has this effect,” says Dr Rooney.
“Is it a secondary effect to sugar making you fatter? Fat cells produce lots of hormones that can influence how we behave and what we eat. Whether they have other effects on how our brains function is a really important question.”
Sugar might also affect the hippocampus, a small region in the centre of the brain responsible for memory and learning. The hippocampus in particular needs to constantly make lots of new neurons; high blood sugar levels might block this process.
“The hippocampus seems to be especially vulnerable to damage. Poor sleep, epilepsy, diabetes, depression, stress – and many many other things too – all seem to start by damaging the hippocampus,” says Professor Stevenson.
All the research is starting to add up, says Monash University’s Associate Professor Zane Andrews, who researches the way sugar controls the parts of our brain that makes us hungry.
“You just need to look at Alzheimer’s, a condition characterised by memory loss. Some people call Alzheimer’s ‘type 3 diabetes’. That’s due to the severe insulin resistance caused by high sugar levels that’s a signature of that disease,” he says.
Others, including Professor Amanda Lee, are more reserved. She’s an expert on assessing the evidence on diet and health, and helped put together the Australian Dietary Guidelines.
“These studies all add to the evidence picture, and that’s great,” she says.
“But I’m not aware of much work at all on sugar and memory in humans. It’s really an emerging area, a new frontier.
“You need to have human-specific evidence. This work suggests a theory that’s really fascinating.”
The Australian Beverages Council, which represents the soft-drinks industry, says suggestions that sugary drinks are bad for our brains should be taken with a pinch of salt.
“We urge extreme caution in applying the results of a rat study to any implications for humans,” the council’s chief executive Geoff Parker said.
“The study’s small sample size and design, particularly that rats were fed unrestricted sugar solution, are fundamentally flawed; humans consume a variety of foods and beverages as part of a varied diet.”
Mr Parker said most Australians did not binge on sugary soft drinks.