As concern grows about the long‑term health effects of modern diets, new research led by the University of Technology Sydney (UTS) has examined how changes in what we eat affect memory and brain function.
Published in Nutritional Neuroscience, the study focuses on whether memory function can recover after diets high in fat and sugar are replaced with healthier nutrition.
To address this question, researchers analysed evidence from controlled experiments in rodents to examine what happens to cognitive function when animals fed high‑fat and high‑sugar diets are returned to healthier nutrition.
‘Our results show that improving diet quality does benefit memory,’ said Dr Simone Rehn, lead author on the study. ‘But those improvements were incomplete. Even after weeks on a healthy diet, memory did not return to the level seen in animals that had never eaten an unhealthy diet.’
To look beyond the results of individual studies, the researchers conducted a systematic review and meta‑analysis of 27 preclinical studies to identify consistent patterns across experiments.
The analysis focused on memory, but also looked at anxiety- and depression-like behaviour, general activity, and motivation for food.
Memory results
Across the studies, animals switched to a healthy diet performed better on memory tasks than those that continued eating unhealthy food. However, memory recovery depended on diet composition, with recovery seen in experiments that used high‑fat diets but not those using diets high in sugar or combined high‑fat and high‑sugar diets.
No consistent improvements were seen for anxiety, activity levels or food motivation, suggesting the effects were specific to memory rather than general behaviour.
‘We saw clearer memory improvements after high‑fat diets were replaced with healthy food,’ Dr Rehn said.
‘But diets high in added sugar, including diets high in both fat and sugar, showed little evidence of recovery. This suggests sugar may be a key factor in limiting memory recovery.’
The memory tasks analysed reflect function of the hippocampus, a brain region essential for learning and memory, and one that is also involved in regulating appetite and food intake.
Dr Mike Kendig, senior author of the paper, said animal models were critical for understanding how diet affects the brain.
‘In humans, changes in diet usually occur alongside changes in exercise, mood and daily routines, which makes it very difficult to separate the effects of diet alone on brain function. There is a common belief that the effects of unhealthy eating are easily reversible,’ he said.
‘These results suggest that, at least for memory, the picture may be more complicated, especially when diets are high in added sugar.’
Dr Kendig said improving diet quality is still worthwhile, but protecting brain health may also depend on avoiding prolonged exposure to unhealthy diets, rather than assuming the effects can always be fully undone later.
