Summary: A comprehensive systematic review and meta-analysis reveal that the cognitive damage caused by unhealthy diets may not be as easily reversible as commonly believed. The study analyzed data across 27 preclinical controlled experiments to evaluate whether memory function could fully recover after switching from high-fat and high-sugar diets to healthy nutrition.
While transitioning to a clean diet consistently improves memory performance compared to sustained unhealthy eating, the recovery remains strictly incomplete. Crucially, the researchers identified added sugar as the definitive barrier to neurological healing, noting that memory deficits induced by high-sugar or combined high-fat/high-sugar diets showed almost no evidence of recovery.
Key Facts
- Incomplete Brain Recovery: Even after weeks of consuming a balanced, healthy diet, memory scores did not return to the baseline levels of subjects that had never been exposed to an unhealthy diet.
- The Sugar Barrier: Memory recovery is entirely dependent on past diet composition. Brains can successfully recover after a period of exclusive high-fat eating, but prolonged exposure to added sugar or combined fat-and-sugar diets leaves lingering, irreversible cognitive impairments.
- Hippocampal Specificity: The persistent memory deficits specifically impact the hippocampus, the core brain region responsible for learning, memory consolidation, appetite regulation, and food intake control.
- Memory-Isolated Impact: The long-term damage appears tightly isolated to cognitive memory circuits; switching back to a healthy diet yielded no consistent behavioral improvements for anxiety, general physical activity levels, or basic food motivation.
Source: University of Technology Sydney
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.
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,” Dr Kendig said.
“These results suggest that, at least for memory, the picture may be more complicated, especially when diets are high in added sugar.”
“Improving diet quality is still worthwhile,” he said. “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.”
Key Questions Answered:
A: Unfortunately, no. This study completely shatters the myth that the neurological consequences of an unhealthy diet can easily be fully undone. While upgrading your diet quality absolutely improves memory compared to continuing to eat junk, the recovery is permanently incomplete. The brain’s memory centers remain fundamentally scarred and never quite make it back to the cognitive performance levels of an inherently clean lifestyle.
A: Added sugar acts as a unique neurological blocker. The meta-analysis revealed clear memory improvements when high-fat diets were replaced with healthy choices, but high-sugar diets showed virtually no cognitive bounce-back. Sugar induces profound, lingering changes inside the hippocampus, the brain’s master switch for memory and learning, that stubbornly resist repair long after the sugar itself is gone.
A: Humans are incredibly messy research subjects. In a human life, changing your diet almost always coincides with changing your exercise habits, shifts in mood, or alterations to your daily routine, making it impossible to isolate diet’s independent impact on the brain. Controlled rodent experiments allow neuroscientists to isolate diet as a standalone variable, exposing the raw, uninterrupted chemical toll that sugar takes on brain tissue.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this memory and diet research news
Author: Jacqueline Middleton
Source: University of Technology Sydney
Contact: Jacqueline Middleton – University of Technology Sydney
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Cognitive and behavioural effects of high-fat, high-sugar diet reversal: a systematic review and meta-analysis of animal studies” by Simone Rehn, Isabella Eikelis, Joanne M. Gladding, Laura A. Bradfield, and Michael D. Kendig. Nutritional Neuroscience
DOI:10.1080/1028415X.2026.2664635
Abstract
Cognitive and behavioural effects of high-fat, high-sugar diet reversal: a systematic review and meta-analysis of animal studies
In addition to their effects on physical and metabolic health, high-fat, high-sugar (HFHS) diets are associated with neurocognitive impairment and behavioural effects. It is not clear whether these impairments persist or recover when the HFHS diet is replaced with a healthy diet, i.e. diet reversal.
We performed a systematic review and meta-analysis of the effects of diet reversal on cognition and behaviour in rodent studies. Our review included 27 studies which fed rodents HFHS diets for a minimum of two weeks, replaced this diet with healthy chow for at least 24 h, and then assessed cognition, anxiety, depression, motivation or locomotor activity.
Meta-analyses revealed that diet reversal significantly improved memory performance relative to rodents maintained on HFHS diets (g = 0.46, 95% CI [0.16, 0.76], p = .004) but did not restore memory to the level of chow-fed controls (g = −0.28 [−0.49, – 0.06], p = .013).
Meta-regressions revealed test- and diet-dependent effects of diet reversal, with significant memory improvements in studies using the novel object location test, or high-fat diets, but not high-sugar or combined HFHS diets.
Diet reversal had no significant effects on tests of anxiety-like or depression-like behaviour, motivation, or locomotor activity. Heterogeneity estimates were moderate to high across domains, and risk of bias was generally low.
Results demonstrate that diet-induced cognitive impairments are amenable to healthy diet intervention in controlled animal models, underscoring the need for public health nutrition strategies designed to reduce intake of foods high in sugar and fat.
