Switching to a healthier diet may help memory recover after a period of unhealthy eating, but the benefits may depend on what the earlier diet looked like.
A new review of animal studies found that rodents moved from unhealthy diets back to healthier food generally performed better on memory tasks than animals that continued eating unhealthy diets. But the recovery was not complete, and the pattern appeared weaker after diets high in sugar or both fat and sugar.
The findings, published in Nutritional Neuroscience, do not prove the same thing happens in people. The analysis was based on controlled experiments in rodents, not human diet trials. Still, the results add to a growing body of research suggesting diet quality may affect parts of the brain involved in learning and memory.
The researchers reviewed 27 preclinical studies that looked at what happened after animals were fed high-fat, high-sugar or combined high-fat and high-sugar diets, then switched back to healthier nutrition. They focused mainly on memory, while also looking at anxiety-like behavior, depression-like behavior, general activity and motivation for food.
“Our results show that improving diet quality does benefit memory,” said Dr. Simone Rehn, lead author of the study.
The important caveat is that the improvement did not always bring memory performance back to the level seen in animals that had never eaten the unhealthy diet.
Across the studies, animals switched to healthier food did better on memory tasks than animals that stayed on unhealthy diets. The clearest improvement was seen after high-fat diets were replaced with healthier food.
The picture was different for diets high in sugar or both fat and sugar. In those studies, the researchers found less evidence that memory fully bounced back after the animals returned to healthier food.
“We saw clearer memory improvements after high-fat diets were replaced with healthy food,” Rehn said.
The researchers said diets high in added sugar, including those high in both fat and sugar, showed little evidence of recovery. That suggests sugar may play an important role in limiting how much memory improves after a diet change, at least in animal studies.
The memory tasks in the review were tied to the hippocampus, a part of the brain involved in learning and memory. The hippocampus also plays a role in appetite and food intake, which makes it a key area of interest for researchers studying how diet affects the brain.
The review did not find consistent improvements in anxiety-like behavior, activity levels or food motivation after the switch to healthier diets. That suggests the effects were more specific to memory than to overall behavior.
The study’s biggest strength is that it pulled together evidence from multiple controlled animal experiments rather than relying on one small study. Animal models can help researchers isolate the effects of diet in ways that are much harder to do in people, where changes in food often happen alongside changes in exercise, sleep, stress, medication, mood and daily routines.
But the animal-study limitation is important. Rodent studies can help explain possible biological mechanisms, but they cannot tell people exactly how their own memory would respond after cutting back on sugar or improving diet quality.
The findings also should not be read as a warning that every dessert or sweet food causes lasting brain damage. The studies involved controlled diets designed for research, not the occasional sweet in an otherwise balanced eating pattern.
At the same time, the findings raise the possibility that prolonged exposure to diets high in added sugar may be harder for the brain to recover from than researchers once assumed.
“There is a common belief that the effects of unhealthy eating are easily reversible,” said Dr. Mike Kendig, senior author of the paper.
These findings suggest the answer may be more complicated, especially when diets are high in added sugar. More research, especially in people, is needed to understand whether the same pattern applies to human memory and what kinds of dietary changes may be most protective.
For now, the message is less about fear and more about prevention. A healthier diet may help support memory, but waiting to fix the problem later may not always be the same as protecting brain health along the way.
The study was supported by an Australian Research Council Discovery Project. Dr. Mike Kendig is supported by an Australian Research Council DECRA Fellowship.