Most people know the feeling that follows a stretch of unhealthy eating: too much fast food, sugary drinks, and late-night snacking.
Then comes the promise to eat better. Most people assume the body and brain will recover once healthier habits return.
But is recovery really that easy? A new study suggests the answer is more complicated.
Researchers looked at whether switching from unhealthy diets to healthy ones could reverse brain-related damage caused by junk food.
Their findings showed that some brain functions improved, especially memory, but recovery was not complete. The study also pointed to one major problem: sugar.
Junk food affects memory
Scientists have known for years that ultra-processed foods can harm physical health. Diets high in fat and sugar increase the risk of obesity, heart disease, and type 2 diabetes.
Now, researchers are paying closer attention to the brain.
Studies in humans show that people who eat more ultra-processed food often have poorer memory and faster decline in thinking skills.
Brain scans have also linked these diets to shrinkage in the hippocampus, an important brain region involved in learning and memory.
Animal studies have shown similar results. When rodents eat high-fat and high-sugar diets, their memory and learning abilities decline.
Researchers wanted to know whether those effects could be reversed by switching back to a healthy diet.
How the study worked
The research team reviewed 27 rodent studies that followed a similar pattern.
First, the animals were fed unhealthy diets rich in fat, sugar, or both. After several weeks, researchers replaced the junk food with healthy food.
The animals then completed tests measuring memory, anxiety, depression-like behavior, motivation, and activity levels.
The scientists compared the recovering animals with two groups: animals that continued eating unhealthy food and animals that had always eaten healthy diets
This allowed researchers to measure both improvement and complete recovery.
Healthy diets helped memory
The clearest improvement appeared in memory tests. Animals that switched to healthy diets performed better than those that continued eating junk food.
“Our results show that improving diet quality does benefit memory,” said Dr. Simone Rehn, lead author of the study from the University of Technology Sydney.
This is encouraging news because it suggests the brain can recover at least partly after unhealthy eating.
However, recovery was incomplete. When researchers compared the recovering animals with those that had always eaten healthy food, the recovering group still performed worse.
“But those improvements were incomplete,” Dr. Rehn said. “Even after weeks on a healthy diet, memory did not return to the level seen in animals that had never eaten an unhealthy diet.”
In simple terms, eating better helped the brain recover, but it did not completely erase the earlier damage.
Damage in the memory center
One important memory test in the study focused on spatial memory, or remembering where things are located. This type of memory depends heavily on the hippocampus.
The hippocampus is especially sensitive to unhealthy diets because it reacts strongly to inflammation and oxidative stress in the body.
Scientists believe this may explain why memory problems often appear after long periods of unhealthy eating.
It may also explain why the hippocampus showed some improvement once the diet became healthier again.
Sugar caused bigger problems
One of the most important findings involved sugar.
Researchers found that animals recovering from high-fat diets showed clearer memory improvement. But animals recovering from high-sugar diets showed little improvement.
“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.”
This finding matches earlier human studies that linked sugary drinks to poorer cognitive health. Sugar may have longer-lasting effects on the brain than researchers once believed.
Longer damage, larger recovery
The study also found something surprising.
Animals that spent longer periods eating unhealthy food sometimes showed larger improvements after switching to healthy diets.
Researchers think this may simply be because those animals started from a worse condition, leaving more room for improvement.
Still, the finding suggests the brain may remain responsive to healthier habits even after long periods of poor eating. That offers some hope.
Mood and mental health
While memory improved, emotional changes were less encouraging.
The study found little evidence that switching to healthy diets improved anxiety, depression-like behavior, motivation, or activity levels.
The researchers believe this could happen because removing highly rewarding junk food may itself create stress, balancing out some of the benefits of healthier eating.
The authors also noted that fewer studies focused on mood, so stronger research is still needed.
What this means for people
Animal studies cannot perfectly predict human outcomes, but they can still provide valuable clues.
The researchers noted that the unhealthy eating periods in these studies were often longer than the recovery periods. This means the animals may simply not have had enough time to fully recover.
“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,” said Dr. Mike Kendig, senior author of the study.
The research offers both good news and a warning.
The good news is that healthier eating can improve memory and support brain recovery. The brain remains adaptable even after unhealthy habits.
The warning is that recovery may not be complete, especially after diets high in added sugar.
Brain damage may linger
“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.”
The findings suggest that protecting brain health early may be more effective than trying to repair damage later.
Healthy eating still matters. The brain can recover to some extent. But science shows that the effects of junk food may last longer than many people expect.
The study is published in the journal Nutritional Neuroscience.
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