Our love of sugar once helped our foraging ancestors get more bang for their buck from rare, high-energy foods.
Today, it drives us back to the store at 10pm to satisfy a cheeky chocolate craving.
We do this even though we know many of the physical and metabolic risks of excess sugar, such as obesity, diabetes, tooth decay, and even memory impairment.
But how well can we undo the damage by changing our diets?
According to a new systematic review and meta-analysis, it is possible to reverse cognitive effects of a sugary diet in rodents – but perhaps only partially.
Some impairments persist even after adopting healthier eating habits.
Reducing sugar intake can improve memory in rodents previously fed high-sugar diets, the study found, but it doesn't restore memory to the level of peers consistently fed healthy diets.
Previous research has linked high-fat, high-sugar (HFHS) diets with an array of neurocognitive and behavioral effects, but it remains unclear how persistent those effects are if diet improves.
For the new study, researchers analyzed results from 27 preclinical studies involving controlled experiments on rats and mice, hoping to illuminate how shifting from HFHS diets to healthier food affects cognition.
"Our results show that improving diet quality does benefit memory," says lead author Simone Rehn, a biopsychologist at the University of Technology Sydney (UTS) in Australia.
"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."
By conducting a systematic review and meta-analysis, Rehn and her colleagues sought to reveal broad patterns arising from more than two dozen studies.
They focused on diet and memory, but also tracked other potential indicators of cognitive effects such as general activity, food motivation, and anxiety- or depression-like behaviors.
Among rodents fed unhealthy diets for at least two weeks, those who were switched to healthier food for at least 24 hours consistently fared better on memory tests than those continuing to chow down on junk food, the study found.
No other cognitive measures showed consistent improvements associated with the dietary changes, suggesting the effects were specific to memory.
The memory effects varied across diet subtypes, with a significant boost seen in models using high-fat diets, but not in high-sugar or HFHS diets.
"We saw clearer memory improvements after high‑fat diets were replaced with healthy food," Rehn says.
"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."
Rodent models were instrumental in isolating the effect of diet on memory, explains senior author Mike Kendig, biopsychologist at UTS.
"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," Kendig says.
All this seems to center around the hippocampus, a brain region that plays vital roles in memory and learning – and has a hand in regulating appetite.
HFHS diets in humans have been previously linked with reduced hippocampal volume and functioning, a connection supported by the new study.
"Our findings imply that diet reversal primarily improves hippocampal-dependent spatial memory, underscoring evidence that the hippocampus is particularly sensitive to changes in diet and other environmental factors," the researchers write in the new study.
Despite the memory damage, the point isn't to despair about all the sugar you've already eaten, but to recognize the high stakes involved and cut back on sugar sooner rather than later.
Related: Common Sweetener May Damage Critical Brain Barrier, Risking Stroke
"There is a common belief that the effects of unhealthy eating are easily reversible. These results suggest that, at least for memory, the picture may be more complicated, especially when diets are high in added sugar," Kendig says.
"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."
The study was published in Nutritional Neuroscience.