We all know that eating well is important for our health. But what about to indulge in an unhealthy meal or two on weekends?
A study conducted by researchers from the University of New South Wales (UNSW) in Australia suggested that a diet pattern of ‘clean’ eating interspersed with ‘cheat’ meals of junk food could not only result in weight gain but could also have an impact on brain function and gut health in rodents.
Rodents that were fed mostly healthy food but occasionally indulged in high-sugar and saturated-fat foods exhibited significant cognitive impairment, particularly in tests of spatial memory, and experienced negative changes in gut bacteria.
“This sort of work is considered critical to make us contemplate the maintenance of our brain’s health as we age,” explains neuroscientist Margaret Morris from UNSW.
The study builds upon previous research conducted by the team members, who found a correlation between a poor diet and impaired long-term spatial memory, offering new insights into the effects of ‘diet cycling’.
The swings in nutrition, sometimes seen as a compromise between extremes of healthy and unhealthy eating, might come with a cost.
In each of 3 different experiments, a healthy control group of 12 rats were provided with standard rat food, and they were compared to 3 experimental groups of 12 rats who were also occasionally given processed food high in fat and sugar.
The unhealthy food was administered to the experimental groups for the same total number of days, either consecutively or distributed across different length ‘cycles’.
The rats’ short-term memory was assessed by the researchers before and after the periods of diet cycling, and the microbiota in their feces was measured. Additionally, the rats’ weights were recorded before and after the experiments, and the amount of each food type consumed was tracked.
Rats that consumed a poor diet for any duration exhibited a less diverse gut microbiome, with an increase in bacteria associated with obesity and a decrease in beneficial bacteria linked to weight control. These changes worsened with prolonged exposure to an unhealthy diet.
Over time, cognitive impairment became more pronounced, particularly in the rats that had been fed an unhealthy diet for multiple days. They performed the poorest in memory tests that required recalling object placement.
“The extent of memory impairment was found to correlate with the levels of two specific bacteria,” notes Mike Kendig, a medical scientist at UNSW at the time. “This suggests a potential connection between the effects of diet cycling on cognition and the microbiota.”
Unsurprisingly, the rats that consumed a high-fat, high-sugar diet for an extended period gained more weight compared to the rats in the control group. However, the duration of the unhealthy eating cycles did not appear to directly influence weight gain.
This implies that the impact on gut and memory health may not be solely attributed to the weight gain typically associated with an unhealthy diet.
“In humans, it is known that a diet that increases inflammation seems to have a lesser beneficial effect on brain function,” says Morris. “And in the past, it has been demonstrated in rats that these cognitive deficits are actually correlated with inflammation in the brain.”
These findings contribute to the growing body of evidence that establishes a connection between gut health, diet, and brain health.
“We are aware that the gut is strongly linked to our brain,” adds Morris. “Changes in the microbiome resulting from our diet might impact our brain and behavior.”
Although these findings may not be desirable, the scientists emphasize that maintaining a healthy eating pattern over an extended period seems to yield more favorable outcomes for both gut and brain health.
Other studies indicate that adopting a nutritious diet reduces the risk of diseases and may increase lifespan, so it could be worthwhile to substitute high-fat and sugary treats with healthier alternatives.
“If a healthy diet can be maintained – such as the Mediterranean-type diet with high diversity, fruits, vegetables, low saturated fats, and good proteins – we have a better chance of preserving our cognition,” concludes Morris.
The effects of diet cycling on cognition and fecal microbiota are not well understood.
Method and Results
Adult male Sprague-Dawley rats were cycled between a high-fat, high-sugar “cafeteria” diet (Caf) and regular chow. The impairment in place recognition memory produced by 16 days of Caf diet was reduced by switching to chow for 11 but not 4 days. Next, rats received 16 days of Caf diet in 2, 4, 8, or 16-day cycles, each separated by 4-day chow cycles. Place recognition memory declined from baseline in all groups and was impaired in the 16- versus 2-day group. Finally, rats received 24 days of Caf diet continuously or in 3-day cycles separated by 2- or 4-day chow cycles. Any Caf diet access impaired cognition and increased adiposity relative to controls, without altering hippocampal gene expression. Place recognition and adiposity were the strongest predictors of global microbiota composition. Overall, diets with higher Caf > chow ratios produced greater spatial memory impairments and larger shifts in gut microbiota species richness and beta diversity.
Results suggest that diet-induced cognitive deficits worsen in proportion to unhealthy diet exposure, and that shifting to a healthy chow for at least a week is required for recovery under the conditions tested here.