A new animal study suggests that short-term exposure to refined, low-fiber diets may alter memory-related brain regions in older rats, potentially through changes in gut-derived compounds linked to inflammation.
The research, published in Brain, Behavior, and Immunity, found that aged rats fed several types of refined diets for just three days showed impairments in emotional memory tasks associated with the amygdala, a brain region involved in learning from negative experiences. Young rats on the same diets did not show the same degree of impairment.
Importantly, the refined diets varied in fat and sugar content. Some were high fat, some high sugar and others lower in both. The common feature across all of them was a lack of fiber.
“The amygdala is important for learning the association between something fearful and a bad outcome. And we found that all of the refined diets, whether they were high fat, high sugar, low fat, low sugar, it didn’t matter. They all impaired memory that’s governed by the amygdala,” said co-lead author Ruth Barrientos, an investigator at The Ohio State University.
When researchers examined the animals’ gut and blood samples, they found reduced levels of butyrate, a molecule produced when gut microbes break down dietary fiber. Butyrate has anti-inflammatory properties and can cross the blood-brain barrier.
“What our study really brings to light is the complexity of diet and how it affects so many different things, even the brain,” said co-lead author Kedryn Baskin, assistant professor of physiology and cell biology at Ohio State. “There’s not a magic bullet, but in this case, low butyrate, as a result of a lack of fiber, is a culprit.”
At the cellular level, the researchers observed changes in microglia, immune cells in the brain that play a role in memory processes. In aged rats, mitochondria within these cells showed reduced respiratory capacity when exposed to metabolic stress, suggesting older brains may be less adaptable to dietary challenges.
“These effects on the brain after you eat something are pretty rapid,” Barrientos said. “You can experience this unhealthy cognitive dysfunction well before you reach obesity.”
The study does not show that short-term changes in human diet cause immediate cognitive decline. It was conducted in rats under tightly controlled laboratory conditions, and the diets used were simplified formulations rather than real-world eating patterns. Still, the findings add to growing evidence that aging brains may be particularly sensitive to dietary composition and that fiber intake could play a protective role through its influence on the gut microbiome.
Rather than pointing to fat or sugar alone, the study highlights the importance of overall diet quality and the potential role of fiber in maintaining gut-derived metabolites that influence brain function.
This work was supported by the National Institute on Aging, the National Heart, Lung, and Blood Institute, the National Center for Advancing Translational Sciences and the Foods for Health Research Initiative at The Ohio State University.