A recent study indicates that both high-fat and high-sugar diets cause changes in gut bacteria that appear to be related to a significant loss of “cognitive flexibility,” or the power to adapt and adjust to changing situations.

This effect was most serious on the high-sugar diet, which also showed an impairment of early learning for both long-term and short-term memory.

The findings are consistent with other studies about the impact of fat and sugar on cognitive function and behavior, and suggest that some of these problems may be linked to changes in the microbiome –  the approximate 100 trillion microorganisms in the digestive system.

The research, conducted at Oregon State University, was done with mice that consumed different diets and then faced a variety of tests (such as water maze testing) to monitor changes in their mental and physical function, and the associated impacts on various types of bacteria.

“It’s increasingly clear that our gut bacteria, or microbiota, can communicate with the human brain,” said Kathy Magnusson, a professor in the OSU College of Veterinary Medicine and principal investigator with the Linus Pauling Institute.

“Bacteria can release compounds that act as neurotransmitters, stimulate sensory nerves or the immune system, and affect a wide range of biological functions,” she said. “We’re not sure just what messages are being sent, but we are tracking down the pathways and the effects.”

Magnusson said that mice have proven to be a particularly good model for studies relevant to humans, on such topics as aging, spatial memory, obesity, and other issues.

After just four weeks on a high-fat or a high-sugar diet, the performance of mice on various tests of mental and physical function began to drop, compared to animals on a normal diet. One of the most pronounced changes was in what researchers call cognitive flexibility.

“The impairment of cognitive flexibility in this study was pretty strong,” Magnusson said. “Think about driving home on a route that’s very familiar to you, something you’re used to doing. Then one day that road is closed and you suddenly have to find a new way home.”

A person with high levels of cognitive flexibility would immediately adapt to the change, determine the next best route home, and remember to use the same route the following morning, all with little problem. But with impaired flexibility, it might be a long, slow, and stressful trip home.

What’s often referred to as the “Western diet,” or foods that are high in fat, sugars, and simple carbohydrates, has been linked to a range of chronic illnesses, including obesity, type II diabetes, and an increased incidence of Alzheimer’s disease.

“We’ve known for a while that too much fat and sugar are not good for you,” Magnusson said. “This work suggests that fat and sugar are altering your healthy bacterial systems, and that’s one of the reasons those foods aren’t good for you. It’s not just the food that could be influencing your brain, but an interaction between the food and microbial changes.”