The team found diets high in sugar or fat triggered alterations to the gut microbiome linked to reduced cognitive functioning.
Published in the journal Neuroscience, the study reveals that high-fat and high-sugar diets trigger changes in gut bacteria that are largely associated with loss of "cognitive flexibility" - the ability to adapt to changing situations.
In addition, the high-sugar diet was associated with poorer short- and long-term memory.
Principal investigator Prof. Kathy Magnusson, of the College of Veterinary Medicine and the Linus Pauling Institute at Oregon State, notes there is an increasing amount of evidence emerging that gut bacteria can communicate with the brain.
"Bacteria can release compounds that act as neurotransmitters, stimulate sensory nerves or the immune system, and affect a wide range of biological functions," Prof. Magnusson explains. "We're not sure just what messages are being sent, but we are tracking down the pathways and the effects."
One example of how gut bacteria may interact with the brain was revealed in a study published in the journal Cell in April, in which researchers from the California Institute of Technology (Caltech) found that gut bacteria influence the production of serotonin - a neurotransmitter responsible for maintaining mood balance.
Reduced cognitive flexibility with high-fat, high-sugar diets
Prof. Magnusson and colleagues reached their findings using 2-month-old male mice, which were randomized to be fed either a high-fat diet (42% fat, 43% carbohydrate), a high-sugar diet (12% fat, 70% carbohydrate - mainly from sugars) or normal chow.
Prior to dietary intervention and 2 weeks after, the researchers analyzed the feces of the mice in order to establish the composition of their gut bacteria.
The short- and long-term memory and cognitive flexibility of the mice were assessed before and after dietary intervention via water maze testing and novel object and location tasks.
Compared with mice fed normal chow, mice fed the high-fat or high-sugar diets experienced a significant reduction in cognitive functioning - particularly in cognitive flexibility.
Explaining what cognitive flexibility is, Prof. Magnusson asks us to imagine driving home using a route that is very familiar. One day, the road is closed, meaning we need to find a different route.
An individual with a high level of cognitive flexibility would adapt to the situation straight away, immediately seeking out an alternative route. But a person with impaired cognitive flexibility may find the unexpected change in situation very stressful, causing them to become flustered and take longer getting home.
Reduction in cognitive flexibility was strongest for mice fed the high-sugar diet, according to the researchers, and this diet was also found to reduce short- and long-term memory.
Diets altered gut microbiome of mice
The team believes the reduction in cognitive functioning following diets high in fat or sugar was driven by alterations to the composition of gut bacteria, or the gut microbiome.
Both diets were linked to an increase in bacteria called Clostridiales and a reduction in bacteria known as Bacteroidales, with such changes associated with reduced cognitive flexibility.
Mice fed the high-sugar diet experienced the highest increases in Clostridiales and the biggest reductions in Bacteroidales, consistent with the largest reductions in cognitive flexibility.
The team says their findings are consistent with some previous studies suggesting that a Western diet - typically high in fat and sugar - may negatively impact cognitive functioning. Past research has associated a Western diet with greater risk of Alzheimer's disease, for example. Their study indicates that such a diet may affect cognitive functioning via alteration of the gut microbiome.
Prof. Magnusson says:
"We've known for a while that too much fat and sugar are not good for you. 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."
Medical News Today recently reported on a study in which researchers have identified a gut microbe - Akkermansia muciniphila - that could prove effective for improving leanness and metabolic health among people who are overweight or obese.