The new research, published in the journal Neuroscience, investigated the impact of diets rich in sugary and fatty foods on brain functions, the make-up of the gut microbiome and the gut-brain axis that is thought to interconnect the two.
Led by principal investigator Professor Kathy Magnusson of Oregon State University and the Linus Pauling Institute, the research team found that when compared to a normal diet, mice fed a high-fat and a high-sugar diet had changes in gut bacteria that appeared 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,” said the team.
"It's increasingly clear that our gut bacteria, or microbiota, can communicate with the human brain," said Magnusson. "Bacteria can release compounds that act as neurotransmitters, stimulate sensory nerves or the immune system, and affect a wide range of biological functions."
"We've known for a while that too much fat and sugar are not good for you," she added. "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.”
The team used a sample of 18 two-month-old male mice, which were randomly assigned to either a high-fat (42% fat, 43% carbohydrate (CHO), high-sucrose (12% fat, 70% CHO (primarily sucrose) or normal chow (13% kcal fat, 62% CHO) diets.
The team performed faecal microbiome analysis, step-down latency, novel object and novel location tasks prior to and 2 weeks after diet change. Water maze testing for long- and short-term memory and cognitive flexibility was conducted during weeks 5–6 post-diet change, they said.
Magnusson and her colleagues found that 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 – with one of the most pronounced changes in cognitive flexibility.
"The impairment of cognitive flexibility in this study was pretty strong," commented Magnusson. "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."
According to the lead researchers, 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.
However, with impaired flexibility, it might be a long, slow, and stressful way home.
The team said such findings are consistent with some other studies investigating the impact of fat and sugar on cognitive function and behaviour - and suggest that some of these problems may be linked to alteration of the microbiome.
Volume 300, 6 August 2015, Pages 128–140, doi: 10.1016/j.neuroscience.2015.05.016
“Relationships between diet-related changes in the gut microbiome and cognitive flexibility”
Authors: K.R. Magnusson, et al