The typical American diet is loaded with fat and sugar, and it may be hurting not only our physical health, but also our ability to think clearly.
New research from Oregon State University finds a high-sugar, high-fat diet causes changes in gut bacteria that seem to lead to significant losses in cognitive flexibility, a measurement of the brain's ability to switch between thinking about one concept to another, and to adapt to changes in the environment.
The study, which was conducted on mice and published this week in the journal Neuroscience, found that a high-sugar diet was particularly detrimental to brain function, leading not only to decreased cognitive flexibility but also to impairments in short- and long-term memory.
“We’ve known for a while that too much fat and sugar are not good for you,” Dr. Kathy Magnusson, a biomedical scientist at the university and the study's lead author, said in a statement. “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.”
As Magnusson suggests, the findings aren't entirely surprising. They're the latest to join a growing body of research that has shown that the trillions of bacteria living in the gut can have a major influence on brain function and mental health. The upshot? Diet could play an important role in neurological and mental health, both for better and for worse.
The Experiment
For the study, the researchers fed groups of mice a high-fat diet, a high-sugar diet or a normal diet, and gave them tests measuring various physical and mental functions. To assess changes to the gut microbiome -- the community of trillions of bacteria living in the gut -- the researchers also analyzed fecal samples prior to the implementation of the diet and again five weeks after adopting the diet.
What did they find? After just four weeks on the high-fat or high-sugar diet, the mice performed worse on a series of mental and physical tests, compared to the mice on a normal diet. The mice on both diets performed particularly poorly on the test of cognitive flexibility, in which they were expected to find a new escape route from their cages.
The mice on the high-sugar diet also performed poorly on tests on working memory and long-term memory.
The microbiome analysis revealed that higher percentages of "bad" gut bacteria and lower amounts of healthy bacteria among the high-sugar and high-fat groups were directly correlated with worse performance on the tests of cognitive flexibility.
In humans, poor cognitive flexibility can manifest in difficulty understanding new concepts or adapting to new experiences.
“Think about driving home on a route that’s very familiar to you, something you’re used to doing," Magnussion explained in a statement. "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 be quick to find an alternative route, while someone with lower cognitive flexibility would be more likely to struggle to find a new way home.
How does it work? Scientists aren't yet sure, but there are a number of possible mechanisms by which gut bacteria can "talk" to the brain. For one, these bacteria can activate an inflammatory response of the immune system.
"There's also evidence that the bacteria can release chemicals that are used as transmitters in the brain," Magnusson said in an email to The Huffington Post. "So they're producing things like serotonin, and we have receptors in our brain that will react to serotonin."
Your Brain On Sugar
The findings join a growing body of research that warns of the possible negative effects of excessive sugar consumption on brain health.
The mounting evidence has even led some scientists to refer to Alzheimer's disease as "Type 3 diabetes," because of the association with chronically elevated blood sugar levels and insulin resistance with cognitive decline.
There's an important caveat here. Cutting back on sugar doesn't mean that it's a good idea to switch to artificial sweeteners. Recently, scientists found that regular consumption of artificial sweeteners can have a devastating effect on the microbiome, and can lead to metabolic disorders and obesity.
"Artificial sweeteners are likely even worse [than sugar] because they have a huge effect on changing the microbiome," neurologist Dr. David Perlmutter, author of Brain Maker: The Power of Gut Microbes to Heal and Protect Your Brain for Life, told The Huffington Post. "Your risk of diabetes is twice as great if you’re consuming diet drinks, which is far greater than if you’re drinking sugar-sweetened beverages."
But the good news is that a balanced diet that's relatively low in sugar, artificial sweeteners and unhealthy fats -- and high in probiotics, the "good" bacteria that support digestion, immune health and even mental health -- can support the health of both the gut and the brain.
"A brain-healthy diet is one that’s very low in sugar and carbohydrates that welcomes fat back to the table, but only good fat -- not trans fats, not the modified fats," said Perlmutter. "It’s mostly vegetables on the plate -- colorful, nutrient-dense vegetables that are rich in fiber."