Trillions of microorganisms – mainly bacteria and yeast – inhabit your intestines and make up your microbiome.

Don’t worry – most of them are generally harmless, and even sometimes beneficial. Many of these microbes help us obtain nutrients from food we normally can’t digest, and they also help protect us from opportunistic infections caused by pathogenic (disease-causing) bacteria.

However, we are only beginning to understand the importance of the microbiome in maintaining our health.

If you don’t know about the microbiome, we have a primer in our research newsletter. In short, the microbiome refers to the composition of the community of microorganisms living in your intestine and may also refer to the environment in which they live. It’s comprised of many different species of bacteria, yeast, fungi, and even viruses.

The microbiome isn’t fixed – it develops over time and changes in response to its environment. The food you eat has a big influence on the microbiome. Since different microorganisms prefer different food, some species may grow or decline in numbers depending on what you choose to eat.

Changes can happen rapidly – within days of incorporating new food in the diet. You can even stimulate the growth of particular microorganisms by eating carbohydrates that are fuel for bacteria, known as prebiotics, or foods containing live bacteria and/or yeast, known as probiotics. However, probiotics must be consumed regularly to maintain those bacteria; otherwise, the microbiome can revert to what it was before.

We know that promoting the growth of some bacterial species found in your intestines can be helpful. In the intestines of breast-fed infants, certain bacteria metabolize the oligosaccharides in milk, improving immunity and health. Bacteria and yeast found in some fermented foods like yogurt or kefir have been useful in treating certain inflammatory bowel diseases.  The introduction of bacteria from healthy people into the intestinal tract of people who suffer from a C. difficile infection can reduce disease severity and this treatment has successfully treated other gastroinstestinal diseases as well. Other bacteria have been associated with maintaining a healthy weight and may be involved in energy expenditure and metabolism.

However, there are types of intestinal bacteria that don’t cause disease but are still associated with poor health. Recently, researchers at the Linus Pauling Institute led by Dr. Kathy Magnusson asked if high-fat or high-sugar diets (generally considered unhealthy) could alter the composition of gut microbiota in mice to a degree that would affect their behavior.

To answer this question, the Magnusson group fed high-fat or high-sugar diets to mice and monitored the diet-related changes in their gut microbiome. Not surprisingly, both the experimental diets caused dramatic changes in the composition of the microbiome – some bacteria loved fat, others loved sugar, and some did not thrive under either condition.

The researchers also began testing the animals for changes in anxiety, memory, or cognitive measures. Mice eating a high-sugar or high-fat diet showed impairments in certain learned tasks, what the researchers termed “a loss of cognitive flexibility,” when compared to animals on the control diet. The high-sugar diet also appeared to impair memory.

The most striking finding from this study was how the microbiome changes appeared to directly influence animal behavior. An increased number of one group of intestinal bacteria and a loss of two others related to the degree of cognitive impairment in these animals. In other words, the data suggests that the bacteria themselves somehow brought about these changes in the brain.

This introduces a new and very complex dimension in nutrition. If we can influence our gut microbiome through the food we eat, and some of these microorganisms can metabolize that food into something different that we then absorb, then how much will the combination of food and bacteria affect our health?

Already, studies are examining how gut bacteria and yeast may act as little metabolic factories – transforming compounds from plants and other nutrients into new types of chemicals. And these don’t just stay in the gut, there’s evidence that they can enter the blood stream, but just how much these metabolic products influence our health is still unknown.

The Magnusson lab study, is trying to find out exactly how high-fat and high-sugar diets affect mouse behavior and whether the same processes happen in humans. Bacteria can release compounds that act as neurotransmitters, stimulate nerves, or alter the immune system – all of which can affect brain function, even in subtle ways. Could fats and carbs be transformed by the microbiome into some new compounds that enter the blood stream and affect cognition?

This is certainly a very complicated field, and we are only beginning to scratch the surface with these studies in animals. Researchers at the LPI, Oregon State University, and many institutions around the world are investigating how the gut microbiome may influence the relationship between diet and health.

As always, we will bring you the results of promising new research as it develops.