The 100 trillion bacteria that live inside the human digestive tract are the subject of many studies currently underway. Researchers the world over are trying to figure out which microbes constitute a “healthy microbiome,” which microbes protect our health, and which cause disease. Many advances have already been made, but there is still much we don’t know about the creatures that inhabit us.

In a recent study published in the journal Genome Biology, researchers collected daily bowel movements and tracked the lifestyle habits of two people over the course of a year. They tracked habits such as diet, sleep, mood, leisure activity, and exercise. They found that the bacterial populations are relatively stable over time, but fluctuate on a daily basis in response to certain lifestyle factors.

“On any given day, the amount of one species could change manifold, but after a year, that species would still be at the same median level,” noted Eric Alm, PhD, lead researcher. “To a large extent, the main factor we found that explained a lot of the variance was the diet.”

It’s nice to know that we have some control over our bacterial balance by the foods we eat. Eating more dietary fiber increased certain beneficial populations of bacteria, such as Bifidobacteria. High-fiber foods were found to change the gut bacterial composition rapidly. Interestingly factors such as mood and sleep did not have as much of an effect on bacteria populations.

During the course of the study, each of the two subjects experienced an event that changed their bacterial balance significantly. One subject experienced food poisoning that completely altered gut balance so much so that it did not fully recover. The other subject visited a third-world country and experienced diarrhea for two weeks, resulting in a change in bacteria balance that eventually returned to normal after return to the United States.

This study gives us a small glimpse into the dynamic world of our inner inhabitants. It shows that we can positively affect our gut balance by eating the right foods, but it also shows us the vulnerability of our gut bacteria to disease. Eating healthy not only for our own sake, but for the sake of our bacteria, is becoming more important than ever.