The human body contains tenfold more microbial cells than human cells. The digestive tract, however, is by far the most densely populated with microorganisms. There are thousands of microbial species that thrive in the human intestine. One might even dare call them an "organ", an organ essential for proper immunity, digestion, metabolism, and other physiological processes.
They extract energy from several types of indigestible carbohydrates, especially certain fibers. Their activity leads to the production of several key nutrients, such as vitamin K, B12, folic acid, short-chain fatty acids, and even amino acids. 7
As far as immunity goes, these friendly bacteria defend our bodies against certain pathogens. Some of them actually produce anti-microbial chemicals to protect our body from harmful bacteria.
Finally, these intestinal bacterial residents help maintain the health and function of our digestive tract's own immunity and barrier against the outside world. 1
Still, we currently only understand its most basic features and functions. There is still so much to explore and discover about the human gut microflora.
What We Do Know
One thing we do know is that our diet (what we eat) influences the diversity and number of bacteria living in our digestive tract. 10 Interestingly, as early as 1962 we find research pointing to a similar conclusion. For example, one important conclusion was that the microbiota of almost all breastfed infants contain high populations of Bifidobacterium. 4, 8
Another study in 1998 showed that the composition of intestinal bacteria varies signifcantly among indiviuals. One reason is because of genetics. 3 Disease can alter and even lead to an imbalance in the body's microbiota. Also, with age, the microbiota can change significantly.
As aforementioned, however, our diets can have a huge impact on how our microbiota looks.
One recent study named a few factors that have led to significant changes in the composition of microbes in the human gut - antibiotics, C-sections, a drop in breastfeeding, sedentary behavior, and diets low in fiber-packed fruits, veggies, and whole grains (fiber being the main food source of these bacteria).
Researchers closely examined one of the few remaining traditional hunter-gatherer populations in the world - the Hadza in Tanzania. They found that the microbiota of the Hadza is more diverse than of those living in industrialized countries. Their microbiotas also change with the seasons as their diet does. Interestingly, study findings show they get as much as 100 grams of fiber a day in their food. 6,10
The average American only gets 15.
These results go hand in hand with what a study by Stanford University School of Medicine found - that low-fiber diets may deplete the complex microbial environment in human guts to the point of being irreversible. 6
In this study, mice were either given a fiber-rich diet or a diet almost devoid of fiber. Though initially the two groups' fecal samples were similar, after a couple of weeks a significant change was observed in the diversity of bacterial species in their guts. Some species disappeared, others dropped significantly. 6
Easy Ways to Boost Fiber Intake
Clearly, there is a link between our dietary choices and the fate of those friendly, hard-working bacteria residing inside us. This is especially true of our fiber intake.
Is there a way to increase fiber intake without even feeling like you're trying?
Simple solutions include adding a fruit to every meal, throwing in a tablespoon of chia or flax seeds with your yogurt, smoothie, or salads, saving the skin of your fruits and potatoes, and switching to whole grains instead of white ones (white bread, rice, refined flours). Adding just one quarter cup of beans to your next salad or pasta dish will add an extra 5 grams of fiber.
One recent study even found that an increase in intake of anthocyanin-rich red berries led to an increase of Bifidobacterium populations. 2
We've long known that the many bacteria residing in the human gut work wonders for our health. It turns out, though, that our choices can have a huge impact on their health. Yes, it's obvious that our diets influence the type and number of bacteria in our gut.
While there is still a need for more studies on how each individual food influences them, we know enough to realize just how crucial it is to "feed" our friendly residents the food they, you might say, deserve for all their hard work.
1. Aliment Pharmacol Ther. 2008 Jan 15; 27(2):104-19.
2. Am J Clin Nutr. 2012 Jun; 95(6):1323-34.
3. Appl Environ Microbiol. 1998 Oct; 64(10):3854-9.
4. Curr. Issues Intest. Microbiol. (2001) 2(1): 1-15.
5. Eckburg, P. B., Bik, E. M., Bernstein, C. N., Purdom, E., Dethlefsen, L., Sargent, M., … Relman, D. A. (2005). Diversity of the Human Intestinal Microbial Flora. Science (New York, N.Y.), 308(5728), 1635–1638. http://doi.org/10.1126/science.1110591
6. Goldman, B. (1970, January 13). Low-fiber diet may cause irreversible depletion of gut bacteria over generations. Retrieved August 30, 2017, from https://med.stanford.edu/news/all-news/2016/01/low-fiber-diet-may-cause-irreversible-depletion-of-gut-bacteria.html
7. J Clin Gastroenterol. 2006 Mar; 40(3):235-43.
8. Rosebury, T. 1962. Microorganisms Indigenous to Man. McGraw-Hill, Microbial Biota of the Human Intestine 15 New York, pp. 1-435.
9. Savage DC. Microbial ecology of the gastrointestinal tract. Annu Rev Microbiol. 1977;31:107–133.
10. Stanford University School of Medicine, news release, Aug. 24, 2017. https://medlineplus.gov/news/fullstory_168008.html