Can you guess which nutrient 95% of Americans are not getting enough of?

It’s not protein, nor is it any vitamin or mineral – it’s fiber. Virtually the entire U.S. population is deficient in dietary fiber, also known as roughage or bulk. ₁ The standard American diet, filled with animal products and junk food, is largely to blame, but even people trying to eat healthier often overlook the importance of fiber, or have an outdated idea of what a high-fiber diet entails (what, just prunes and bran flakes?). Eating a high-fiber diet is extremely good for our health. But this is even more apparent when it comes to our gut bacteria, collectively referred to as our gut microbiota.

"THE FORGOTTEN ORGAN"

Our gut microbiota consists of trillions of bacteria cells, which are mostly located in our large intestine. Once overlooked as a useless bystander tagging along for a free ride, we now know that our gut microbiota has a major influence on our health, giving it the nickname, “the forgotten organ.” The microbes living in our large intestine are now thought to play a role in some of our most common chronic diseases including, obesity, type-2 diabetes, heart disease, nonalcoholic fatty liver disease, inflammatory bowel disease and even certain cancers. ₂ And it turns out that certain day-to-day choices can heavily influence our gut microbiota.

Although many things affect our gut microbiota (past antibiotic use, where we live, even how we were delivered as a baby), something we can do on a daily basis is change what we eat. Research consistently shows that consuming a high-fiber, plant-based diet leads to favorable changes in our gut microbiota, whereas consuming a high-fat, low-fiber, meat-based diet leads to unfavorable changes. ₃₋₈

A HIGH VS LOW FIBER DIET

Fiber intake, or lack thereof, is thought to explain most of these bacterial changes seen between plant-based and meat-based diets. Humans can’t digest fiber, but our gut bacteria can. In fact, it’s their favorite food. So when we eat a high-fiber diet, our microbes feast on an all-you-can-eat buffet, which supports a healthy, happy, diverse and well-populated bacterial community. In return, these well-fed bacteria produce beneficial nutrients that work to keep us healthy, such as short-chain fatty acids, which reduce inflammation and help regulate our immune system. ₆ In other words, choosing a high-fiber diet supports a healthy gut microbiota, which in turn, works to keep us healthy. A true symbiotic relationship. 

Unfortunately, the opposite is true as well. A low-fiber diet centered on junk food and animal products starves our gut bacteria. Such a diet causes a decrease in microbial diversity and because there’s no fiber, the starving microbes that remain begin to feed on proteins, or even our body’s own intestinal cells. This leads to a decrease in short-chain fatty acid production and an increase in detrimental byproducts, such as secondary bile acids, ammonia and hydrogen sulfide, which promote inflammation and increase our risk of colorectal cancer. ₆ If we want to avoid this situation, we need to consume a high-fiber diet. 

HIGH-FIBER FOODS

Animal products do not contain fiber as it’s only found in plants. Great sources include:

• Legumes (beans, peas, lentils, soy products)
• Whole grains (oats, barley, quinoa, whole wheat, brown rice)
• Fruit
• Vegetables
• Nuts and seeds 

These plant foods should be consumed in their whole, unrefined state. For example, we should choose baked or boiled potatoes instead of fried potato chips, fruit instead of Froot Loops and black beans instead of jelly beans.

Beyond fiber, other nutrients found in plants may also feed our gut bacteria. Polyphenols, found in many fruits and vegetables, as well as tea, dark chocolate and red wine, can provide a nice snack for our microbiota. Certain starch molecules, referred to as “resistant starch,” can also be a good food source for our gut bacteria. Foods rich in these starches include potatoes and corn, as well as most whole grains and beans.

CAN I EAT TOO MUCH FIBER?

Although a high-fiber diet is optimal for our gut microbiota, a sudden and drastic upswing in fiber consumption can lead to some unwanted gastrointestinal side effects, like gas and bloating. A few tips to minimize these symptoms include:

• Gradually introduce high-fiber foods into your diet
• Choose cooked vegetables instead of raw
• Choose sprouted whole grains and whole grain products
• Chew your food well and eat slowly
• Give it time: As your gut bacteria evolve and adapt, symptoms generally improve

THE BOTTOM LINE

Our gut bacteria are totally dependent on us for food. What we eat, they eat. To keep them well-fed, we should consume a high-fiber diet rich in plants. Doing so will keep them, and in turn, us, happy and healthy.    

References:

1. Quagliani D, Felt-Gunderson P. Closing America's Fiber Intake Gap: Communication Strategies From a Food and Fiber Summit. Am J Lifestyle Med. 2016;11(1):80-85. Published 2016 Jul 7. doi:10.1177/1559827615588079
2. Wang et al. The Human Microbiota in Health and Disease. Engineering 2017; 3(1):71-82. doi: https://doi.org/10.1016/J.ENG.2017.01.008
3. David, L., Maurice, C., Carmody, R. et al.Diet rapidly and reproducibly alters the human gut microbiome.Nature 505, 559–563 (2014). https://doi.org/10.1038/nature12820
4. Stephen J. D. O'Keefe, Dan Chung, Nevine Mahmoud, Antonia R. Sepulveda, Mashudu Manafe, Judith Arch, Haytham Adada, Tian van der Merwe, Why Do African Americans Get More Colon Cancer than Native Africans?,The Journal of Nutrition, Volume 137, Issue 1, January 2007, Pages 175S–182S, https://doi.org/10.1093/jn/137.1.175S
5. O’Keefe, S., Li, J., Lahti, L. et al.Fat, fibre and cancer risk in African Americans and rural Africans. Nat Commun 6, 6342 (2015). https://doi.org/10.1038/ncomms7342
6. Makki K, Deehan EC, Walter J, Bäckhed F. The Impact of Dietary Fiber on Gut Microbiota in Host Health and Disease. Cell Host Microbe. 2018;23(6):705-715. doi:10.1016/j.chom.2018.05.012
7. Wong MW, Yi CH, Liu TT, et al. Impact of vegan diets on gut microbiota: An update on the clinical implications. Ci Ji Yi Xue Za Zhi. 2018;30(4):200-203. doi:10.4103/tcmj.tcmj_21_18
8. Tomova A, Bukovsky I, Rembert E, et al. The Effects of Vegetarian and Vegan Diets on Gut Microbiota. Front Nutr. 2019;6:47. Published 2019 Apr 17. doi:10.3389/fnut.2019.00047