The Superfood You (And Your Microbes) Aren’t Getting Enough Of
With all the noise about superfoods and fad diets, it’s easy to forget about the fundamentals of healthy eating—including a macronutrient that evades classification, and, too often, public awareness (or aversion): fiber.
With all the noise about superfoods and fad diets (have you met a breatharian1 yet?) it’s easy to forget about the fundamentals of healthy eating.
These include maintaining a well-balanced diet made up of mostly whole foods, and seeking out essential nutrients that cannot be synthesized by the body—vitamins, minerals, select fatty acids and amino acids, to name a few. They also include getting enough of a macronutrient that evades classification, and, too often, public awareness (or aversion): fiber.2 (Note: Access to healthy, whole foods is a human right, however, it is clear that access is not equitable.)
A 2015 survey found that just 4% of adult males and 13% of adult females were meeting their daily recommended fiber intake.3 This doesn’t just create problems in the bathroom. Studies have shown that fiber can reduce your risk for heart disease and stroke, combat obesity and type 2 diabetes, and even help prevent certain cancers.4,5,6,7 This shouldn’t come as a surprise, considering the influence the digestive system has on whole-body health.
Fiber isn’t just a true-to-life superfood; it’s the original one.8 According to researchers at the Dietary Guidelines of America, it’s also “an underconsumed nutrient of public health concern”.9 But before you go raiding your local cereal aisle, it’s important to learn what fiber is, and how it’s used by our bodies. It’s also important to learn the differences between various types of fiber, because they are not all created equal.
Let’s dig in.
Dietary fiber is actually a fairly broad category. It refers to the part of plant-based carbohydrates that cannot be directly digested or absorbed by your body (unlike, say, sugars and most starches). Fiber can be found in fruits and vegetables, legumes, nuts and seeds, and whole grains, and comes in numerous forms: insoluble and soluble (easily dissolved in water), which includes viscous (gel-forming) and fermentable (digestible only by bacteria) fibers. Those fermentable fibers—a relatively recent discovery—are particularly fascinating, due to the myriad ways in which they interact with our gut microbiome.
Blueberry microscopy. One cup (148 grams) of blueberries provides 3.6 grams of fiber.
Fiber, we’re taught, is necessary for bowel movements. And it’s true—the roughage produced by indigestible fiber passes relatively intact/unmodified through your digestive system and helps both solidify and soften stool, which can lead to more regular and, well, pleasant bowel movements, as well as helping to prevent hemorrhoids and diverticulitis. Fiber also had a brief moment in our mixed-up diet culture: foods rich in fiber may help you feel full longer, leading to weight loss.10
But the benefits of a high-fiber diet go far beyond the time you spend on the toilet. Recent studies have shown that increasing your intake of soluble fiber can lower the amount of low-density lipoprotein (or “bad”) cholesterol in your body; fiber has also been linked to a decrease in blood pressure.11,12 These effects indicate that fiber intake plays a role in preventing heart disease and heart attacks.
In fact, a recent review in The Lancet concluded that for every 1,000 people that transitioned to a high-fiber diet, six cases of heart disease would be prevented.13 The Lancet review also demonstrated consistently lower rates of type 2 diabetes in individuals with higher fiber intake, and a lesser incidence of bowel cancer.
Other studies have demonstrated a lower risk of colorectal cancer, and researchers are now looking into whether fiber intake can prevent other types of cancers, including cancers of the mouth, pharynx, larynx, esophagus, colon, rectum, and stomach.14,15 The bottom line? Eating more fiber can lower your mortality rate.16
It’s only recently that fiber’s role in the health of our microbiome has become a focus of study.
We know that for our ancestors, fiber was essential — and essentially unavoidable, as whole foods (mainly fruits, vegetables, and legumes) dominated the majority of human diets. Only in the last 100 years did fiber cease to be a cornerstone of our nutritional intake.17 That means that for hundreds of thousands of years, our diverse ancestral microbiomes adapted to a steady dose of fiber.
Hunter-gatherers relied on the microbiota-accessible carbohydrates (MACs) found in dietary fiber to shape and sustain their microbial ecosystems. Recently, research has discovered that low-fiber diets can cause microbial extinction in just a few generations. That means when we don’t get enough fiber from diverse sources, microbes disappear, lowering gut microbial diversity. And when they’re gone, they don’t come back.18
Studies have found that immigrants to the U.S. experience a rapid loss in gut microbiome diversity as they start consuming a low-fiber Western diet, and that over generations, that loss of diversity is associated with obesity in these populations.19 Bottom line? The American diet, with its focus on processed foods that lack a healthy diversity of plant fibers, is slowly destroying the rich community of beneficial microbes inside of us—undoing thousands of years of co-evolution with our ancestors.
So, what kind of fiber do your microbes really need?
Recent studies that focus on the interaction of fiber and our microbiome have discovered what may be the key to understanding just how important fiber is to our overall health.
Remember those microbiota-accessible carbohydrates (MACs) found in fiber? Without MACs, our gut bacteria are unable to generate short-chain fatty acids (SCFAs) like acetate and butyrate. SCFAs are gut miracle workers, stimulating blood flow and increasing mucus production throughout our digestive systems, and have been suggested as a means to prevent and treat diseases such as ulcerative colitis, diabetes, and cancer.20,21 And it’s those SCFAs that are first to vanish when our bodies are starved of dietary fiber.
As we learn more about our miraculous microbiomes, one thing becomes clear: we want to treat them well, which means feeding them what they want to eat. And while dietary fiber isn’t as buzzy (or as marketable) as kombucha, fiber is the thing for which our microbiome clamors, and crave.
The National Institute of Medicine recommends a daily fiber intake of 25 grams for women and 38 grams for men.22 The average American gets only 16 grams a day. If you’re part of the 95% not meeting that target, you’re missing out on a health panacea. So what can you do about it?
First, understand that not all fiber is created equal. Fructo-oligosaccharides, for example, is a type of fiber found in fruit and vegetables. Certain fibers are also prebiotics: they are not digested in the small intestine, but are digested via fermentation by our gut microflora in the colon (part of the large intestine).
Inulin, another type of fiber that is more difficult to obtain from food alone (it is found in limited sources such as artichokes and chicory root) has been found to encourage the growth of Bifidobacteria and Lactobacteria, two broad bacterial genera that make up most of the bacterial gut microbiome.23
A wide-ranging study by the American Gut Project found that people who ate more than 30 different plant types each week have significantly higher gut microbial diversity.24 The best way to close your own fiber gap? Work on eating a diverse diet of whole foods. Legumes, nuts, whole grains, fruits and vegetables—all of these are fabulous sources of fiber that are easy to incorporate into a daily diet.
- Kerr, B. (2017, September 7). Breatharians: The People Who Think Air Is Food. GQ. https://www.gq.com/story/breatharians-the-people-who-think-air-is-food
- Kohn, J. B. (2016). Is Dietary Fiber Considered an Essential Nutrient? Journal of the Academy of Nutrition and Dietetics, 116(2), 360. https://doi.org/10.1016/j.jand.2015.12.004
- USDA. (2015, February). Scientific Report of the 2015 Dietary Guidelines Advisory Committee, Advisory Report to the Secretary of Health and Human Services and the Secretary of Agriculture.
- Threapleton, D. E., Greenwood, D. C., Evans, C. E., Cleghorn, C. L., Nykjaer, C., Woodhead, C., Cade, J. E., Gale, C. P., & Burley, V. J. (2013). Dietary Fiber Intake and Risk of First Stroke. Stroke, 44(5), 1360–1368. https://doi.org/10.1161/strokeaha.111.000151
- Dietary fiber protects against obesity and metabolic syndrome, study finds. (2018, January 22). ScienceDaily. https://www.sciencedaily.com/releases/2018/01/180122184723.htm
- Zhao, L., Zhang, F., Ding, X., Wu, G., Lam, Y. Y., Wang, X., Fu, H., Xue, X., Lu, C., Ma, J., Yu, L., Xu, C., Ren, Z., Xu, Y., Xu, S., Shen, H., Zhu, X., Shi, Y., Shen, Q., . . . Zhang, C. (2018). Gut bacteria selectively promoted by dietary fibers alleviate type 2 diabetes. Science, 359(6380), 1151–1156. https://doi.org/10.1126/science.aao5774
- Aune, D., Chan, D. S. M., Lau, R., Vieira, R., Greenwood, D. C., Kampman, E., & Norat, T. (2011). Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies. BMJ, 343(nov10 1), d6617. https://doi.org/10.1136/bmj.d6617
- Pontzer, H., Wood, B. M., & Raichlen, D. A. (2018). Hunter-gatherers as models in public health. Obesity Reviews, 19, 24–35. https://doi.org/10.1111/obr.12785
- Clemens, R., Kranz, S., Mobley, A. R., Nicklas, T. A., Raimondi, M. P., Rodriguez, J. C., Slavin, J. L., & Warshaw, H. (2012). Filling America’s Fiber Intake Gap: Summary of a Roundtable to Probe Realistic Solutions with a Focus on Grain-Based Foods. The Journal of Nutrition, 142(7), 1390S-1401S. https://doi.org/10.3945/jn.112.160176
- Thompson, S. V., Hannon, B. A., An, R., & Holscher, H. D. (2017). Effects of isolated soluble fiber supplementation on body weight, glycemia, and insulinemia in adults with overweight and obesity: a systematic review and meta-analysis of randomized controlled trials. The American Journal of Clinical Nutrition, 106(6), 1514–1528. https://doi.org/10.3945/ajcn.117.163246
- Gunness, P., & Gidley, M. J. (2010). Mechanisms underlying the cholesterol-lowering properties of soluble dietary fibre polysaccharides. Food & function, 1(2), 149–155. https://doi.org/10.1039/c0fo00080a
- Aleixandre, A., & Miguel, M. (2016). Dietary fiber and blood pressure control. Food & function, 7(4), 1864–1871. https://doi.org/10.1039/c5fo00950b
- Reynolds, A., Mann, J., Cummings, J., Winter, N., Mete, E., & te Morenga, L. (2019). Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. The Lancet, 393(10170), 434–445. https://doi.org/10.1016/s0140-6736(18)31809-9
- Kunzmann, A. T., Coleman, H. G., Huang, W. Y., Kitahara, C. M., Cantwell, M. M., & Berndt, S. I. (2015). Dietary fiber intake and risk of colorectal cancer and incident and recurrent adenoma in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. The American journal of clinical nutrition, 102(4), 881–890. https://doi.org/10.3945/ajcn.115.113282
- Collins, K. (2020, August 19). Facts on Fiber and Whole Grains. American Institute for Cancer Research. https://www.aicr.org/resources/blog/ask-the-dietitian-get-your-facts-right-on-fiber-and-whole-grains/
- Yang, Y., Zhao, L. G., Wu, Q. J., Ma, X., & Xiang, Y. B. (2015). Association Between Dietary Fiber and Lower Risk of All-Cause Mortality: A Meta-Analysis of Cohort Studies. American Journal of Epidemiology, 181(2), 83–91. https://doi.org/10.1093/aje/kwu257
- Belluz, J. (2019, July 15). Fiber: The superfood 95 percent of Americans fail to eat enough of. Vox. https://www.vox.com/2019/3/20/18214505/fiber-diet-weight-loss
- Sonnenburg, E. D., Smits, S. A., Tikhonov, M., Higginbottom, S. K., Wingreen, N. S., & Sonnenburg, J. L. (2016). Diet-induced extinctions in the gut microbiota compound over generations. Nature, 529(7585), 212–215. https://doi.org/10.1038/nature16504
- Vangay, P., Johnson, A. J., Ward, T. L., Al-Ghalith, G. A., Shields-Cutler, R. R., Hillmann, B. M., Lucas, S. K., Beura, L. K., Thompson, E. A., Till, L. M., Batres, R., Paw, B., Pergament, S. L., Saenyakul, P., Xiong, M., Kim, A. D., Kim, G., Masopust, D., Martens, E. C., . . . Knights, D. (2018b). US Immigration Westernizes the Human Gut Microbiome. Cell, 175(4), 962–972.e10. https://doi.org/10.1016/j.cell.2018.10.029
- Henningsson, S., Björck, I., & Nyman, M. (2001). Short-chain fatty acid formation at fermentation of indigestible carbohydrates. Näringsforskning, 45(1), 165–168. https://doi.org/10.3402/fnr.v45i0.1801
- Baxter, N. T., Schmidt, A. W., Venkataraman, A., Kim, K. S., Waldron, C., & Schmidt, T. M. (2019). Dynamics of Human Gut Microbiota and Short-Chain Fatty Acids in Response to Dietary Interventions with Three Fermentable Fibers. mBio, 10(1). https://doi.org/10.1128/mbio.02566-18
- The National Academies of Sciences, Engineering, Medicine. (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. The National Academies Press.
- Slavin, J. (2013). Fiber and Prebiotics: Mechanisms and Health Benefits. Nutrients, 5(4), 1417–1435. https://doi.org/10.3390/nu5041417
- Buschman, H., PhD. (2018, May 15). Big Data from World’s Largest Citizen Science Microbiome Project Serves Food for Thought. UC Health – UC San Diego. https://health.ucsd.edu/news/releases/Pages/2018-05-15-big-data-from-worlds-largest-citizen-science-microbiome-project-serves-food-for-thought.aspx