Prebiotic: Specialized fiber that supports the beneficial microbes that are present in your gut.
Prebiotics are a special type of fiber that pass through the stomach and feed the good bacteria that live in your large intestine (3). Fiber is arguably the most important factor in maintaining a healthy and balanced gut microbiome.
Dietary fibers are carbohydrates with both soluble and insoluble parts.
· Insoluble fiber is resistant to digestion; you’ve probably heard Grandma call it roughage. Insoluble fiber has many benefits, including preventing constipation and possibly reducing the risk of diverticular disease (4).
· Soluble fibers can be broken down by bacteria in the gut to produce active byproducts such as short-chain fatty acids, which are associated with the health benefits linked to fiber.
The Institute of Medicine (5) currently recommends an average of 25 grams of fiber per day for women and 38 grams per day for men. However, the average American consumes just 15 grams per day.
Courtesy of CSIRO.
Not all fiber is prebiotic
There are three criteria for classifying a compound as prebiotic:
It is resistant to stomach acid, digestive enzymes and is not absorbed in the gastrointestinal tract
It is fermented by intestinal bacteria
It selectively promotes the growth of intestinal bacteria associated with health and well-being
Many fiber components meet these requirements, but not all. Examples of prebiotics:
Inulin (Fructo-oligosaccharide, FOS)
Galactose (Galacto-oligosaccharide, GOS)
Xylose (Xylo- oligosaccharide, XOS)
What foods contain prebiotic fiber?
Prebiotic fiber is found in many fruits and vegetables such as:
It should be easy to get enough prebiotic fiber from all these different sources, right?
Unfortunately no, it’s not so easy. These foods contain relatively small amounts of prebiotics per serving. The foods with the highest amounts (such as chicory root) only contain 1-2g per serving. You’re going to need to eat a lot of chicory to meet those daily fiber intake goals!
The primary goal of prebiotics is to increase the numbers of healthy bacteria in your microbiome and stimulate the production of the short chain fatty acid butyrate.
The most studied of these prebiotics is Inulin (FOS), and it’s ability to stimulate healthy bacteria and butyrate production is well documented. However research interest into other types of prebiotics with these same benefits is rapidly increasing.
Health benefits associated with prebiotic consumption include:
Increased stool frequency
Increased absorption of dietary minerals in the colon (good for bone health)
Increased secretion of satiety hormones (feeling full)
Increased antioxidant production
Reduced blood cholesterol levels
How do prebiotics work?
Perhaps a better way to think of this is why is a low-fiber diet bad for our health?
A recent study has shown that when microbes inside the digestive tract of mice don't get the natural prebiotic fiber that they rely on for food, they begin to munch away the natural layer of mucus that lines (and protects) the gut, eroding it to the point where dangerous invading bacteria can infect the colon wall. The protection of the mucus layer only happened when the mice were fed a diet containing fiber from a variety of sources, giving the mice only single, purified prebiotics still resulted in degradation of the mucus layer. This reiterates why the complete fiber approach is necessary for gut microbiome health. The scientists note however, that mixes of prebiotic fibers might be more effective, which is something they will be studying in future. It is also worth noting here that the mice receiving the prebiotic only received no other type of fiber and this simply wouldn’t happen to someone in real life. So, adding in a single prebiotic supplement as part of a balanced diet should still be beneficial.
While this evidence comes from work in mice, the results mirror everything that doctors and nutritionists have been telling us for years:
Eat lots of fiber from diverse sources!
Blaser, M. J. Who are we? Indigenous microbes and the ecology of human diseases. EMBO Rep.7, 956–960 (2006).
Strachan, D. P. Hay fever, hygiene, and household size. BMJ299, 1259–1260 (1989).
Roberfroid, M. Prebiotics: the concept revisited. J. Nutr.137, 830S–7S (2007).
Strate, L. L. Lifestyle factors and the course of diverticular disease. Dig Dis30, 35–45 (2012).
Trumbo, P., Schlicker, S., Yates, A. A., Poos, M.Food and Nutrition Board of the Institute of Medicine, The National Academies. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. J Am Diet Assoc102, 1621–1630 (2002).
Mariat, D. et al. The Firmicutes/Bacteroidetes ratio of the human microbiota changes with age. BMC Microbiol.9, 123 (2009).
Cook, S. I. & Sellin, J. H. Review article: short chain fatty acids in health and disease. Alimentary Pharmacology & Therapeutics12, 499–507 (1998).
Nicholson, J. K. et al. Host-gut microbiota metabolic interactions. Science336, 1262–1267 (2012).
Frank, D. N. et al. Molecular-phylogenetic characterization of microbial community imbalances in human inflammatory bowel diseases. Proc. Natl. Acad. Sci. U.S.A.104, 13780–13785 (2007).
Wang, T. et al. Structural segregation of gut microbiota between colorectal cancer patients and healthy volunteers. ISME J6, 320–329 (2012).
Wong, J. M. W., de Souza, R., Kendall, C. W. C., Emam, A. & Jenkins, D. J. A. Colonic health: fermentation and short chain fatty acids. J. Clin. Gastroenterol.40, 235–243 (2006).
Samuel, B. S. et al. Effects of the gut microbiota on host adiposity are modulated by the short-chain fatty-acid binding G protein-coupled receptor, Gpr41. Proceedings of the National Academy of Sciences105, 16767–16772 (2008).
Desai, Mahesh S. et al. A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility. Cell ,Volume 167, Issue 5,1339 - 1353.e21 (2016).