Microbes provide new reasons to love your fibre
Kristina Campbell, MSc
Fibre does a lot more than keep us regular. Discover the important role it plays in our gut.
“Eat more fibre” is the oldest nutritional advice in the book. Yet scientists have long been perplexed by how fibre carries out its beneficial work. It turns out the real power of fibre is that it feeds the trillions of micro-organisms living in our intestinal tract, which support our health in many ways.
Fibre is a type of carbohydrate that is impossible for the human body to digest, but that doesn’t mean it’s irrelevant to health. When you take a bite of an apple, for example, the fibre components pass into your stomach and small intestine without being digested. Then they pass into your large intestine, where the microbiota take over and begin to ferment it.
The process of fermentation works like this: bacteria use enzymes to “chop up” the fibre and produce short-chain fatty acids (SCFAs) and several other products. The key output of fermentation is one SCFA called butyrate. Cells of the body are greedy for butyrate and use it as an energy source.
An established body of research has linked higher fibre consumption to numerous health benefits, including
As scientists begin to uncover the mechanisms of action, it appears the microbiota mediate many, if not all, of these benefits.
We know that a diverse microbiome is linked to better health. We also know that groups around the world living traditional hunter-gatherer lifestyles have a more diverse set of microbes than those living a typical Western lifestyle.
Many factors probably contribute to this discrepancy, but Stanford researchers recently proposed that a lack of “microbiota-accessible carbohydrates” in the diet is one of the biggest factors that keep those in the Western world microbially poor.
A diet low in fibre may weaken the body’s defences: the immune system becomes imbalanced when the microbiota don’t produce enough SCFAs, which help the body react appropriately to harmful stimuli. Low intake of fibre, and thus, low SCFA production, also damages the protective mucous layer in the colon and increases the likelihood that colitis will develop.
Numerous studies are also linking low-fibre diets to the risk of colon cancer. In one recent study, researchers took groups of African-Americans and rural South Africans and switched their habitual diets: the African-Americans ate African-style foods low in fat and high in fibre, while the Africans consumed a typical American diet (including plenty of hamburgers and barbecued meats) high in fat and low in fibre. After two weeks, those consuming the low-fibre diet had increased biomarkers of colon cancer risk; for example, lower quantities of SCFAs.
This and other studies show that adding or taking away fibre from the diet can cause rapid changes in how many microbes proliferate and how they function, so it’s vital to feed them their roughage on a regular basis.
How much fibre will get your microbes through the day? It’s much more than the 15 g per day consumed on a typical Western diet. Canadian Dietary Reference Intakes recommend adult males get 38 g per day. This can be achieved by eating a cup of granola (11 g) with blueberries (4 g) for breakfast, bean salad (14 g) for lunch, and a serving of broccoli (9 g) for dinner. For females, the targets are slightly lower, at 25 g per day. Pregnant women should boost this to 28 g, and lactating mothers to 29 g. Slightly less fibre is required for men and women over the age of 50.
Microbes seem to need both soluble and insoluble fibre to confer their health benefits. Soluble fibre, found in oatmeal, nuts, legumes, and fruits, generally helps regulate blood glucose levels, while the insoluble fibre that is abundant in whole grains and vegetables can help food move through the digestive system and promote bowel regularity.
|probiotics (live bacteria)||many potential health benefits throughout the lifespan, primarily relating to inflammation, the immune system, and digestive health|
|prebiotics (food for live bacteria)||possible benefits include reducing the risk of gastrointestinal infection and inflammation, inflammatory bowel disease and irritable bowel syndrome, and promoting weight loss|
|synbiotics (probiotics and prebiotics together)||benefits derive from the actions of the two components; more research is needed on potential synergistic effects|
|dietary fibre||fibre found naturally in foods|
|functional fibre||a substance added to food and drinks to boost their fibre content; it has been extracted from plants or animals, or commercially produced|
|soluble fibre||readily dissolves in water; binds with fatty acids and slows the release of blood sugars|
|insoluble fibre||does not dissolve in water; helps food move smoothly through the gastrointestinal tract, providing bulk to the stool and reducing constipation|
|microbiota-accessible carbohydrates (MACs)||carbohydrates (namely, certain types of fibre that come from plants, fungi, or animal tissues) that can be metabolically used by gut microbes|
|prebiotics||special kinds of fibre that have the ability to change the composition or activity of bacteria in the colon and result in a health benefit; often act as food for Lactobacilli and Bifidobacteria; common prebiotics include inulin (found naturally in leeks, asparagus, chicory, Jerusalem artichokes, garlic, onions, wheat, oats, and soybeans), oligofructose, and galactooligosaccharides (added to foods such as yogurts, cereals, breads, cookies, ice cream, spreads, and drinks)|