Fiber is the primary source of fuel for our gut bacteria. Human cells cannot digest dietary fiber, its sole purpose is to feed gut bacteria. Fiber increases the diversity and abundance of the gut microbiota.
When fiber is fermented by gut bacteria it results in the production of short-chain fatty acids (SCFAs). SCFAs are responsible for a lot of the amazing, beneficial impacts of the microbiota. SCFAs have a role in metabolism, insulin sensitivity, gut transit time, inflammation, reducing appetite and improving satiety…just to name a few!
How Much Fiber Per Day?
Health Canada suggests a minimum of 25g/day for women and 38g/day for men. But I recommend aiming for closer to 50 grams of fiber per day as recommended by the well-established Mediterranean diet.
Types of Fiber
Dietary fiber is divided into two main types: insoluble or soluble fiber. Plant foods generally contain both insoluble and soluble fiber in varying amounts.
Dietary fiber also contains many types of different bioactive plant components. Some common ones that you may have heard of include non-starch polysaccharides, cellulose, dextrins, inulin, lignins, resistant starch, pectins, beta-glucans and oligosaccharides.
What is the Difference Between Soluble and Insoluble Fiber?
Soluble fiber (dissolves in water):
- Slows digestion, therefore increasing satiety and preventing blood sugar spikes by reducing the rate at which your body absorbs sugar from the foods you eat. This type of fiber is particularly beneficial for the management of diabetes and prediabetes.
- Lowers LDL (bad) cholesterol by binding with fatting acids and helping to flush them out of the body (rather than being absorbed)
- Fermented in the colon into gasses and produces physiologically active by-products such as short-chain fatty acids (SCFAs) produced by gut bacteria
- Examples: inulin oligofructose, beta-glucans, pectins and gums, psyllium, dextrin
- Food sources: oats and oat bran, barley rye, chia, flax, apples, bananas
Insoluble fiber (does not dissolve in water):
- Promote bowel regularity/adds bulk to stools by helping to hydrate and move waste through your digestive tract
- May be beneficial in treating constipation, diverticular diseases and some types of IBS and may reduce the risk of colon cancer
- Examples: lignins, resistant starches, cellulose, hemicellulose
- Food sources: bran, whole grains, whole-wheat and wheat bran, most vegetables, skins of fruits and vegetables, legumes, nuts, berries
Functional fiber/Fiber Supplements: extracted from natural (food) sources, then added to supplements or fortified foods
The most common fiber supplements are metamucil (100% psyllium fiber) and inulin, both of which are sources of soluble fiber. Metamucil is commonly used for the treatment of constipation as it assists in bulk-forming through the absorption of liquid in the intestines and therefore assists in the elimination of stool. Inulin is a popular prebiotic that has been associated with beneficial health outcomes in clinical studies including decreased inflammation, improved gut barrier function, reduced body fat and appetite regulation.
Although there is not enough evidence available for me to support the routine use of fiber supplements, the demonstrated health benefits of inulin supplementation are promising. As always, before taking a supplement I would recommend focusing on including lots of prebiotic fiber-containing foods in your diet first!