Beyond control of blood sugar and reduction of blood cholesterol, fiber — specifically soluble fiber's — claim to fame is intimately tied to the complex world of intestinal bacteria, so-called probiotics. In this framework, particular forms of soluble fiber have taken on an alternate identity and are known as prebiotics.
Prebiotics are more than oddities of structure that convey certain mechanical advantages; they're genuine food, valuable sources of energy that contribute to what is now recognized as "the prebiotic effect."
According to an article published last summer in The British Journal of Nutrition, the prebiotic effect — a now well-established scientific fact — is technically defined as "the selective stimulation of growth and/or activity(ies) of one or a limited number of microbial genus(era)/species in the gut microbiota that confer(s) health benefits to the host." That's a mouthful, but what it means is an ingredient can be defined as a prebiotic if it activates or stimulates the growth of healthy bacteria in the gastrointestinal tract.
Fiber typically is bundled into two broad categories, soluble and insoluble. Insoluble fibers garnered most of the attention because they took on so many different roles. Unlike their sawdust-like cousins that whisk food remnants through the colon in record time, soluble fibers swell with water and slow the movement of food through the small intestine. In the process, the entrance of sugar into the blood is delayed. Soluble fibers also trap bile acids, which are made from cholesterol by the liver, and prevent their absorption.
While there are healthy bacteria throughout the human digestive tract, food moves through too quickly in the upper regions for the residing bacteria to make much use of prebiotics. So it's in the colon where healthy bacteria (probiotics) take advantage of the prebiotic source of energy derived from both the diet and naturally occurring products of normal metabolism. The vast majority of the bacteria in the colon are strict anaerobes, which means they don't use oxygen during metabolism and thus derive energy from a process called fermentation.
The soluble fibers that provide friendly bacteria with tasty meals are carbohydrates that evade upper intestinal digestion and absorption. They include such goodies as non-starch polysaccharides (such as pectins, inulin, arabinogalactans, gums, hemicelluloses); resistant starch; non-digestible oligosaccharides (such as raffinose, stachyose, galactans and mannans); resistant dextrins, as well as undigested portions of disaccharides (such as lactose) and sugar alcohols.
What makes all these forms of soluble fiber beneficial are the products that result from their breakdown by healthy bacteria and the effects these products have on our health. The major fermentation products are short chain fatty acids (SCFA), mainly acetate, propionate and butyrate. SCFAs tend to acidify the colon, which suppresses the growth of pathogens. They are rapidly absorbed by the cells that line the colon and contribute some usable energy.
Acetate is metabolized in the muscles, kidney, heart and brain. Propionate taken up by the liver could be used to create new glucose and may inhibit cholesterol synthesis. It also might play a role in how we store fat. Butyrate is metabolized by the colon cells providing energy and acting as a regulator of cell growth and maturity, which may reduce the risk of colon cancer by promoting cell death at a normal rate.
And of course these forms of soluble fiber can break down into a number of other metabolites, for instance lactate, pyruvate, ethanol and certain gases — hydrogen, carbon dioxide, methane and hydrogen sulfide.
When soluble fibers break down, their fermentation products are generally beneficial to health as noted above, which is why we try to increase their impact on our diet. The same cannot be said when it comes to feeding different bacteria the undigested portions of proteins. The subunits of proteins are amino acids, and the breakdown products of their metabolism in the colon produce potentially toxic products such as ammonia, amines and some harmful phenolic compounds.
That's probably why excessive fermentation of proteins, especially in the distal colon, has been linked with colon cancer and irritable bowel syndrome. That makes it doubly advantageous to increase soluble fiber, not only for the products that result from the metabolism of prebiotics, but also for the reduction of less beneficial products.
No matter how you get them -- straight up in fruits and vegetables, grains, beans and potatoes or hidden away in one or more of the modern invisible soluble fibers that can contribute texture to so many different types of foods -- soluble fibers prove to be a prebiotic boon to health.