Defining Insoluble and Soluble Fibers
Sources of dietary fiber are usually divided into categories of "insoluble" and "soluble" fibers. Both types are present in all plant foods, with varying degrees of each according to a plant's characteristics. Insoluble refers to the inability to dissolve in water and soluble indicates a fiber source that would readily dissolve in water.
As you will soon see, those definitions are too limiting, especially because soluble fiber undergoes active metabolic processing via fermentation that yields end products with broad, significant health effects.
To conceptualize insoluble and soluble fibers, consider the segments of a plum (or prune). The plum skin is an example of an insoluble fiber source, whereas soluble fiber sources are inside the pulp. Other sources of insoluble fiber include: whole wheat, wheat or corn bran, flax seed lignans, and vegetables like carrots, celery, green beans and potato skins.
One of the most versatile sources of dietary fiber is the husk (hull) of seeds from psyllium grain (Plantago ovata), a fiber source with clinically demonstrated properties of lowering blood cholesterol when it is regularly included in a human diet. Psyllium seed husk is 34% insoluble fiber and 66% soluble fiber, providing an optimal division of both fiber types that make it a valuable food additive.
The American Association of Cereal Chemists defined soluble fiber this way: "The edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine".
There are several key words in this statement that inspire analysis and comment for considering fermentable fiber. Let's break it down.
Edible parts of plants
This phrase indicates that all parts of a plant we eat - skin, pulp, seeds, stems, leaves, roots - contain fiber. Both insoluble and soluble sources are in those plant components.
Complex carbohydrates, such as long-chained sugars also called starch or polysaccharides, are excellent sources of soluble fiber.
Resistant to digestion and absorption in the human small intestine
Foods providing nutrients are digested by enzymes and acids in the stomach and small intestine where the nutrients are released and then absorbed through the intestinal wall for transport via the blood throughout the body. A food resistant to this process is undigested, as both insoluble and soluble fibers are. They pass to the large intestine only affected by their absorption of (insoluble fiber) or dissolution in water (soluble fiber).
Complete or partial fermentation in the large intestine.
The large intestine is comprised mainly of a segment called the colon within which additional nutrient absorption occurs through the process of fermentation. Fermentation occurs by the action of colonic bacteria on the food mass, producing gases and short-chain fatty acids. It is these short-chain fatty acid--butyric, acetic, propionic, and valeric acids--that have such significant health properties.
Short-chain Fatty Acids
Short-chain fatty acids are absorbed through the intestinal wall into portal blood (from the intestine to the liver) that transports them into the general circulation. Particularly butyric acid has extensive physiological actions that promote health effects among which are:
Stabilizing blood glucose levels by acting on pancreatic insulin release and liver control of glycogen breakdown.
Suppressing cholesterol synthesis by the liver, thereby reducing blood levels of low-density lipids (LDL cholesterol) and triglycerides responsible for atherosclerosis.
Lowering colonic pH (i.e., raise the acidity levels in the colon) which protects the colon lining from cancer polyp formation and increases absorption of minerals such as calcium, magnesium and iron.
Stimulating production of T helper cells, antibodies, leukocytes, splenocyte cytokines and lymph mechanisms having crucial roles in immune protection.
Increasing proliferation of colonic bacteria beneficial for intestinal health--bifidobacteria and lactobacilli (providing a probiotic value).
Improving barrier properties of the colonic mucosal layer, inhibiting inflammatory and adhesion irritants.
To summarize these effects, fermentable fibers yield the important short-chain fatty acids that affect blood glucose and lipid levels. They also improve the colonic environment and regulate immune responses.
Regulatory Guidance on Fiber Products
Americans and Canadians consume less than 50% of the dietary fiber levels required for good health. Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the US FDA have given approvals to food products making health claims for fiber. In clinical trials to date, these fiber sources were shown to significantly reduce blood cholesterol levels and so are important to cardiovascular health.
The Soluble (fermentable) fiber sources gaining FDA approval are:
Psyllium seed husk (7 grams per day)
Beta-glucan from oat bran, whole oats, oatrim or rolled oats (3 grams per day)
Beta-glucan from whole grain or dry-milled barley (3 grams per day)
Other examples of fermentable fiber sources used in functional foods and supplements include:
Cellulose Guar gum
Oligofructose Oligo- or polysaccharides
Consistent intake of fermentable fiber through foods like berries and other fresh fruit, vegetables, whole grains, seeds and nuts is now known to reduce the risk of some of the world's most prevalent diseases.
These diseases include:
High blood cholesterol
Numerous gastrointestinal disorders (constipation, inflammatory bowel disease, ulcerative colitis, Crohn's disease, diverticulitis and colon cancer).
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