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Dietary Fiber

Dietary fibers are long-chain carbohydrates (polysaccharides) that are indigestible by the human digestive tract.

Uses of dietary fibre

There are two principal types of dietary fiber: soluble and insoluble. Insoluble fiber is simply bulk that changes little as it passes through the body. Soluble fiber, on the other hand, forms a soft gel in solution with water. Most foods provide a mixture of both, but are listed as mostly one or the other. Soluble fiber has been shown to be able to bind bile salts which may reduce blood cholesterol levels. It also may slow the absorption of glucose from the intestine, thereby requiring less insulin secretion.

Fiber may decrease spasms in the gastrointestinal tract by keeping the lumen distended. The main value of dietary fiber is that it provides bulk to the bolus moving through the digestive tract. There are two great advantages to this: by bulking up the bolus, eventually increasing the weight of the stool, it's easier for the digestive system to move it through, and the bulkier stool also tends to retain normal amounts of moisture to make it easier to eliminate with less straining and abrasion. The moisture content of human stool does not change when more fiber is consumed, except marginally from psyllium husk (Eastwood et. al & Prynne et. al). Because the bowel regulation is mostly due to bulking and not to increased water in the stool, it is very unlikely to cause diarrhea unless if taken in massive amounts (this is as long as one does not consider synthetic sugars in this category).

Fiber is an important part of the treatment and prevention of diabetes, colorectal cancer, gastrointestinal disorders, high cholesterol, heart disease and obesity. High-fiber foods help move waste through the digestive tract faster and easier, so possibly harmful substances do not have as much contact with the gastrointestinal tract and reduce straining. Many cause blood sugar or cholesterol absorption to decrease in amplitude of the plotted absorption or decrease the amount absorbed by slowing or decreasing the absorption.

Harmful effects

An overdose of soluble fiber can cause diarrhea and worsen irritable bowel syndrome. Negative effects of dietary fiber include a reduced absorption of vitamins, minerals, proteins, and calories from the gut. Some insoluble fibers can bind to certain minerals: calcium, magnesium, phosphorus, and iron. This is unlikely to be harmful in the average adult, but guidelines for the US have been established.

The American Dietetic Association (ADA) recommends 20-35g/day for a healthy adult depending on calorie intake (eg. 2000 calorie a day diet should include 25g of fiber/day). Their recommendations for a child was that intake should equal their age in numbers plus 5 g/day for children (example a 4 year old should consume 9g/day). No guidelines have yet been established for the elderly or very ill. Patients with current constipation, vomiting, and abdominal pain should see a physician. Certain bulking agents are not commonly recommended with the prescription of opioids because the slow transit time mixed with larger stools may lead to severe constipation, pain, or obstruction.


The ADA recommends trying to get most of your dietary fiber from foods you eat, as an important part of consuming variety, nutrition, synergy between nutrients, and possibly phytonutrients. Soluble fiber is found in some fruits (particularly oranges, also apples and bananas), oats, legumes, (peas, soybeans, and other beans), other vegetables, such as broccoli and carrots, and psyllium seed. Legumes also typically contain shorter-chain carbohydrates that are indigestible by the human digestive tract but which are digested by bacteria in the small intestine, which is a cause of flatulence.

Sources of insoluble fiber include whole grain foods, bran, nuts and seeds, vegetables such as green beans, cauliflower, and potato skins, and the skins of fruit and root vegetables.

Fiber supplements

There are many types of soluble fiber supplements: psyllium, methylcellulose, pectins, vegetable gums and polycarbophil. Psyllium is digested and does contain calories, may reduce LDL, absorbs water, and may cause gas. Methylcellulose is created from the cell wall of plants. Methylcellulose is undigestible, unfermentable and doesn't cause gas or have calories that humans can use. Both of these can be used for IBS, constipation, diverticulitis. Polycarbophil is also plant based and is similar to methylcellulose. It usually causes even less bloating.

Choosing your supplement
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Most supplements come in tablet and chewable forms. You can start at a lower dose and work your way to recommended dosing or level of fiber in your diet to reduce your risk of side effects (eg. bloating, gas). The bloating and gas may adjust, but it may take a few weeks. Over the counter supplements are able to be mixed because a person takes it for the fiber content and not for other reasons (this is not recommended for most medications), although one needs to keep track of the fiber consumed and not the number of pills. Not all supplements are equal in the amount of fiber and what other ingredients they contain. Companies are under no obligation to include only the fiber and often contain sugars or substitutes that may cause more gas or bloating than others with the same active ingredient, read the label carefully.

If you are just trying to treat constipation or diarrhea, use of the smallest dose effective should be your goal. If you do not tolerate the addition of one you can try another with different chemicals, different forms of the same supplement, or you can try starting at a lower dose and working your way up over 2-3 days or even a week. If cholesterol is your concern, beet fiber, guar gum, karaya gum, konjac glucomannan, locust bean gum, pectin, psyllium seed husk, soybean polysaccharide and xanthan gum may all lower LDL cholesterol. Beta glucan in oats and psyllium husk, have been sufficiently studied for the FDA to promote that foods containing 0.75 g or 1.7 g of these soluble fibers has enough of an effect to be able to reduce the risk of heart disease (J Am Diet Assoc 2002).

Taking a fiber supplement may make you feel more full if taken before meals. When starting a fiber supplement start with a low dose, to let your body adjust to the new amount. It may take a month before the supplement induces many of its effects. While taking fiber supplements it is important to understand that you will need to drink plenty of water in order to get an appropriate response. If you don't then you may end up constipated because the fiber absorbed too much water. The equivalent of 8, 8oz glasses of fluid (eg. water) per day is recommended. Supplements of any kind should be discussed with your doctor, as they may influence other medications you are taking.

Some common supplements in the US include: Metamucil and Fybogel (psyllium), Citrucel (methylcellulose), Equalactin and Fibercon (polycarbophil), and Benefiber (guar gum & cellulose).


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