Fri, 23 Sept. 2011 - 7:10 p.m. MT
Credit: ARA Staff - American Running Association
The indigestible threadlike structures in fruits, vegetables, and grains that we know as fiber have been heralded for decades as an important part of a healthy diet. Fiber is arguably the best food ingredient specifically for improving and maintaining good colon function, but there are of course other well-known benefits, including lower cholesterol, blood glucose, and diabetes and heart disease risk.
Should you supplement?
Sadly, dieticians, government officials, and clinical researchers consistently report that Americans fall far short of the recommended daily intake for fiber. The American Dietetic Association (ADA) and USDA both suggest 25 grams daily for women and 38 grams daily for men. Most Americans are said to get only 10 to 18 grams. It’s a good idea, then, for many people to consider adding a fiber supplement to their diet. Here we look at types of fiber supplements, and also give an overview of which types of fiber perform what actions within the body, to better alert you as to what fiber supplements can and cannot do. Generally speaking, the more various your fiber sources, the better off you are, and so remember that a supplement shouldn’t be expected to do all the work—but a supplement may certainly help you meet the recommended daily intake of fiber.
Note that in their 2010 Dietary Guidelines for Americans, the USDA lowers the daily intake recommendation for fiber after age 50, from 25 to 21 grams for women and from 38 to 30 for men. Your need for fiber is influenced by the number of calories you consume, which accounts for the lower numbers. The thinking is that folks over 50 need less calories, in part due to a slowing of the metabolism, but the USDA should not assume people are less active after 50, particularly among the Running & FitNews® readership. In any case, the numbers are a guide. If you find yourself exceeding the recommendations, that is most likely to your benefit, so long as you don’t suffer from bloating and are consuming plenty of water on a daily basis to account for its absorption by the fiber. The government’s recommendation for children is that intake should equal age in years plus 5 grams per day. With these recommendations in mind, let’s examine the two different types of dietary fiber more closely.
Soluble vs. insoluble fiber
Water-soluble fiber is viscous and composed mainly of plant pectin, gum, and mucilage. The first two are parts of the cell structure of plant walls and naturally coat seeds; they have gelling properties when mixed with water. Mucilage is a gelatinous glycoprotein produced by plants. Insoluble fiber is composed of cellulose and its less crystalline counterpart hemicellulose. Because it does not dissolve in water, this latter type of fiber bulks up as it absorbs water. Soluble fiber, on the other hand, becomes gelatinous and viscous. It too bulks up, but its primary role in bodily health is to bind to the bile acids in the small intestine. Soluble fiber takes these acids with it as it passes through the body, which in turn lowers LDL cholesterol and blood glucose levels by slowing glucose absorption in the small intestine. Researchers are still discovering the precise mechanism at work here, but when people with hyperlipidemia were given a medication that binds to and removes bile acids, their bad cholesterol improved and their blood glucose level lowered; soluble fiber has the same binding ability.
Insoluble fiber mainly eases the passage of foods through the digestive system and cleanses intestinal walls, whereas soluble fiber promotes actual changes in intestinal pH. Also, because soluble fiber ferments, it stimulates the growth of “friendly” bacteria in the gut. It is therefore known as pre-biotic. Insoluble fiber is by contrast inert. Both types increase food volume without increasing caloric content, providing satiety, and so they are widely associated with weight management.
Plant foods contain both types of fiber in varying degrees, according to the plant's characteristics. For example, the skin of a plum is insoluble, the flesh soluble. Whole grains, nuts, and seeds are plentiful in insoluble fiber. Beans, lentils, and peas are excellent sources of soluble fiber. Just as in the plum example, you can learn to decipher which type of fiber you’re getting by keeping in mind whether the food remains crystalline or reacts to the addition of water. Celery is high in insoluble fiber—it can absorb water, but the fiber doesn’t expand when it is introduced.
When is fiber high? "High-fiber" is defined by the FDA as a product containing at least 20% of the Daily Value. In its dietary guidelines, the USDA lists an appendix of foods high in fiber. Topping the list are any kind of cooked beans (up to 9.6 grams per ½ cup, mostly soluble), cooked lentils, chickpeas, and split peas (5.6 to 8.1 grams, mostly soluble), and 100% bran cereal (both). Bran is not a whole grain—it is made from pieces of grain husk separated from flour after milling—but at 9.1 grams per ½ cup it is indeed a high-fiber food.
Cooked artichoke hearts (7.2 grams, soluble) and pears (5.5 grams, medium-sized, mostly soluble) also scored surprisingly high. Other unlikely sources include tomato paste (2.7 grams, soluble) and, at 3.4 grams per ½ cup, sauerkraut has more fiber than a medium-sized banana or ½ cup whole wheat spaghetti (both contain 3.1 grams).
In addition to these foods, there are countless high-fiber cereals on the market today, ranging from 7 grams per ½ cup all the way up to 14 grams. As always, let the nutrition information on the box be your guide. Half a cup of oatmeal—whether old-fashioned or quick oats—contains 4 grams total, with equal parts soluble and insoluble fiber.
The ADA recommends eating mostly whole instead of refined grain breads, rice and cereal, and consuming at least five servings of fruits and vegetables daily. When choosing bread, remember the word “whole” is more important than the word “grain” or “wheat,” so look for “whole” to appear in the first ingredient.
The seed husks of the psyllium plant, in particular, have been recognized as health-beneficial by the FDA. Several supplements on the market make their powders, pills, and other products from psyllium, which contains mainly mucilage.
Fiber supplement types
Both types of fiber are important. Due to its scouring of intestinal walls, insoluble fiber plays a big role in reducing the risk of colon-related cancers and other problems. Because soluble fiber also creates bulk but additionally facilitates the growth of good bacteria, it reduces the risk of colorectal cancer in more than one way. Of the two, then, it makes the most sense to supplement soluble fiber, which is indeed the more commonly offered product. Some fiber supplement manufacturers do offer insoluble fiber powder, which is made from plant cellulose. Because it does not ferment, this type of fiber may be easier on the stomach and less likely to cause bloating than soluble fiber supplements. However, this type of fiber supplement will give you none of the benefits of fermentation.
Fiber supplements come in capsules, powders, and even wafers. All major manufacturers offer it in several forms. Before deciding which form you prefer, you’ll want to know whether the product is soluble fiber or not, and from there determine from what plant substance the fiber was derived. Soluble fiber supplements can be made from psyllium seed husk, dextrin, vegetable gums like xanthan gum or guar gum, or inulin. Cellulose is most often the ingredient of insoluble fiber products.
Determining which product your stomach tolerates best may take some trial and error. Each of the major brands has different arguments on offer as to why theirs is best.
Metamucil is made from 100% psyllium seed husk (often shortened to just “psyllium”). Their website claims the product acts like a sponge, trapping cholesterol and gently removing waste. The FDA would seem to agree, as psyllium is specifically endorsed by the organization. Other fiber products are likely as effective, but simply lack as many rigorous clinical trials to date.
Benefiber relies on 100% wheat dextrin. The improvement they claim to offer is dextrin’s mild taste and ease of dissolution in soft foods and non-carbonated beverages, which they assure us won’t thicken after the supplement is added. Both Metamucil and Benefiber ferment.
If you prefer a milder insoluble fiber supplement, UniFiber is made entirely from plant cellulose. It does not ferment and so will not lead to the gas and bloating sometimes associated with soluble fiber intake.
Binding with bile without the bacteria. Do be aware that there are non-fermentable yet soluble fiber supplements presently on the market. One example is Citrucel. Their website states: “Citrucel with SmartFiber contains only 100% non-fermentable fiber, so none of it ferments to cause excess gas like the fermentable fiber in Metamucil. This is based on laboratory tests. Results may vary in humans.” If you’re looking for a middle-of-the-road option, Citrucel might work well for you. It provides some of the binding with bile associated with soluble fiber, but does not cause the gas that sometimes leads to cramping and abdominal pain in sensitive GI tracts. Just remember it will not create the digestion-aiding bacteria that the fermenting fiber supplements will. Discuss with your doctor which supplement, if any, might work best within your existing diet. Then, be sure to read closely which type you are indeed purchasing.
A few caveats
As noted, one disadvantage of a diet high in fiber is the potential for significant intestinal gas production and bloating. Also, for people on opioids or similar medications, fiber supplements may be contraindicated, as they can slow down the digestive process. These patients may already be experiencing a significant slowing, and so constipation can result. Constipation can also occur if insufficient fluid is consumed with a high-fiber diet.
Never abruptly increase your fiber intake; rather gradually work up to the recommended grams per day over the course of many weeks. As you do, remember to increase the amount of water you drink. Though most fiber supplements are nearly tasteless or pleasant tasting nowadays, if you find the experience undesirable or you otherwise decide to hold off on a fiber supplement, you can simply try adding 1/4 cup of wheat bran to foods, such as cooked cereals, applesauce, and meat loaf. And do eat beans each week.
Fermentable means preventable. Consistent intake of fiber—fermentable fiber in particular—is well known to reduce risk of several common but preventable diseases: diabetes, obesity, IBS, Crohn’s disease, cardiovascular disease, high blood cholesterol, diverticulitis, and colorectal cancer are among the most prevalent.
USDA, Center for Nutrition Policy and Promotion, 2010 Dietary Guidelines for Americans, http://www.cnpp.usda.gov/dietaryguidelines.htm
J. Am. Diet Assoc., 2008, Position of the American Dietetic Association: Health
Implications of Dietary Fiber, 108, pp. 1716-1731
(RUNNING & FITNEWS® July / August 2011 • Volume 29, Number 4)
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