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Interesting article on fibers....
(with its references)
Dietary fibers are the portions of plants that cannot be digested by the human digestive tract. The different kinds of plant fiber are divided into two classifications - water soluble fiber consists of pectin, gums, mucilages, and some hemicelluloses; and insoluble fiber consists of lignin, cellulose and the remaining hemicelluloses. The primary action of fiber is in the gastrointestinal tract, but different forms of fiber have different physiological effects and health benefits (ADA Fiber Position Paper, 1997; Van Horn, 1997).
Soluble fiber delays gastric emptying and intestinal transit. Viscous soluble fiber sources also slow the appearance of glucose in the blood and decrease serum cholesterol. Soluble fibers are rapidly broken down (fermented) by bacteria in the large bowel and, with the exceptions of oats (which contain up to 50 percent soluble fiber as beta-glucan) and psyllium seed husks, do not promote laxation. Major food sources of soluble fiber include oats, beans, dried peas, and legumes (ADA Fiber Position Paper, 1997).
Insoluble fibers speed up intestinal transit and increase stool weight. They promote laxation and are either slowly or not fermented. However, insoluble fibers have no effect on serum cholesterol. Major food sources of insoluble fiber include wheat bran, whole grain products, and vegetables. Fruits, vegetables, and barley are sources of both insoluble and soluble fiber. Nearly all fiber containing foods have more insoluble than soluble dietary fiber. About two-thirds to three-fourths of the dietary fiber in typical mixed-food diets is insoluble fiber (ADA Fiber Position Paper, 1997).
Americans presently consume 14 to 15 gm of dietary fiber daily. Recommendations for dietary fiber for adults usually are in the range of 20 to 35 gm per day or 10 to 13 gm of dietary fiber per 1,000 calories. The American Dietetic Association recommends a diet consistent with the Food Guide Pyramid that makes use of a variety of plant foods to achieve adequate fiber intake. People should include at least two to three servings of whole grains as part of the daily 6 to 11 servings of grains, 5 servings of fruits and vegetables daily, and legumes at least once or twice per week. In addition to fiber, minimally processed plant foods provide vitamins, minerals, and phytochemicals that have positive health effects (ADA Fiber Position Paper, 1997). Fatal and non-fatal myocardial infarctions have been inversely related with total fiber intake (Rimm, 1996).
Many studies have shown the beneficial effect that soluble fiber has on plasma cholesterol and lipoprotein levels. In general, LDL decreases and HDL is unchanged. The magnitude of the cholesterol reduction is related to the person's initial cholesterol level - individuals with higher cholesterol levels respond more to soluble fiber than individuals with lower cholesterol levels. This makes soluble fiber a valuable tool for reducing cholesterol in those who would benefit the most (Krummel et al, 1998).
Oat fiber is easily added to the diet, and is well tolerated by most people, producing very little gastrointestinal distention, bloating or gas. In response to a health claim petition filed by the Quaker Oats company in 1995, the Food and Drug Association (FDA) reviewed more than 37 studies that investigated the effects of oatmeal and oat bran on total cholesterol and LDL levels (Hasler, 1996). Based on this extensive body of research, in 1997 the FDA approved the first food specific health claim for oatmeal and heart disease: "Soluble fiber from oatmeal, as part of a saturated fat, low cholesterol diet, may reduce the risk of heart disease."
Oat products must contain at least 0.75 grams of oat soluble fiber (beta-glucan) per serving and be low in total fat, saturated fat, and cholesterol. The 0.75 grams amount represents one-fourth of the three gram amount required to achieve a 5% reduction in total cholesterol for most people. The benefits are greatest for those individuals with elevated baseline cholesterol levels.
An intake of 3 grams of beta-glucan is equivalent to approximately 60 grams of uncooked oatmeal or 40 grams of uncooked oat bran (Hasler, 1996). In practical terms, this represents one and one-half cups of cooked oatmeal or one cup of cooked oat bran.
Small amounts of beans are also readily incorporated into the diet and lower plasma total cholesterol and LDL. Although some people note increased flatulence initially, adding beans to the diet gradually helps to decrease this complaint. There are also supplements to help prevent flatulence (one is trademarked Beano). The following cooking tips also help: canned beans should be rinsed before use. Dried beans should be boiled, then allowed to soak for four hours, then drained and cooked with fresh water.
Fiber supplements made from pectin, guar gum, locust bean gum, and pysllium seed husks may also reduce total cholesterol and LDL. However, viscosity, taste characteristics, and gastrointestinal side effects may be deterrents to their long term use.
The laxative Metamucil (made from the husks of psyllium seed) is a rich source of soluble fiber. Metamucil has four to five times as much soluble fiber per ounce as oat bran. Psyllium was found to lower total cholesterol by 15 percent and LDL cholesterol by 20 percent in hypercholesterolemic men, as shown below (Anderson et al, 1988). The subjects took a teaspoon of Metamucil three times a day (before meals) over an eight-week period. Serum cholesterol dropped from an average of nearly 250 to about 215 mg/dl. Remember that the LRC-CPT study found that a 1 percent decrease in serum cholesterol reduced the risk of CHD by 2 percent. The 15 percent decrease in total cholesterol in the Metamucil study could theoretically reduce CHD risk by 30 percent in hypercholesterolemic people.
Glore and colleagues recently conducted a literature review of 77 studies to evaluate the impact of soluble fiber on serum lipids (Glore et al., 1994). Out of 77 studies, 69 (88 percent) reported significant reductions in total cholesterol, 41 of 49 (84 percent) reported significant reductions in LDL, 43 of 57 (75 percent) reported no change in HDL, and 50 of 58 (86 percent) reported no change in triglycerides. Although the sources and amounts of soluble fiber varied, the authors noted that a significant cholesterol-and LDL-lowering effect can be expected with daily consumption of the following: 6 to 40 gm of pectin, 8 to 36 gm of gums (e.g. guar gum), 100 to 150 gm of dried beans, 25 to 100 gm of dry oat bran, 57 to 140 gm of dry oatmeal, or 10 to 30 gm of psyllium.
How does soluble fiber lower serum cholesterol? Several theories have been proposed. The predominant theory involves the process by which soluble fiber affects bile acid circulation.
In the liver, cholesterol is converted into bile acids, which are used to emulsify fats in the intestine for digestion and absorption. When bile acids are reabsorbed and come back to the liver (via the portal blood flow from the intestines), they block further production of bile acids, because the body doesn't need to produce more. This leaves more cholesterol in the intestines, which is dumped into the blood in the form of LDL cholesterol. This process, called the enterohepatic circulation of bile, is very effective in conserving bile.
Soluble fiber interferes with this process -- it tends to bind with and increase the excretion of bile acids. When the excretion of bile acids is increased, the liver converts more LDL cholesterol into bile acids. This additional cholesterol is most easily obtained from the intestines, leaving less available for absorption. By causing more cholesterol to be used for bile acid production, soluble fiber helps take cholesterol out of circulation in the bloodstream. This is the major way that soluble fiber is thought to lower serum cholesterol (ADA Fiber Position Paper, 1997).
Other compounds in fiber sources may also affect cholesterol. The gamma analogue of tocotrienol found in oats, barley, and bran is a potent inhibitor of endogenous cholesterol biosynthesis and lowers blood cholesterol concentrations (ADA Fiber Position Paper, 1997).
An American Heart Association Science Advisory recommends a total dietary fiber intake of 25 to 30 grams per day from food (not supplements) to ensure nutritional adequacy and accentuate the lipid-lowering effects of a reduced-fat diet. Adding a fiber supplement to a diet otherwise high in saturated fat and cholesterol provides dubious cardiovascular benefits. However, the soluble fiber found in oats, barley, beams, soy products, and pectin-rich fruits and vegetables provides additional cholesterol lowering benefits that are beyond those achieved with reductions in total fat and saturated fat (Van Horn, 1997).
The importance of dietary fiber should be stressed in all diet counseling, not just cholesterol-lowering programs. It's recommended that people consume high-fiber foods, rather than load up on oat bran or psyllium, because fiber-rich foods also provide nutrients and phytochemicals. Some people with hypercholesterolemia may benefit from fiber supplements when diet modification is not sufficient or practical.
There is concern that dietary fibers may bind with certain minerals and decrease their bioavailability. This is unlikely to occur unless fiber intake is excessive and mineral intake inadequate. Excessive fiber intake may cause abdominal distress, gas, and diarrhea. Dietary fiber should be increased gradually and accompanied by an adequate fluid intake. This allows the digestive system to adapt to fiber's physiologic actions.
References
Anderson JW, Settwoch N, et al. Cholesterol lowering effects of psyllium hydrophilic muciloid for hypercholesterolemic men. Arch Intern Med, 148: 292, 1988.
Glore SR, Van Treeck D, et al. Soluble fiber and serum lipids: a literature review. J Am Diet Assoc, 94: 425, 1994.
Hasler, C. Oat beta-glucan, coronary heart disease risk, and health claims. SCAN's Pulse. Vol. 15, No. 4, Fall 1996.
Krummel, D et al. Medical nutrition therapy for cardiovascular disease and associated risk factors: specific diets for prevention and treatment. In: Kris-Etherton PM and Burns JH ed: Cardiovascular Nutrition. American Dietetic Association. 1998.
Position of the American Dietetic Association: Health Implications of Dietary Fiber. J Am Diet Assoc, 97: 1157, 1997.
Rimm EB, et al. Vegetable, fruit and cereal fiber intake and risk of coronary heart disease among men. JAMA. 275: 447, 1996.
Van Horn, L. Fiber, lipids, and coronary heart disease. Circulation. 95:2701, 1997.
(with its references)
Dietary fibers are the portions of plants that cannot be digested by the human digestive tract. The different kinds of plant fiber are divided into two classifications - water soluble fiber consists of pectin, gums, mucilages, and some hemicelluloses; and insoluble fiber consists of lignin, cellulose and the remaining hemicelluloses. The primary action of fiber is in the gastrointestinal tract, but different forms of fiber have different physiological effects and health benefits (ADA Fiber Position Paper, 1997; Van Horn, 1997).
Soluble fiber delays gastric emptying and intestinal transit. Viscous soluble fiber sources also slow the appearance of glucose in the blood and decrease serum cholesterol. Soluble fibers are rapidly broken down (fermented) by bacteria in the large bowel and, with the exceptions of oats (which contain up to 50 percent soluble fiber as beta-glucan) and psyllium seed husks, do not promote laxation. Major food sources of soluble fiber include oats, beans, dried peas, and legumes (ADA Fiber Position Paper, 1997).
Insoluble fibers speed up intestinal transit and increase stool weight. They promote laxation and are either slowly or not fermented. However, insoluble fibers have no effect on serum cholesterol. Major food sources of insoluble fiber include wheat bran, whole grain products, and vegetables. Fruits, vegetables, and barley are sources of both insoluble and soluble fiber. Nearly all fiber containing foods have more insoluble than soluble dietary fiber. About two-thirds to three-fourths of the dietary fiber in typical mixed-food diets is insoluble fiber (ADA Fiber Position Paper, 1997).
Americans presently consume 14 to 15 gm of dietary fiber daily. Recommendations for dietary fiber for adults usually are in the range of 20 to 35 gm per day or 10 to 13 gm of dietary fiber per 1,000 calories. The American Dietetic Association recommends a diet consistent with the Food Guide Pyramid that makes use of a variety of plant foods to achieve adequate fiber intake. People should include at least two to three servings of whole grains as part of the daily 6 to 11 servings of grains, 5 servings of fruits and vegetables daily, and legumes at least once or twice per week. In addition to fiber, minimally processed plant foods provide vitamins, minerals, and phytochemicals that have positive health effects (ADA Fiber Position Paper, 1997). Fatal and non-fatal myocardial infarctions have been inversely related with total fiber intake (Rimm, 1996).
Many studies have shown the beneficial effect that soluble fiber has on plasma cholesterol and lipoprotein levels. In general, LDL decreases and HDL is unchanged. The magnitude of the cholesterol reduction is related to the person's initial cholesterol level - individuals with higher cholesterol levels respond more to soluble fiber than individuals with lower cholesterol levels. This makes soluble fiber a valuable tool for reducing cholesterol in those who would benefit the most (Krummel et al, 1998).
Oat fiber is easily added to the diet, and is well tolerated by most people, producing very little gastrointestinal distention, bloating or gas. In response to a health claim petition filed by the Quaker Oats company in 1995, the Food and Drug Association (FDA) reviewed more than 37 studies that investigated the effects of oatmeal and oat bran on total cholesterol and LDL levels (Hasler, 1996). Based on this extensive body of research, in 1997 the FDA approved the first food specific health claim for oatmeal and heart disease: "Soluble fiber from oatmeal, as part of a saturated fat, low cholesterol diet, may reduce the risk of heart disease."
Oat products must contain at least 0.75 grams of oat soluble fiber (beta-glucan) per serving and be low in total fat, saturated fat, and cholesterol. The 0.75 grams amount represents one-fourth of the three gram amount required to achieve a 5% reduction in total cholesterol for most people. The benefits are greatest for those individuals with elevated baseline cholesterol levels.
An intake of 3 grams of beta-glucan is equivalent to approximately 60 grams of uncooked oatmeal or 40 grams of uncooked oat bran (Hasler, 1996). In practical terms, this represents one and one-half cups of cooked oatmeal or one cup of cooked oat bran.
Small amounts of beans are also readily incorporated into the diet and lower plasma total cholesterol and LDL. Although some people note increased flatulence initially, adding beans to the diet gradually helps to decrease this complaint. There are also supplements to help prevent flatulence (one is trademarked Beano). The following cooking tips also help: canned beans should be rinsed before use. Dried beans should be boiled, then allowed to soak for four hours, then drained and cooked with fresh water.
Fiber supplements made from pectin, guar gum, locust bean gum, and pysllium seed husks may also reduce total cholesterol and LDL. However, viscosity, taste characteristics, and gastrointestinal side effects may be deterrents to their long term use.
The laxative Metamucil (made from the husks of psyllium seed) is a rich source of soluble fiber. Metamucil has four to five times as much soluble fiber per ounce as oat bran. Psyllium was found to lower total cholesterol by 15 percent and LDL cholesterol by 20 percent in hypercholesterolemic men, as shown below (Anderson et al, 1988). The subjects took a teaspoon of Metamucil three times a day (before meals) over an eight-week period. Serum cholesterol dropped from an average of nearly 250 to about 215 mg/dl. Remember that the LRC-CPT study found that a 1 percent decrease in serum cholesterol reduced the risk of CHD by 2 percent. The 15 percent decrease in total cholesterol in the Metamucil study could theoretically reduce CHD risk by 30 percent in hypercholesterolemic people.
Glore and colleagues recently conducted a literature review of 77 studies to evaluate the impact of soluble fiber on serum lipids (Glore et al., 1994). Out of 77 studies, 69 (88 percent) reported significant reductions in total cholesterol, 41 of 49 (84 percent) reported significant reductions in LDL, 43 of 57 (75 percent) reported no change in HDL, and 50 of 58 (86 percent) reported no change in triglycerides. Although the sources and amounts of soluble fiber varied, the authors noted that a significant cholesterol-and LDL-lowering effect can be expected with daily consumption of the following: 6 to 40 gm of pectin, 8 to 36 gm of gums (e.g. guar gum), 100 to 150 gm of dried beans, 25 to 100 gm of dry oat bran, 57 to 140 gm of dry oatmeal, or 10 to 30 gm of psyllium.
How does soluble fiber lower serum cholesterol? Several theories have been proposed. The predominant theory involves the process by which soluble fiber affects bile acid circulation.
In the liver, cholesterol is converted into bile acids, which are used to emulsify fats in the intestine for digestion and absorption. When bile acids are reabsorbed and come back to the liver (via the portal blood flow from the intestines), they block further production of bile acids, because the body doesn't need to produce more. This leaves more cholesterol in the intestines, which is dumped into the blood in the form of LDL cholesterol. This process, called the enterohepatic circulation of bile, is very effective in conserving bile.
Soluble fiber interferes with this process -- it tends to bind with and increase the excretion of bile acids. When the excretion of bile acids is increased, the liver converts more LDL cholesterol into bile acids. This additional cholesterol is most easily obtained from the intestines, leaving less available for absorption. By causing more cholesterol to be used for bile acid production, soluble fiber helps take cholesterol out of circulation in the bloodstream. This is the major way that soluble fiber is thought to lower serum cholesterol (ADA Fiber Position Paper, 1997).
Other compounds in fiber sources may also affect cholesterol. The gamma analogue of tocotrienol found in oats, barley, and bran is a potent inhibitor of endogenous cholesterol biosynthesis and lowers blood cholesterol concentrations (ADA Fiber Position Paper, 1997).
An American Heart Association Science Advisory recommends a total dietary fiber intake of 25 to 30 grams per day from food (not supplements) to ensure nutritional adequacy and accentuate the lipid-lowering effects of a reduced-fat diet. Adding a fiber supplement to a diet otherwise high in saturated fat and cholesterol provides dubious cardiovascular benefits. However, the soluble fiber found in oats, barley, beams, soy products, and pectin-rich fruits and vegetables provides additional cholesterol lowering benefits that are beyond those achieved with reductions in total fat and saturated fat (Van Horn, 1997).
The importance of dietary fiber should be stressed in all diet counseling, not just cholesterol-lowering programs. It's recommended that people consume high-fiber foods, rather than load up on oat bran or psyllium, because fiber-rich foods also provide nutrients and phytochemicals. Some people with hypercholesterolemia may benefit from fiber supplements when diet modification is not sufficient or practical.
There is concern that dietary fibers may bind with certain minerals and decrease their bioavailability. This is unlikely to occur unless fiber intake is excessive and mineral intake inadequate. Excessive fiber intake may cause abdominal distress, gas, and diarrhea. Dietary fiber should be increased gradually and accompanied by an adequate fluid intake. This allows the digestive system to adapt to fiber's physiologic actions.
References
Anderson JW, Settwoch N, et al. Cholesterol lowering effects of psyllium hydrophilic muciloid for hypercholesterolemic men. Arch Intern Med, 148: 292, 1988.
Glore SR, Van Treeck D, et al. Soluble fiber and serum lipids: a literature review. J Am Diet Assoc, 94: 425, 1994.
Hasler, C. Oat beta-glucan, coronary heart disease risk, and health claims. SCAN's Pulse. Vol. 15, No. 4, Fall 1996.
Krummel, D et al. Medical nutrition therapy for cardiovascular disease and associated risk factors: specific diets for prevention and treatment. In: Kris-Etherton PM and Burns JH ed: Cardiovascular Nutrition. American Dietetic Association. 1998.
Position of the American Dietetic Association: Health Implications of Dietary Fiber. J Am Diet Assoc, 97: 1157, 1997.
Rimm EB, et al. Vegetable, fruit and cereal fiber intake and risk of coronary heart disease among men. JAMA. 275: 447, 1996.
Van Horn, L. Fiber, lipids, and coronary heart disease. Circulation. 95:2701, 1997.
