Dietary fiber consists of complex carbohydrates found in plant-based foods that are indigestible and unabsorbed.
Fibers are primarily components in the plant cell wall, such as cellulose, hemicellulose and pectin, and a variety of gums, mucilages and algal polysaccharides.
Fibers contribute negligible calorie value to the diet.
There are two types of fibers, soluble and insoluble. Insoluble fibers are cellulose, hemicellulose, and lignin. Soluble ones are pectins, gums and mucilages.
Fibers have little nutritional value but may have physiological effects in the digestive system.
The health claim on fiber that is approved by the FDA is that diets low in saturated fat and cholesterol and high in fruits, vegetables, and grain products that contain fiber, particularly soluble fiber, reduce the risk of coronary heart disease.
Daily intakes of fibers in the US have been estimated to be 15 g a day.
Fibers are not digested in the upper digestive tract, but may be fermented by intestinal microflora in the lower digestive tract, producing acetic and butyric, propionic acids and gases such as methane and hydrogen.
Among short chain fatty acids, butyric acid can be used by enterocytes. Propionic and acetic acids are transported into circulation and utilized by the liver and many tissues.
Insoluble fibers provide bulk to the gastrointestinal contents. Fibers like lignins adsorb cholesterol and some types of toxic compounds, inhibit their absorption into the bloodstream and excrete them to feces. This type of fiber stimulates peristalsis and results in shorter passage time (more frequent defecation).
Soluble fibers slow down gastric emptying due to a gel-forming capacity, delaying the digestion and absorption of sugars, starches and fats.
Fibers alter population of intestinal microflora, probably inducing functional changes in the digestive system, including intestinal immunity.
Based on physiological functions mentioned above, fiber may be used to prevent and treat chronic diseases such as constipation, hypercholesterolemia and diabetes.
Although there is no RDA for dietary fiber, the American Diabetes Association, the American Dietetic Association, the American Medical Association and other health organizations have recommended dietary intake of fibers, ranging from 20 to 35 g a day. For children 2 years and older, daily intake of an amount of dietary fiber equal to or greater than their age plus 5 g is recommended. 12
To reduce serum total cholesterol, 10 g of soluble fiber should be consumed daily.
For weight loss, 14 g of fiber a day (insoluble or soluble) may be required.
Not known. Excess intake of insoluble fiber, especially from cereal grains, may reduce the absorption of minerals such as calcium, magnesium, zinc and iron. This reduction is attributed to the high phytate content of such foods which binds to minerals and prevents their absorption.
Caution with pregnancy or nursing, consult physician before using.
Information on the relationship between substances and disease is provided for general information, in order to convey a balanced review of the scientific literature. In many cases the relationship between a substance and a disease is tentative and additional research is needed to confirm such a relationship.
Risk for Diabetes: More than 65,000 U.S. women, 40 to 65 years of age, were followed for 6 years to see whether dietary glycemic index, glycemic load and dietary fiber were associated with development of type 2 diabetes. It was found that cereal fiber intake was inversely related to risk of diabetes. The combination of high glycemic load and a low cereal fiber content further increased risk of diabetes.8
In a randomized, crossover study, 13 patients with type 2 diabetes were given two fiber diets, each for 6 weeks. One diet contained 24 g of total fiber (8 g soluble and 16 g insoluble fiber), as recommended by the ADA, and the other 50 g of total fiber (25 g of soluble and 25 g of insoluble fiber). As a result, preprandial plasma glucose concentrations were 13 mg lower as compared with the ADA diet. The high fiber diet decreased urinary excretion of glucose significantly. In addition, plasma total cholesterol, VLDL cholesterol and triacylglycerol concentrations were significantly reduced by the 50 g fiber diet. The authors concluded that high fiber intake, especially soluble fiber, improved glycemic control and plasma lipid profile.9
Coronary Heart Disease: A study on the relation between dietary fiber and risk of coronary heart disease (CHD) among women was conducted in the Nurses' Health Study. During the 10 year follow up, 591 major incidents of CHD were documented. The age-adjusted relative risk for major CHD was 0.53 for women in the highest quintile of the total dietary fiber intake (22.9 g/day), compared with women in the lowest quintile (11.5 g/day). Only cereal fiber was strongly associated with a reduced risk of CHD among different sources of dietary fiber.10
Colon: Some types of fiber have been shown to bind carcinogens or dilute fecal concentrations of bile acids, which suggests that dietary fiber may reduce risk of cancer, especially colorectal cancer. A study was reported on 88,000 women followed for 16 years in the Nurses' Health Study to identify the association of colorectal cancer, adenoma and dietary fiber. During the 16 year follow up, 787 cases of colorectal cancer and 1012 cases of adenoma were found. After adjustment of confounding factors, no significant association between fiber intake and risk of colorectal adenoma was observed.11 Furthermore, another paper reported similar results. Diets low in fat and high in fiber, fruits and vegetables did not affect the risk of recurrent of colorectal adenomas in people who had previously confirmed colorectal adenomas.12
Gastric Cardia Cancer: In a large-scale population-based case-control study, risk factors on adenocarcinoma of the gastric cardia and esophagus and squamous cell carcinoma of the esophagus were determined. A strong inverse relation was found between total dietary intake of fiber, especially from cereal, and gastric cardia adenocarcinoma. Cereal fiber also showed a protective effect against adenocarcinoma of the esophagus.13
Cholesterol-lowering Action: In a meta-analysis, consumption of 10.2 g/day of soluble fiber, including psyllium, reduced total cholesterol by 4% and LDL cholesterol by 7% as compared to placebo in subjects. No significant effect was observed on serum HDL cholesterol and triacylglycerol concentrations.3
A meta-analysis including 67 controlled trials was performed to determine if dietary fibers affected blood lipid. Soluble fiber, 2 - 10 g/day exerted small but significant effects on total cholesterol and LDL cholesterol concentrations. There was no difference on cholesterol-lowering action among major fibers such as oat, psyllium and pectin. However, plasma triacylglycerol and HDL cholesterol concentrations were not influenced by dietary fibers.3
Stroke: In the Health Professionals Follow-up Study, the association of various nutrient intake and risk of stroke was analyzed. In addition to potassium, intake of dietary fiber, especially from cereal, was inversely correlated to the incidence of stroke. This relation was stronger in hypertensive than normotensive men. 14
Weight Reduction: It has been suggested that dietary fiber may have an affect on weight control. However, definitive conclusions still remain unanswered. In a review, researchers at Tufts University analyzed data on the effect of dietary fiber on hunger, satiety, energy intake and body composition in healthy individuals. The beneficial effects of fiber were found with both soluble and insoluble fibers. Researchers concluded that consumption of an additional 14 g/day of fiber (regardless of solubility) for > 2 days is associated with a 10% decrease in energy intake and body weight loss of 1.9 kg over 3.8 months. The efficacy was greater in obese people than lean people. 4
The dietary supplement information contained on this site has been compiled from published sources thought to be reliable, but it cannot be guaranteed. Efforts have been made to assure this information is accurate and current. However, some of this information may be purported or outdated due to ongoing research or discoveries. The authors, editors and publishers cannot accept responsibility for errors or omissions or for any consequences from applications of the information in this site and make no warranty, expressed or implied, with respect to the contents herein.
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