The bacteria that live in the human body outnumber the body’s cells 10 to 1.
Bacteria live on the skin, in the mouth and in the nose, but the great majority live in the gut, primarily the large intestine.
About 500 different species of bacteria live in the intestine, totaling about 100 trillion cells. These gut bacteria are also known as the gut flora.
This is not a bad thing. In fact, there is a mutually beneficial relationship between you and some of the bacteria that live in your digestive system.
You provide food, shelter and a safe habitat for the bacteria. In return, they take care of some things that the human body cannot do on its own.
Of the many different kinds of bacteria, some are crucial for various aspects of your health, including digestive health, weigh and blood sugar control, immune function and even brain function.
You may wonder what this has to do with fiber. Just like any other organism, bacteria need to eat to get energy to survive and function.
The problem is that most carbs, proteins and fats are absorbed into the bloodstream before they make it to the large intestine, leaving little for the gut flora.
This is where fiber comes in. Human cells don’t have the enzymes to digest fiber, so it reaches the large intestine relatively unchanged.
However, intestinal bacteria do have the enzymes to digest many of these fibers.
This is the most important reason that (some) dietary fibers are important for health. They feed the “good” bacteria in the intestine, functioning as prebiotics.
In this way, they promote the growth of “good” gut bacteria, which can have various positive effects on health.
The friendly bacteria produce nutrients for the body, including short-chain fatty acids like acetate, propionate and butyrate, of which butyrate appears to be the most important.
These short-chain fatty acids can feed the cells in the colon, leading to reduced gut inflammation and improvements in digestive disorders like irritable bowel syndrome, Crohn’s disease and ulcerative colitis.
When the bacteria ferment the fiber, they also produce gases. This is the reason high-fiber diets can cause flatulence and stomach discomfort in some people. These side effects usually go away with time as your body adjusts.
SUMMARY Consuming adequate amounts of soluble, fermentable fiber is very important for optimal health because it optimizes the function of the friendly bacteria in the gut.
Oregon State University Study Summary
- Dietary fiber is a diverse group of compounds, including lignin and complex carbohydrates, which cannot be digested by human enzymes in the small intestine.
- Although each fiber type is chemically unique, fibers can be classified according to their solubility, viscosity, and fermentability in order to better understand their physiological effects.
- Soluble gel-forming fibers, such as psyllium and fibers found in oat products, can lower serum LDL cholesterol concentrations and normalize blood glucose and insulin responses.
- Large/coarse fibers (e.g., wheat bran) and nonfermentable, soluble gel-forming fibers (e.g., psyllium) can have a potential laxative effect.
- Large prospective cohort studies consistently report inverse associations between consumption of diets rich in fiber and risks of cardiovascular disease and type 2 diabetes mellitus.
- A review of the most recent meta-analyses of observational studies suggests that dietary fiber consumption is inversely associated with the risk of cancer of the esophagus, stomach, colon, pancreas, ovary, and breast.
- Both consumption of fiber-rich diets and supplementation with soluble gel-forming fibers could help improve glycemic control in individuals with type 1 or type 2 diabetes mellitus.
- The Adequate Intake (AI) recommendation for total daily fiber intake is 38 g/day for men and 25 g/day for women. However, the average American consumes only about 17 g/day of dietary fiber, and dietary fiber intake might be closer to 10 g/day in those following a low-carbohydrate diet.
- Some strategies for increasing dietary fiber intake include increasing fruit and nonstarchy vegetable intake, increasing intake of legumes, eating whole-grain cereal or oatmeal for breakfast, substituting whole grains for refined grains, and substituting nuts or popcorn for less healthy snacks.
Institute of Medicine: dietary, functional, and total fiber
Before establishing intake recommendations for fiber in 2001, a panel of experts convened by the Institute of Medicine (now the National Academy of Medicine) developed definitions of fiber that made a distinction between fiber that occurs naturally in plant foods (dietary fiber) and isolated or synthetic fibers that may be added to foods or used as dietary supplements (functional fiber). However, these distinctions are controversial, and there are other classification systems for dietary fiber.
According to the Institute of Medicine’s definition, functional fiber “consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans” (4). Functional fibers may be nondigestible carbohydrates that have been isolated or extracted from a natural plant or animal source, or they may be manufactured or synthesized. However, designation as a functional fiber by the Institute of Medicine requires the presentation of sufficient evidence of physiological benefit in humans. Fibers identified as potential functional fibers by the Institute of Medicine include:
- Isolated or extracted forms of the dietary fibers listed above.
- Psyllium: Psyllium refers to viscous, gel-forming mucilage, which is isolated from the outer coat (husk) of psyllium seeds — known in India as ispaghula husk — from the plant Plantago ovata or blond psyllium (4).
- Chitin and chitosan: Chitin is a polysaccharide polymer extracted from the exoskeletons of crustaceans, such as crab and lobster. It is a polymer of more than 5,000 acetylated glucosamine units linked by β-1,4 glycosidic bonds. Deacetylated chitin or chitosan thus consists in unbranched chains of glucosamine (8).
- Fructooligosaccharides: Fructooligosaccharides are short, synthetic fructose chains terminating with a glucose unit. They are used as food additives (9).
- Galactooligosaccharides: Galactooligosaccharides are produced through the enzymatic conversion of lactose and are classified as prebiotics (10).
- Polydextrose and polyols: Polydextrose and polyols are synthetic carbohydrates. Polydextrose is made of glucose and sorbitol (a sugar alcohol) and may be used as bulking agent in food. Polyols are non-sugar molecules containing multiple hydroxyl groups (-OH). Polydextrose and polyols are used as sugar substitutes in food (4).
- Resistant dextrins: Resistant dextrins, also called resistant maltodextrins, are synthetic indigestible polysaccharides formed when starch is heated and treated with enzymes. They are used as food additives (4).
Total fiber is defined by the Institute of Medicine as “the sum of dietary fiber and functional fiber” (4).
Other classification systems
Fibers can be classified into four clinically meaningful categories according to their physiochemical properties, i.e., their solubility, viscosity, and fermentability (reviewed in 11):
Soluble, viscous/gel forming, readily fermented fibers
E.g., β-glucans from oats and barley, raw guar gum
Soluble fibers dissolve in water, while insoluble fibers do not. Viscous fibers thicken in the presence of water, forming very viscous solutions or even visco-elastic gels. Fermentable fibers are readily metabolized by the gut microbiota (i.e., bacteria that normally colonize the large intestine). Fermentation of fiber results in the formation of short-chain fatty acids (acetate, propionate, and butyrate) and gases (1). Short-chain fatty acids can be absorbed and metabolized to produce energy. Interestingly, the preferred energy source for colonocytes (epithelial cells that line the colon) is butyrate. Fermentation of fiber is estimated to contribute up to 10% of daily energy intake (12). Fibers that are fermentable and can stimulate the growth and/or activity of beneficial gut bacteria are called prebiotic fibers (13). Fibers that are soluble, viscous, and fermentable have been shown to improve glycemic control and to lower blood cholesterol concentration. However, their water-holding capacity is lost when they are fermented in the colon such that they have no laxative effect (see Biological Activities).
Soluble, viscous/gel forming, nonfermented fibers
These fibers can dissolve in water and form viscous gels. They can improve glycemic control and lower blood cholesterol concentration. In addition, they retain their water-holding/gel-forming capacity in the large intestine since they are resistant to fermentation. As a consequence, they can exert a stool-normalizing effect, preventing constipation or softening hard stool as well as firming loose/liquid stool in diarrhea and fecal incontinence (see Biological Activities).
Soluble, nonviscous, readily fermented fibers
E.g., inulin, wheat dextrin, oligosaccharides, resistant starches
Although these fibers can dissolve in water, they cannot provide any health benefits associated with fiber viscosity. They are fully fermented and thus do not exert a laxative effect. They can nonetheless exert a prebiotic effect by influencing the composition of the gut microbiota. In vitrostudies have shown inulin to selectively stimulate the proliferation of beneficial bacteria and limit the growth of potentially pathogenic bacteria ) (see Isolated fibers and supplements) (reviewed in 14). However, no health benefit is currently associated with this fiber-driven prebiotic effect.
Insoluble, poorly fermented fibers
E.g., wheat bran, cellulose, lignin
These fibers do not dissolve in water, do not trap water, and are poorly fermented. Large/coarse fiber particles can have a laxative effect. They can irritate the large intestine mucosa and trigger the secretion of mucus and water, which increases the water content of stools. Small, insoluble fiber particles (e.g., finely ground wheat bran) have no laxative effect and can actually have a constipating effect by adding only to the dry stool mass (see Improving regularity in stool elimination).
Fiber Improves Human Biome and Disease Prevention
Improving glycemic control
Lowering serum cholesterol
Lowering blood pressure
Improving regularity in stool elimination
Decrease in Cardiovascular disease
Type 2 diabetes mellitus
Sources of Fiber
Good sources of dietary fiber include legumes, nuts, whole grains, bran products, fruit, and nonstarchy vegetables. Legumes (e.g., dry beans and peas), nuts, seeds, and whole grains are generally more concentrated sources of fiber than fruit and vegetables (138). These higher fiber foods are currently underconsumed, contributing to only about 6% of total dietary fiber intake (137). Although refined grains are often perceived as being poor sources of fiber, they can provide as much fiber as either fruit or vegetables when comparable serving sizes are consumed (138). In addition, not all whole grains are good sources of fiber, yet they provide key micronutrients and phytochemicals that contribute to the health benefit associated with whole grain consumption (see the article on Whole Grains) (12).
All plant-based foods contain a mixture of soluble and insoluble fiber (138). Bran flaxseed, oat cereal, legumes, nuts, fruit, and vegetables are good sources of soluble viscous and nonviscous fiber. Wheat bran, brown rice, barley, cabbage, celery, and whole grains are rich sources of insoluble fiber. The total fiber content of some fiber-rich foods is presented in Table 3. Some strategies for increasing dietary fiber intake include increasing fruit and nonstarchy vegetable intake, increasing intake of legumes, eating whole-grain cereal or oatmeal for breakfast, substituting whole grains for refined grains, and substituting nuts or popcorn for less healthy snacks. For more information about the fiber content of specific foods, search the USDA National Nutrient Database.
|Navy beans, cooked, boiled||½ cup||9.6|
|Split peas, cooked, boiled||½ cup||8.1|
|Lentils, cooked, boiled||½ cup||7.8|
|Refried beans, canned||½ cup||5.7|
|Kidney beans, canned||½ cup||5.5|
|Cereal and grains|
|All-bran (wheat) cereal||½ cup||19.5|
|Bulgur, cooked||½ cup||4.1|
|Cereal, instant oatmeal||½ cup||4.0|
|Pearled barley, cooked||½ cup||3.0|
|Oat bran, cooked||½ cup||2.8|
|Quinoa, cooked||½ cup||2.6|
|Rice, long-grain, brown, cooked||½ cup||1.6|
|Winter squash, butternut, cooked, baked||1 cup||6.6|
|Artichoke hearts, cooked||½ cup||4.8|
|Spinach, frozen, cooked||½ cup||3.5|
|Mushrooms, white, cooked from fresh||1 cup||3.4|
|Brussel sprouts, frozen, cooked||½ cup||3.2|
|Plums, dried (prunes), uncooked||½ cup, pitted||6.2|
|Guava, fresh||½ cup||4.5|
|Pear||1 small pear||4.6|
|Asian pear||1 small pear||4.4|
|Raspberries, fresh||½ cup||4.0|
|Blackberries, fresh||½ cup||3.8|
|Plums, fresh||2 plums||1.8|
|Nuts and Seeds|
|Almonds||1 ounce (23 kernels)||3.5|
|Pistachios||1 ounce (49 kernels)||3.0|
|Pine nuts||1 ounce||3.0|
|Hazelnuts||1 ounce (21 kernels)||2.7|
|Pecans||1 ounce (19 halves)||2.7|
Some suggestions for increasing fiber intake
- Eat at least five servings of fruit and vegetables daily (see the article on Fruit and Vegetables).
- Substitute whole grains for refined grains (see the article on Whole Grains).
- Eat oatmeal, whole-grain cereal, or bran cereal for breakfast.
- Eat beans, split peas, or lentils at least once weekly (see the article on Legumes).
- Substitute nuts or popcorn for less healthful snacks like potato chips or candy (see the article on Nuts).
Adopting one of the USDA healthy dietary patterns (i.e., healthy US-style, healthy Mediterranean-style, and healthy vegetarian dietary patterns) recommended in the 2015-2020 Dietary Guidelines for Americans will help meet the recommendations for total fiber intake (110). Fruit, vegetables, and whole grains available in the USDA dietary patterns contribute nearly 90% of the recommended dietary fiber intake. Within the vegetable group, beans, peas, and starchy vegetables are the main contributor of total fiber intake (22%). Refined grains provide 9% of total fiber intake (110).
Some people experience abdominal cramping, bloating, or gas when they abruptly increase their dietary fiber intake (146, 147). These symptoms can be minimized or avoided by increasing intake of fiber-rich foods gradually and increasing fluid intake to at least 64 oz/day (~2 liters or 2 quarts/day). There have been rare reports of intestinal obstruction related to large intakes of oat bran or wheat bran, primarily in people with impaired intestinal motility or difficulty chewing (148-151). The National Academy of Medicine (formerly, the Institute of Medicine) has not established a tolerable upper intake level (UL) for dietary or functional fiber (4).
Isolated fibers and fiber supplements
Gastrointestinal symptoms: The following fibers have been found to cause gastrointestinal distress, including abdominal cramping, bloating, gas, and diarrhea: guar gum, inulin and oligofructose, fructooligosaccharides, polydextrose, resistant starch, and psyllium (4). It is recommended to gradually introduce a new fiber supplement, not exceeding 3 to 4 g/day the first week, in order to minimize gastrointestinal symptoms (152). In subjects who are constipated, the initiation of a fiber supplement should start once the hard stool is cleared (152). Use of a guar gum-containing supplement for weight loss has been associated with esophageal and small bowel obstruction (153). Additionally, several cases of intestinal obstruction by psyllium have been reported when taken with insufficient fluids or by people with impaired swallowing or gastrointestinal motility (154, 155).
Colorectal adenomas: One randomized controlled trial in patients with a history of colorectal adenomas (precancerous polyps) found that supplementation with 3.5 g/day of psyllium for three years resulted in a significant increase in colorectal adenoma recurrence compared to placebo (see Colorectal cancer) (72).
Allergy and anaphylaxis: Since chitin is isolated from the exoskeletons of crustaceans, such as crabs and lobsters, and chitosan is derived from chitin, people with shellfish allergies should avoid taking chitosan supplements (8). Anaphylaxis has been reported after intravenous (IV) administration of inulin (156), as well as ingestion of margarine containing inulin extracted from chicory (157). Anaphylaxis has also been reported after the ingestion of cereal containing psyllium, and asthma has occasionally been reported in people with occupational exposure to psyllium powder (158).
Gel-forming fibers (e.g., β-glucan, psyllium, raw guar gum, pectin) have the potential to slow the absorption of drugs if taken at the same time. Psyllium may reduce the absorption of lithium, carbamazepine (Tegretol), digoxin (Lanoxin), and warfarin (Coumadin) when taken at the same time (8). Guar gum may slow the absorption of digoxin, acetaminophen (Tylenol), and bumetanide (Bumex) and decrease the absorption of metformin (Glucophage), penicillin, and some formulations of glyburide (Glynase) when taken at the same time (159). Pectin may decrease the absorption of lovastatin (Mevacor) when taken at the same time (160). Concomitant administration of a kaolin-pectin antidiarrheal suspension has been reported to decrease the absorption of clindamycin, tetracycline, and digoxin, but it is not known whether kaolin, pectin, or both were responsible for the interaction (8). In general, medications should be taken at least one hour before or two hours after fiber supplements and gel-forming dietary fibers (e.g., oatmeal).
The addition of cereal fiber to meals has generally been found to decrease the absorption of iron, zinc, calcium, and magnesium in the same meal, but this effect appears to be related to the phytate present in the cereal fiber rather than the fiber itself (161). In general, dietary fiber as part of a balanced diet has not been found to adversely affect the calcium, magnesium, iron, or zinc status of healthy people at recommended intake levels (4). Evidence from animal studies and limited research in humans suggests that inulin and oligofructose may enhance calcium absorption (162, 163). The addition of pectin and guar gum to a meal significantly reduced the absorption of the carotenoids β-carotene, lycopene, and lutein from that meal (164, 165).