Dietary Fiber: a Fact Sheet

In this Article (Index)

• Fiber: plants and carbohydrates
• Dietary Fiber
• What is Dietary Fiber?
• Types of Dietary Fiber?
• Benefits of Fiber
• Health Benefits of Soluble Fiber
• Health Benefits of Insoluble Fiber
• Side Effects of Fiber
• Phytates
• The energy content of fiber
• Fiber content in some foods
• Fiber a key prebiotic: Health Benefits
• Cholesterol and Glucose benefits
• Prebiotics
• Short-chain Fatty Acids or SCFAs
• Weight control
• Recommended Dietary Allowance for Fiber
• Adding More Fiber to your Diet
• Added Fiber & Fiber supplements
• Inulin
• Fiber Supplements
• Fiber and Constipation
• Low-fiber diets may promote constipation

Fiber: plants and carbohydrates

Plants convert sunlight and use its energy to combine water and atmospheric carbon dioxide to manufacture sugars (mainly glucose). This process, known as photosynthesis, is vital. The plants then break down the sugar in their cells converting it into oxygen and water, producing energy to keep them alive.

Sugars are relatively simple chemical compounds that belong to a group known as carbohydrates and are stored as starches and simple sugars. They are also the building blocks of more complex molecules that form the plant's structure and tissue (stems, leaves, seeds, flowers).

Carbohydrates

These are the digestible parts of a plant, that can be broken down in our small intestine and absorbed. Carbohydrates are an important part of our diet. Each gram of carbohydrate releases 4 kcal of energy.

Polysaccharides

Plants combine different types of sugar molecules using them to produce compounds called polysaccharides, named after the Greek word "poly" = many, and the Latin word "saccharum" = sugar, hence "many sugars."
Polysaccharides are in fact polymers like plastics, where small units known as monomers (the sugars) are bound in a sequential linear or branched arrangement forming a polymer (the polysaccharide).

To clarify the terminology, sugars have 1 or 2 monomers (like lactose made up of two monosaccharides), oligosaccharides have 3 to 9 monomers and most of them are digestible, while polysaccharides have more than 10 monomers and are mostly non-digestible.

Wood, on the other hand, is made of another plant-based polymer, lignin, but unlike carbohydrates its monomers are not sugars, they are natural aromatic compounds.

There are more than 10 different simple sugars that can serve as monomers for polysaccharides such as arabinose, xylose, glucose, fructose, mannose, and galactose among others.

There are different kinds of polysaccharides, and serve different purposes, depending on their physical properties (flexibility, rigidity, etc).

There are many types of carbohydrates, and animals, fungi, and microorganisms have learned how to use them as energy sources, breaking them down into their sugar building blocks.

The chemical bond linking the monomers can be either an α- or a β-glycosidic one. The former can be digested by humans, the latter can only be broken down by bacteria, some worms and insects manufacture an enzyme (cellulase) that breaks the bonds, and some animals like termites and herbivores (cows, sheep) that use bacteria and enzymes in their stomachs to do the job.

Microbes and animals also synthesize their own polysaccharides like glycogen to store energy, or chitin as the structural component of insects, shrimp, and crabs among others.

Some Types of Plant Polysaccharides:

• Cellulose. Linear chains of thousands of D-glucose monomers linked by β-glycosidic (1-4) bonds.
• Starch or amylum. Made up of amylose and amylopectin, which in turn are long chains of glucose bound by α-glycosidic bonds (they differ in the bond type, and that makes one lineal and the other branched. This type of bond can be digested by humans, and starches form part of our diet.
• Fructans. The best known is inulin, which is a natural polysaccharide that is also produced as an isolated functional fiber.
• β-Glucans. With β-glycosidic (1-3) and (1-4) bonds linking glucose. Also found in mushrooms and yeasts; and in oats, barley, wheat, and sorghum.
• Hemicelluloses. Composed of xylose and arabinose.
• Pectins. Fruits like apples, oranges, grapefruits, and lemons contain more pectin than others. They are rich in galacturonic acid (a sugar) plus other monosaccharides (arabinose) linked by digestible α-glycosidic bonds.
• Resistant starches. These are normal starches that have been modified by heat. They can only be fermented in the gut by the bacteria that live there.

Fibers are polysaccharides.

Dietary Fiber

What is Dietary Fiber?

Dietary fiber, or fiber, are the polysaccharides that are bound in such a way that they cannot be digested in the small intestine, and can only be fermented by the microbes that live in the colon. (15)

The European Food Safety Authority (EFSA) defines dietary fiber as non-digestible carbohydrate plus lignin.

Types of Dietary Fiber?

• Soluble Fiber. It dissolves in water and forms a viscous gel that has cholesterol-lowering properties. You will find it in oat bran, nuts, lentils, peas, and beans. This fiber can be digested, at least in part, by the bacteria in the gut. The insoluble fibers are lignin, cellulose, and resistant starches.
• Insoluble Fiber. It does not dissolve in water, and is mostly made up of cellulose, you can find it in wheat bran, whole grains, and vegetables. It passes through the gut without being digested.

Benefits of Fiber

Health Benefits of Soluble Fiber

• Absorbs water, and turns into a gel during digestion. Traps carbohydrates and slows down glucose absorption, smoothing blood sugar spikes after a meal.
• This gel also absorbs dietary cholesterol and fat, reducing blood levels of LDL ("bad") cholesterol and the risk of heart disease.
• Regulates the acidity of the colon, and stimulates fermentation there, by bacteria that produce Short-chain Fatty Acids or SCFAs, which could help reduce the risk of colorectal cancer
• Adds bulk, making you feel satiated faster. Could help reduce appetite and stay satisfied longer.

Health Benefits of Insoluble Fiber

• Speeds the passage of food through the gut. Improving regularity.
• Adds bulk to stool. Relieves constipation.
• Adds bulk, making you feel satiated faster. Could help reduce appetite and stay satisfied longer.

There is scientific evidence that fiber helps control weight, and reduce irritable bowel syndrome (IBS) symptoms. It also has positive effects on heart disease, diabetes, and constipation.

Fiber does not bind to the vitamins and minerals in your diet, so it does not interfere with their absorption, however, a plant-based component, phytate, blocks their uptake.

Side Effects of Fiber

Adding a lot of fiber to your diet quickly can cause an increase in intestinal gases (due to its fermentation-boosting effects), which causes bloating, abdominal cramps, and flatulence. It can also loosen your stool (diarrhea).

Phytates

Phytates are considered anti-nutrients as they can't be digested by human beings, and they react with some minerals binding with them (Copper, zinc, magnesium, nickel, calcium, iron) forming insoluble compounds that can't be absorbed in the gut. (16)

Phtates are a compound found in plant-based foods. Cereals, beans, nuts, and lentils, are phytate-rich, others like berries, fruits, roots, and tubers contain very low amounts. They are used by plants to store phosphorous in seeds and grain.

However, since fiber-containing plant-based foods are also rich in minerals, this issue shouldn't be a problem.

The Energy content of fiber

Since fiber is not digested, you would expect it to have no impact on your energy intake, however, this is only true for insoluble fiber, which passes through the digestive system without alterations. Soluble fiber, on the other hand, is modified by fermentation in the gut, producing gases and Short-chain Fatty Acids or SCFAs that provide energy to the body.
There is no consensus about the amount of energy absorbed, but it is estimated to be in the range of 2 to 4 kcal per gram of soluble fiber.

For processed foods, in the U.S., soluble fiber is counted on product labels at 4 Cal per gram.

Fiber content in some foods

Total, Insoluble, and Soluble fiber content

Values are in grams (g) per every 100 g of edible portion. (6),(17)

Grains & cereals

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Pulses

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Vegetables

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Fruits

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Nuts and Seeds

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Fiber a key prebiotic: Health Benefits

Prebiotics

Prebiotics are food ingredients that can't be digested in the stomach or small intestine and are fermented in the colon by certain microbes that live there. They can alter the activity and composition of the gut microbiome, providing health benefits, and in general, prebiotics are consumed by bacteria like lactobacilli and bifidobacteria. (1)

All prebiotics are fiber, but not all fiber is a prebiotic. The prebiotic fibers can be found naturally in foods like asparagus, onion, garlic, chicory, Jerusalem artichokes, wheat, oats, soybeans, and bananas.

Short-chain Fatty Acids or SCFAs

We have already mentioned that soluble fiber passes through the small intestine undigested. It is fermented and broken down by the bacteria in the colon, the gut microbiome.

This fermentation produces gases (methane and carbon dioxide) and chemical compounds known as short-chain fatty acids or SCFAs. These short-chain fatty acids are of four types: butyric, acetic (ethanoic), propionic, and valeric acids. They have significant health properties.

Being acidic, they lower the pH in the large intestine and this prevents the growth of pathogenic and putrefactive bacteria. Fiber has a positive influence on the diversity and abundance of the gut microbiome. It promotes the growth of fermenting bacteria like Bifidobacterium and Lactobacillus species that have beneficial health effects. There are many species of fermenting microbes such as Firmicutes, Bacteroidetes, Enterobacteria, Roseburia, and Fecalibacterium.

These bacteria form a barrier that prevents pathogenic bacteria from invading the gastrointestinal tract. Roughly 30 g of bacteria are produced for every 100 g of carbohydrate that is fermented and this also has a stool-bulking effect. (1)

Energy Sources

As mentioned further up the SCFAs are compounds that provide energy; as they are water-soluble are absorbed by the gut and used by the body. SCFAs from the average "Western Diet" which is quite low in fiber, provide roughly 6 to 5% of the total energy needed by a person. Those eating high-fiber diets should obtain even more energy from SCFAs fermented by the gut microbiome. In fact, the quantity of SCFAs is directly proportional to the composition and volume of the microbiome. (16)

Vegan diets contain more fiber and this increases SCFA production, in particular butyrate which reduces inflammation in the gut. (21)

Effect of the SCFAs on your health

Butyrate helps promote the integrity of the intestinal barrier, protecting you from a "leaky gut", and has an important role in brain function. (18)

Experimental studies have proven that regular consumption of fiber (especially the soluble fraction) in the optimal amount, i.e., about 30 g per day, is positively correlated with the production of butyrate by multiple bacteria. Moszak, Szulinska, and Bogdanski (2020) (18)

Butyrate has beneficial effects such as killing colon cancer cells and maintaining the oxygen balance in the gut, keeping the microbiota stable.

Cholesterol and Glucose benefits

Propionate is used by the liver to regulate glucose production and it also helps promote satiety. Acetate seems to regulate appetite and plays a beneficial role by reducing the production of cholesterol; it also prevents the formation of fatty tissues. (19)

Gut microbes also produce indolepropionic acid (more dietary fiber intake promotes more output of this acid), which is a potent antioxidant and seems to reduce the risk of type 2 diabetes.

Weight control

The higher the production of SCFAs by gut bacteria, the lower is diet-induced obesity and insulin resistance. In mice, both propionate and butyrate reduce appetite and food intake by acting on gut-produced hormones. (20)

Fiber reduces Visceral and Subcutaneous Fat

Hairston (2011) (22) conducted a five-year study of a group of 1,114 subjects (roughly 30% African American and 70% Hispanic) and found some interesting relationships between diet, Visceral (VAT), and subcutaneous (SAT) adipose tissue:

• Soluble fiber keeps VAT at bay: "For each 10 g increase in soluble fiber, rate of VAT accumulation decreased by 3.7%."
• Fiber did not impact on SAT.

Recommended Dietary Allowance for Fiber

The Dietary Guidelines for Americans (2020-2025) recommend an intake of 14 grams of fiber per every 1,000 calories in your diet, so the exact number varies with your recommended caloric intake. For 2,000 kcal per day, you'd need to consume 28 g (1 oz) of fiber. (3)

More than 90 percent of women and 97 percent of men do not meet recommended intakes for dietary fiber.
This aligns with intake patterns where fruits, vegetables, and whole grains are underconsumed by more than 85 percent of adults. Dietary Guidelines for Americans (2020-2025) (3)

Adding More Fiber to your Diet

To avoid side effects, you should aim at adding fiber gradually over a period of several weeks. Drinking more water will also help, as fiber absorbs it. Make sure you are drinking your 8 glasses of water a day. (4)

Try to add natural fiber instead of supplements, adding more plant-based ingredients to your dishes is a simple way of doing so.

Peeling fruits and vegetables reduces the fiber content. Leave the skin on potatoes, apart from fiber it has minerals and vitamins. Eat your fruit without peeling them.

Eat whole fruits instead of fruit juice. A 100% fruit juice without added sugars appears healthy, but it is lower in dietary fiber than whole fruit. (3).

Cooking techniques can help improve fiber content in some foods. For instance, the regular starch in pasta, rice, and potatoes is transformed into a type of resistant starch (RS3), that is not digested into glucose in the small intestine, instead, it reaches the colon where it is fermented by the gut microbiota. This happens when these ingredients are cooked and then cooled.

In the case of potatoes, microwave heating, and deep fat frying reduces the digestible starch and significantly increase the resistant starch and water-insoluble dietary fiber (IDF). Cooking does not alter the water-soluble dietary fiber content. (6)

Added Fiber & Fiber supplements

As we have seen, fiber occurs naturally in plant-based foods and this is called Dietary Fiber, it is found in vegetables, grain, fruits, nuts, seeds, and mushrooms as a natural component.

Then there are Functional Fibers that have been processed in factories, extracted from plant sources, or manufactured using bacteria (xanthan gum). Some are obtained from animal sources (chitin).
These fibers aren't a mixture of different types of soluble or insoluble fibers like the natural fibers, instead, they are isolated fibers.

Isolated fiber is used in fiber supplements and as additives in processed foods.

Processed Foods Need Added Fiber

Processed foods contain natural animal and plant-based ingredients that have been treated with heat, chemical, and mechanical processes to break them apart into their building blocks like protein, fats, sugar, fiber, and starches. Some of them are removed, others modified and reassembled into more heavily processed foods with higher content of salt, sugars, fats, and refined carbohydrates, but lacking natural fiber.

To appear healthier to the consumer, to provide texture, and to boost the product's fiber content, added fiber (isolated fiber) is incorporated into these processed foods.

The Food and Drug Administration recognizes several additive ingredients as "fibers"; these are listed below: (23)

• Mixed plant cell wall fibers (a broad category that includes fibers like sugar cane fiber and apple fiber, among many others)
• Arabinoxylan
• Alginate
• Inulin and inulin-type fructans
• High amylose starch (resistant starch 2)
• Galactooligosaccharide
• Polydextrose
• Resistant maltodextrin/dextrin

Inulin

Inulin is a soluble dietary fiber found in natural foods like chicory roots, Jerusalem artichoke, dahlia tubers, yacon, asparagus, leek, onion, banana, wheat, ginger, and garlic.
It is classified as a fructan, with a structure similar to a carbohydrate, but it isn't digestible.

Inulin is also produced as a functional fiber by extracting it from sugar beets and used in processed foods to replace fat or sugar.
It is considered a fermentable oligo-, di-, monosaccharides, and polyols (FODMAP) type fiber. (5).

Inulin safety

The Food and Drug Administration (FDA) classified inulin as "generally recognized as safe" with a recommended maximum daily intake of 15 to 20 g. Side effects of taking inulin include gases, bloating, and nausea, and consuming less than 40 g in healthy adults is safe. Inulin can provoke serious side effects in people suffering from allergies or inflammatory bowel disease (IBD). (24)

Fiber Supplements

Most people believe that consuming fiber supplements on a regular basis will cover the gap in their dietary fiber intake and provide the health benefits that they weren't getting from their low-fiber diet. But this is a misconception.

Isolated functional fibers found in supplements don't necessarily provide health benefits. A study published in 2017 (7) pointed out these shortcomings. It mentions, for the small intestine, the following:

• Only high-viscosity soluble fibers provide health benefits such as lowering cholesterol and controlling blood sugar. This means that gel-forming b-glucan, psyllium, and raw guar gum are effective there.
• Low-viscosity/ nonviscous fibers like low-molecular-weight b-glucan, methylcellulose, inulin, and wheat dextrin, and insoluble fiber like wheat bran and cellulose don't have significant effects on cholesterol or blood sugar levels.

For the large intestine, the study mentions the laxative-constipation effect of fiber:

• Non-fermented, high water-holding gel-forming fiber like psyllium can be used for constipation to soften hard stool and for diarrhea, to firm up loose stool, It is ideal for patients with irritable bowel syndrome.
• On the other hand, low water-holding fine-sized insoluble fiber (fine wheat bran), and fermentable soluble fiber (wheat dextrin) can cause harder stools and constipation.

Natural dietary fiber, on the other hand, isn't just one kind of isolated fiber. Instead, it is a mixture of many different plant-based foods, and each plant has its own specific types of fibers with unique chemical compositions and properties: apples are a source of pectin, bananas contain resistant starch, and fructans like inulin. A diet with varied plant-based foods will provide a variety of dietary fibers and support more diverse gut microbiota. (2)

Fiber and Constipation

Low-Fiber diets may promote constipation

Murakami (2007) (8) studied Japanese women who ate a diet with very low fiber content and found that this low fiber intake coupled with a low water intake was associated with "increased prevalence of constipation."

However, Fitriana (2018) (9) who had studied pregnant Indonesian women, did not find any "significant association between fiber intake and constipation."

A High fiber diet plus fluids helps relieve constipation

Anti (1998) (10) studied 117 patients suffering from chronic functional constipation. The team split them into two groups and gave both groups a standard diet that included 25 g (almost 1 oz.) of fiber per day.

Water intake was monitored: Group 1 consumed 1.1 liters per day (just over 1 quart), while Group 2 had a daily intake that averaged 2 liters.

The results were the following:

• Both groups showed "statistically significant increases in stool frequency and decreases in laxative use during the two-month trial."
• Group 2 (which drank more water) showed greater changes compared to Group 1 and laxative use.

Anti concluded that "A daily fiber intake of 25 g can increase stool frequency in patients with chronic functional constipation, and this effect can be significantly enhanced by increasing fluid intake to 1.5-2.0 liters/day."

Fathallah (2017) (11) favors fiber supplementation with a 20 to 25 g daily intake. Considering it is "the most relevant measure [because] it improves stool frequency and consistency. It has a positive effect on excessive straining and colonic transit time."

Note that for those hot used to eating fiber in their diet, that fiber may cause abdominal pain, flatulence, and bloating (fibers ferment in the gut) so it should be incorporated into the diet gradually over a period of several days.

Fathallah considers additional water intake or exercise as options in special situations such as institutionalized, dehydrated, elderly, or hospitalized patients. For regular subjects, those treatments are "probably anecdotal recommendations."

Borre (2017) (12) agrees, considering that fiber, fluid, and exercise are recommendations based on scarce evidence. Nevertheless, they concede that "Increased intake of fiber will reduce colonic transit time and improve the frequency and consistency of stools in 50% of patients."

Tabbers (2011) (13) in an analysis of 9 studies covering childhood constipation (640 children) concluded that: "There is some evidence that fiber supplements are more effective than placebo. No evidence for any effect was found for fluid supplements, prebiotics, probiotics, or behavioral intervention."

Yet Jing (2012) (14), who performed a meta-analysis (a statistical procedure for combining data from multiple studies) concluded that "Dietary fiber intake can obviously increase stool frequency in patients with constipation. It does not obviously improve stool consistency, treatment success, laxative use and painful defecation."

Closing Comments

Fiber from natural dietary sources should make up an important part of your diet. Eating whole grains, fruit, nuts, seeds, and vegetables not only promotes your intestinal regularity but also contributes to your overall health.