Dietary fiber

Fiber is one of the main reasons why plant-based foods are so healthy. Scientific research shows that adequate fiber intake can aid digestion and reduce the risk of chronic disease. Many of these benefits are mediated by gut flora - the billions of bacteria that live in the digestive tract. However, not all fiber is created equal. There are different types, each with its own health benefits. This article will take a closer look at the scientifically proven benefits of dietary fiber.

What is dietary fiber?

Simply put, dietary fiber is non-digestible carbohydrates that are found in food. The human digestive tract simply lacks the enzymes necessary to break down these carbohydrates into their components, which is why they pass through the digestive tract more or less intact.

There is a wide range of different dietary fibres that can be found in food. One problem is that fiber can be classified in many different ways, which can be quite confusing.

Based on their water solubility, fibres are often divided into two broad categories:

Soluble fibres, which dissolve in water and can be metabolized by the "good" bacteria in the gut.

Insoluble fibres, which are not soluble in water.

Another formal classification of dietary fibers is a distinction according to the following criteria (3):

Naturally occurring dietary fiber

Functional dietary fiber extracted and isolated from whole foods for subsequent addition to processed foods.

Other types of classification include a distinction between viscous and non-viscous dietary fibers. In addition to this, there is a class of nutrients called resistant starch, which is often also classified as dietary fiber.

However, all these classifications have the disadvantage that they say nothing about the health benefits of the fiber so classified. Perhaps a more helpful way to characterize dietary fiber is therefore to distinguish between fermentable and non-fermentable fiber, which refers to whether or not beneficial gut bacteria can use it.

Different types of dietary fiber

It is important to keep in mind that there are many different types of dietary fiber, which we will briefly discuss in the following sections. Some of these have important health benefits, while others are more or less useless.

Soluble vs. insoluble fiber

The solubility of fiber refers to its ability to dissolve in water. Based on this, dietary fibres are characterized as soluble and insoluble fibres:

Soluble fibres mix with water in the digestive tract and form a gel-like substance. This fiber can lower blood sugar spikes and has numerous benefits for metabolic health (4).

Insoluble fiber does not mix with water and passes through the digestive tract mostly intact. They act primarily as "bulking agents" and can help speed the passage of food and waste products through the digestive tract (5).

Soluble fiber includes gummy fiber, pectins, psyllium, beta-glucans and others. Insoluble fibers include lignin and cellulose. Different plant foods contain varying amounts of soluble and insoluble fiber. There is a lot of overlap between soluble and insoluble fiber. Some insoluble fiber can be digested by the "good" gut bacteria and most foods contain both soluble and insoluble fiber.

Summary: Dietary fiber is often characterized based on its solubility in water. Soluble fiber has numerous benefits for metabolic health, while insoluble fiber acts primarily as a bulking agent.

Fermentable fiber

An estimated 100 quadrillion live bacteria colonize the human digestive tract, primarily the large intestine (6). These bacteria are crucial for optimal health. They play numerous important roles in weight management, blood sugar control, immune function, brain function and mental health (7, 8, 9, 10, 11, 12). These bacteria are so important that they are often referred to as the "forgotten organ" (13). Since humans cannot digest dietary fiber, most of it reaches the large intestine unchanged. This is where fermentable fiber comes into play. These fibers are compounds that the "friendly" gut bacteria can digest (ferment) and use as a source of energy (14).

This increases the number of friendly gut bacteria and shifts the balance in the gut towards these bacteria, which also produce short-chain fatty acids that have impressive health benefits (15). Most fermentable fibers are soluble fibers, but there are also some insoluble fibers that work this way. Fermentable fibers include pectins, beta-glucans, guar gum, inulin and oligofructose. The best food sources of fermentable fiber are beans and legumes. One cup often provides up to half the daily recommended intake of fiber. A byproduct of fermenting fiber is gas, which manifests as bloating. This is the reason why foods rich in fermentable fiber can cause bloating and digestive discomfort, especially for people who are not used to eating large amounts of fiber.

Summary: Fermentable fiber is used by the "friendly" gut bacteria as a source of energy. This can result in numerous health benefits.

Viscous fiber

Some types of soluble fiber form a viscous gel when mixed with water. These fibers are known as viscous fibers. In simple terms, the viscosity of a liquid refers to its "thickness". Honey, for example, has a higher viscosity than water. When viscous fiber is ingested, it forms a gel-like substance in the digestive tract. This slows down digestion and the absorption of nutrients, resulting in a longer-lasting feeling of satiety and a reduced appetite (16, 17, 18). A review of 44 studies on dietary fiber treatments concluded that only viscous fiber reduces food intake and causes weight loss (19). Viscous fibers include glucomannan, beta-glucans, pectins, guar gum, and psyllium. Good food sources include legumes, asparagus, Brussels sprouts, oats and flaxseed.

Summary: Viscous fiber forms a viscous gel in the digestive tract that can lead to increased satiety, reduced appetite and weight loss.

Resistant starch

Starches are the primary types of carbohydrates in the diet. They are long chains of glucose molecules that are found in potatoes, cereals and many other foods. Some types of starch are resistant to digestion, which means that they pass through the digestive tract intact and unchanged. This type of starch is called resistant starch and acts like soluble, fermentable fiber in the digestive tract (20). Resistant starch has numerous health benefits. It improves digestive health, increases insulin sensitivity, lowers blood sugar levels and significantly reduces appetite (21, 22, 23, 24, 25).

There are several good sources of resistant starch including green bananas, various legumes, cashews and raw oats. In addition, some starchy foods tend to form large amounts of resistant starch when cooled after cooking. This includes white potatoes and white rice. Raw potato starch is also very rich in resistant starch and is used by some people as a supplement.

Summary: Resistant starch is a type of starch that escapes digestion. It functions like soluble, fermentable fiber and has numerous health benefits.

Special fibers that should not go unmentioned

Multiple fibers have specific effects on health and should therefore be emphasized.

Fructans

Fructan is a term used to describe a short chain of fructose molecules. Oligofructose and inulin are the two primary fructans found in food. They can be metabolized by the "friendly" bacteria in the gut and have been shown to help treat certain forms of diarrhea (26). However, fructans are also classified as FODMAPs - a type of carbohydrate known to cause digestive discomfort in many people (27) In fact, fructans and other FODMAPs can cause adverse symptoms in 3 out of 4 people with IBS. The largest source of fructans in the modern diet is wheat (29).

Beta-glucans

The health benefits of beta-glucans have been extensively documented. These dietary fibers have a specific molecular structure that gives them a high viscosity in the digestive tract. Beta-glucans can improve insulin sensitivity and lower blood sugar levels. They can also significantly reduce cholesterol levels and increase the feeling of satiety (30). The primary sources of beta-glucans are oats and barley.

Glucomannan

Glucomannan is a viscous fiber that is marketed as a weight loss supplement. Numerous studies have shown that glucomannan can induce modest weight loss, help with constipation and improve risk factors for heart disease (31, 32, 33, 34).

Summary: Fructans are dietary fibers that can cause unwanted digestive symptoms in some people. Beta-glucans and glucomannan are soluble, viscous dietary fibers with distinct health benefits.

Health benefits of dietary fiber

Dietary fiber has a number of health benefits, which we will look at in the following sections in conjunction with the current scientific evidence.

Fibre serves as food for the "good" gut bacteria

The number of bacteria living in the human body exceeds the number of body cells by a factor of 10. Bacteria live on the skin, in the mouth and in the nose, but the vast majority live in the digestive tract, mainly in the large intestine (35). About 500 different species of bacteria live in the digestive tract, totaling about 100 trillion cells. These intestinal bacteria are also known as intestinal flora. This is not a bad thing. In fact, there is a beneficial relationship between the body and some of the bacteria that live in the digestive tract.

The body gives these bacteria food, protection and a safe habitat and in return these bacteria take care of some things that the body cannot do itself. Of the many different types of bacteria, some are essential for different aspects of health, which include body weight, blood sugar control, immune function and even brain function (36, 37, 38, 39, 40). Some readers may now be wondering what this has to do with fiber, but gut bacteria, like any other organism, need food to get energy to survive and function.

The problem is that most carbohydrates, proteins and fats are absorbed into the bloodstream before they can reach the large intestine, leaving little for the gut flora. This is where fiber comes into play. The human body does not have the necessary enzymes to digest fiber, which means that fiber reaches the large intestine more or less unchanged. However, intestinal bacteria have the enzymes to digest some of this fiber. This is the main reason why some fiber is important for health. These fibers provide nutrients to the "good" gut bacteria and serve as prebiotics (41). In this way, they promote the growth of the 'good' gut bacteria, which have numerous positive effects on health (42). The 'friendly' gut bacteria produce nutrients for the body, including short-chain fatty acids such as acetate, propionate and butyrate, of which butyrate appears to be the most important (43).

These short-chain fatty acids can provide nutrients to the cells in the gut, which can result in a reduction in intestinal inflammation and relief from digestive disorders such as irritable bowel syndrome, Crohn's disease and ulcerative colitis (44, 45, 46). When the bacteria ferment fiber, they also produce gas. This is the reason why a high-fiber diet can cause digestive discomfort and bloating in some people. However, these side effects usually disappear over time as the body adapts to the increased fibre intake.

Summary: Consuming adequate amounts of soluble, fermentable fibre is very important for optimal health as it optimizes the function of beneficial gut bacteria.

Some types of fiber can help with weight loss

Certain types of fiber can help with weight loss by reducing appetite. In fact, some studies show that increasing fiber intake can cause weight loss by automatically reducing calorie intake (47, 48) Fiber can absorb water in the gut, slow the absorption of nutrients and increase satiety (49). However, this depends on the type of fiber. Some dietary fibers have no effect on weight, while certain soluble fibers can have a significant effect (50, 51, 52, 53). A good example of an effective dietary fiber supplement for weight loss is glucomannan.

Summary: Some types of dietary fiber can induce weight loss by increasing satiety, leading to reduced calorie intake.

Fiber can lower blood sugar levels after a meal

Fiber-rich foods tend to have a lower glycemic index than refined carbohydrate sources, which have been stripped of most fiber. However, scientists believe that only highly viscous, soluble fiber has this property (54). Inclusion of this viscous, soluble fiber in carbohydrate-containing meals could cause weaker increases in blood glucose levels (55). This is important, especially if the diet is high in carbohydrates. In this case, fiber may reduce the likelihood of carbohydrates raising blood glucose levels to harmful levels. If you suffer from problems with blood sugar control, you should also try to reduce your carbohydrate intake, especially low-fiber, refined carbohydrates such as white flour and added sugar.

Summary: Foods that contain viscous fiber have a lower glycemic index and cause a lower rise in blood sugar levels than foods that are low in fiber.

Fiber can lower cholesterol levels, but this effect is not strong

Viscous, soluble fiber can also lower cholesterol levels. However, this effect is not nearly as impressive as one might expect. A review of 67 controlled studies concluded that consuming 2 to 10 grams of soluble fiber per day reduced total cholesterol levels by an average of only 1.7 mg/dl and LDL cholesterol levels by only 2.2 mg/dl (56). This also depends on the viscosity of the dietary fiber. Some studies have observed impressive reductions in cholesterol levels with increased fiber intake (57, 58). Whether this has a significant effect in the long term is unknown, although many observational studies show that people who eat more fiber have a lower risk of heart disease (59).

Summary: Some types of fiber can lower cholesterol levels, but most studies show that this effect is not very strong on average.

Certain dietary fibers may help against constipation

One of the main benefits of increasing fiber intake is a reduction in constipation. It is claimed that fiber helps to absorb water, which increases the bulk of the stool and speeds up the movement of stool through the bowel. However, the scientific evidence is contradictory (60, 61). Some studies show that increasing fiber intake can relieve symptoms of constipation, but other studies show that eliminating fiber can relieve constipation. The effect depends on the type of fiber.

In a study of 63 subjects with chronic constipation, a low-fiber diet was able to solve the problem. The subjects who continued to eat a high-fiber diet experienced no improvement (62). In general, fibers that increase the water content in the stool have a laxative effect, while fibers that contribute to the dry matter of the stool without increasing the water content of the stool may have a constipation-promoting effect.

Soluble fibers that form a gel in the digestive tract and are not fermented by the intestinal bacteria are often effective, a good example being psyllium (63). Other types of fiber, such as sorbitol, have a laxative effect by drawing water into the intestines. Prunes are a good source of sorbitol (64, 65). Choosing the right fiber might help with constipation, but using the wrong supplement can have just the opposite effect. For this reason, you should consult a doctor before using fiber supplements for constipation.

Summary: The laxative effect of fiber varies from type to type of fiber. Some dietary fibers relieve constipation, while others can make it worse. This appears to depend on the individual and the type of fiber.

Fiber could reduce the risk of colon cancer

Colon cancer is the third leading cause of death worldwide (66). Many studies have linked a high-fiber diet to a reduced risk of colorectal cancer (67). However, high-fiber foods such as fruits, vegetables and whole grains also contain numerous other healthy nutrients and antioxidants that could influence cancer risk. For this reason, it is difficult to separate the effects of fiber from other factors of a healthy diet consisting of whole foods. At this time, there is no strong evidence that dietary fiber has a protective effect against cancer (68). However, because fiber can help keep the intestinal wall healthy, many scientists believe that fiber plays an important role (69).

Summary: Studies have linked high fiber intake to a reduced risk of colorectal cancer, but correlation does not equate to causation. To date, no study has proven the direct benefits of fiber in preventing cancer.

Recommended fiber intake

The recommended daily fiber intake is 38 grams for men and 25 grams for women. However, most people consume just half this amount, which corresponds to around 15 to 17 grams per day (1, 2).

Conclusion

Fiber has numerous health benefits. Fiber that is soluble, viscous and fermentable appears to be by far the healthiest, but resistant starch is also surprisingly healthy. This dietary fiber serves as food for the intestinal bacteria and forms short-chain fatty acids that nourish the intestinal wall. In addition to this, viscous soluble fiber can reduce appetite, lower cholesterol levels and reduce the rise in blood sugar levels after a high-carb meal. It's probably not necessary to worry too much about the details at this point. As long as you eat plenty of whole plant foods, getting enough fiber should be a no-brainer. Good sources of healthy fiber include vegetables, fruits, oats, legumes, nuts, dark chocolate, avocados, chia seeds and numerous other foods. Therefore, when aiming for good health, make sure to eat a wide variety of fruits, vegetables and whole grains.