Fiber

Fiber is factually a part of the carbohydrate group. There are two basic types of fiber (found in plants):

• Soluble:

  It is composed of components of plant cells that swell in water (= it is hygroscopic). In the large intestine, they are broken down into simpler components by bacterial action (via fermentation processes). The human digestive tract can digest the soluble fractions of fiber. It is a common component of fruits, vegetables, grains, nuts, and legumes, always in varying proportions. The primary effect of soluble fiber is to absorb excess water from the intestines and increase stool volume. Another important property is its influence on cholesterol metabolism in the blood. In recent years, specific types of soluble forms of fiber, referred to as prebiotics (e.g., fructooligosaccharides – FOS, galactooligosaccharides – GOS, or inulin), which have been shown to have various beneficial health effects, have come to the forefront of interest.

• Insoluble:

  Most represented in grains, vegetables, legumes, and mushrooms. It consists of various components of cell walls and lignins, which bind water but do not swell. The human digestive system cannot break it down (unlike ruminants), so it passes through the digestive tract undigested. Regular intake of this form of fiber supports intestinal function (improves intestinal peristalsis) and the excretion of metabolic residues, including various undesirable substances ingested in the regular diet (the so-called mechanical processes of detoxification).

For fiber in general, the following practically important information applies:

1. Both types of fiber (soluble and insoluble) are practically present in all plants, always in different representations and mutual proportions. For example, in fruit, soluble types predominate, whereas in vegetables, insoluble types prevail.
2. Fiber is extremely important for the proper functioning of the digestive tract – it increases the volume of the digesta and improves its passage through the intestines. It thus determines the states of constipation or diarrhea. In practice, it is therefore necessary to consume enough foods with fiber content, which is very deficient in the regular diet of most people.
3. Fiber affects the glycemic index values of foods (reduces them). It thus plays an important role in preventing the development of insulin resistance and Type 2 diabetes.
4. Fiber has almost no nutritional value. Because its absorption depends on the level of activity of the gut bacteria, its energy value has been determined only as average (less than 2 kcal per gram).
5. The average daily intake of fiber for an adult should range between 25-35 g, but for most people in western civilizations, it reaches half of those values (in the Czech Republic, it is reported as 11-15 g).
6. Fiber is part of many different products intended for weight reduction – it fills the stomach and reduces appetite due to absorbed water. At the same time, it prolongs food digestion and thus induces a feeling of satiety.
7. Excessive fiber intake can cause bloating, flatulence, and abdominal pain.
8. High fiber intake can reduce the effectiveness of certain medications including oral contraceptives.
9. Some foods high in fiber – for example, flaxseeds – contain plant estrogens, which are recommended for reducing the risk of breast cancer in women.