There is no unambiguous definition of food fibers. Before, we used to say that food fibers are all polysaccharides (carbohydrates) in the diet that are not digested by gastrointestinal enzymes, hence do not fuel energy. Recent research has indicated that this is not entirely true. About two thirds of the fibers absorbed in the large intestines are fermented, thereby releasing energy. The average nutritional value of food fibers is 8 kjoule (2 kcal) per 100 gram.
Fibers can be divided into two groups: those that are water-soluble (pectin, guargom, …) and those that are not water-soluble (cellulose, hemicellulose, lignin, …).
The softer parts of the bodies of plants are made up mostly of cellulose. It is the cellulose that enables a plant to bend and have flexibility. Whereas starch provides the food storage for plants, cellulose gives structural support, comparable to the armoring in concrete. As cellulose is hard to break down in nature, it is one of the most common organic compounds on earth. If we think of cellulose fibers as ‘armoring’, then hemicellulose and pectin are the gelatinous substance between the armoring.
Food fiber helps normalize the human gastrointestinal transit time, or the rate at which food moves through the digestive tract. We need a certain amount of fibers in our food for a healthy digestive system. According to research, populations that eat greater amounts of fiber-rich foods are generally less prone to certain diseases of the intestines (constipation, intestinal disorders, and so on). Intestinal diseases are more prevalent in affluent western countries with diets that are low in fibers. Similarly to fats, fibers slow down the emptying of the stomach. The more fiber-full you eat, the more full you will feel.
Food fibers are an important source of fuel for gut flora, which gradually break down cellulose. Food fibers also tend to retain fluid and soften stool. The volume of stool becomes bigger, the stool itself softer.
Food fibers are only present in vegetables and plants: unrefined starch-rich products (cereals), potatoes, vegetables and fruit. A fiber-rich diet consists of whole grain bread, brown rice, potatoes, fresh vegetables (pulse crops) and two pieces of fruit per day.