Dietary fiber is divided in two, as soluble and insoluble fiber, which have different functions in the body, and according to Türkiye Beslenme Rehberi (TÜBER), Türkiye's national dietary guideline, foods contain a mixture of both. What the two share is this: although fiber is in the carbohydrate class it is not turned into energy in the body, and it passes through the digestive system without undergoing a basic change and leaves unused. That is why, even though fiber is counted inside the daily carbohydrate target, it makes no contribution to the energy calculation and is followed with its own gram target.
What does soluble fiber do?
Soluble fiber is the type that dissolves in water, and it is found in fruits such as apple, grapefruit, lemon and orange, in oats, in dried legumes and in many vegetables. In foods it is present in a sticky or viscous form, as in oat bran; gums, mucilage and pectin are in this group.
Its function has to do with the speed of the stomach and the bowel. According to the guideline soluble fiber delays gastric emptying, prevents blood sugar levels from rising fast by slowing the digestion of carbohydrates, and provides control of cholesterol and triglyceride levels.
What does insoluble fiber do?
The fiber types that do not dissolve in water are cellulose, hemicellulose and lignin; these take place in the structure of plant cell walls and make up most of all dietary fiber.
Its function has to do with volume and movement. Insoluble fiber holds water in its structure, increases the movement of waste matter inside the bowel, gives the stool softness and volume so that the bowels work regularly, and shortens the transit time.
How much fiber does each food carry?
The guideline ranks the food groups by their natural fiber content. Dried legumes come first with 11-26 percent; they are followed by nuts with 5-14 percent, grain products with 4-7.5 percent, vegetables with 3-4 percent and fruits with 1-2 percent.
The state of the same food makes a difference too: raw ones contain more dietary fiber than cooked ones, and ones with their skin more than ones without. So two portions of the same number of grams can carry different fiber according to the way they are prepared. In grains, as the processes that separate the bran increase, the fiber content falls to a large extent.
The daily target and the limit
In adults daily fiber intake is recommended to be at least 25 grams; the reference values table gives this value as the adequate intake for women and men over 18.
On the limit side there is a warning. While high fiber diets lower energy intake, they can also lower the absorption of minerals such as iron, calcium and magnesium. So raising the two fiber types has an upper point as well, and that point is drawn by mineral absorption. The reference values table separates the same target by age group too: 19 grams at 11-14, 21 grams at 15-17, and 25 grams at 18 and over.
With dried legumes there is a second practical point. Because their fiber content is high and they are hard to digest, soaking them in cold water the night before and cooking them well prevents gas and digestive trouble. The guideline adds that consuming the same foods together with different groups raises the protein quality of the meal as well.
Fiber's effect on the meal
Even though the two types do separate jobs, some of their effects are shared. According to the guideline the presence of fiber in the diet delays gastric emptying, lowers the desire to eat, and by raising viscosity in the small intestine lowers the absorption of simple carbohydrates.
Another shared function has to do with the gut flora. Foods with fiber contain prebiotics, the nutrients needed by the bacteria that are useful for the bowel, that is by the probiotics; the guideline counts this as important for the health of the gut flora.
These effects also explain why the daily target is given in grams. Because fiber is not turned into energy it does not enter the calorie calculation, but because it changes how long the meal keeps you full and the speed of absorption, it stands with its own line inside the macro table.
Where does it turn into a mistake?
The first mistake is putting the two types in place of each other; insoluble fiber comes forward for bowel regularity and soluble fiber for the speed of blood sugar, and the two are concentrated in different foods. The second mistake is looking for fiber only in grain; in the content ranking dried legumes stand clearly above grain products. Nuts also stand above grain products, but because their portions are small their contribution to the daily total stays limited.
The third mistake is putting fruit juice in place of fruit. On the guideline's list for raising fiber, fruit is recommended instead of fruit juice, whole grain, oat or rye bread instead of white bread, and bulgur instead of rice or pasta; the skin of fruits that can be eaten with their skin is not peeled. You can follow the detail of the daily target from the fiber guide, and its place inside carbohydrate from the simple and complex carbohydrate article.