Types of Dietary Fiber

Dietary fibers are usually divided, at least conceptually, into two major categories: soluble and insoluble fiber.

While all plant foods contain both types of fiber, albeit with variable percentages, some foods are particularly good sources of one or the other type, according to many different criteria such as nutrient density, synergies with other nutritious compounds and cost considerations.

Soluble and insoluble fiber can be recognized macroscopically with a bit of common sense and knowledge: for example, the skin of an apple is a typical insoluble fiber, while the grainy texture of the pulp contains almost only soluble fiber.

Insoluble Fiber

In general, insoluble fiber can absorb water and increase the bulk (and softness) of your stool, reducing transit time through the intestinal tract.

Insoluble fiber tends to be hard, difficult to chew, not very tasty and silky. Green leaves tend to be made of insoluble fiber, as well as filamentous vegetables such as the outer layers of fennel.

Typical sources of insoluble fiber are bran, tomato peel, celery, flax seed lignans and whole wheat shells.

Soluble Fiber

Soluble fiber usually resists digestion and absorption in the small intestine, and is fermented by bacteria in the large intestine (colon),usually via lactic fermentation or alcoholic fermentation, and its by-products have a variety of different properties and health effects (the vast majority being very beneficial).

Depending on the biochemical pathways of the colic bacteria there are different byproducts, but they generally consist of gases and short-chain fatty acids (in particular butyric, ethanoic (acetic), propionic, and valeric acids). These acids are then generally used by the colic mucosa (especially butyrate, which is used extensively as an energy source by colonocytes), transported to the liver (especially propionic acid) or used by peripheral tissues (especially acetate).

Short chain fatty acids have been studied extensively in several medical trials, and they appear to have several beneficial health effects, including:

• Improvement of the digestive mucosa, reducing inflammation and immune functions
• Improvement of glucose regulation, by altering gene expression of glucose transportins
• Reduction of cholesterolemia, in particular concentrations of LDL (bad cholesterol) and triglyceridemia
• Upregulation of immune functions
• Promotion of higher acidity levels in the colon, which generally reduces the number of infections and improves absorption of minerals through increased activity of chelating enzymes
• Upregulation of insulin secretion, resulting in lower glicemia and controlled sugar levels

Soluble fiber tends to be grainy, sweet and easily chewed, like the pulp of most sugary fruits (such as plums, apples, pears and bananas) or the inside of legumes (such as beans and peas).

Typical sources of soluble fiber are peas, beans, apples and carrots.

With these general guidelines, you should be able to deduce at a glance if a specific food contains more soluble or insoluble fiber (and which parts of the same food, in particular).