Fibre is the term that describes remnants of plant cells that are resistant to human digestion. The usual sources are vegetables, cereals, bread, nuts, seeds and fruits. Although medical researchers have been recommending high fibre diets for about twenty years, and sales of metamucil and other bulk laxatives have gone up, there has been no significant increase in fibre consumption from food and the fibre intake of Americans is far below recommended levels. This is unfortunate, because the fibre found in food is far more complex than the purified powders sold in drug stores.
There are many different chemical types of fibre, but the most important distinction is between soluble and insoluble fibre. Soluble fibre dissolves in water, forming a thick gel. Fruit pectin, for example, is a highly solube fibre. Psyllium seed, the commonest source of bulk laxatives, contains fibre that is moderately solube. Wheat bran consists of relatively insoluble fibre that is most readily evident as “roughage”. Although all fibre adds bulk to bowel movements, the chemical effects of the different types of fibre can be opposite.
Soluble fibre feeds the intestinal bacteria, which ferment it to produce chemicals called short chain fatty acids (SCFAs). SCFAs have a number of positive effects on the body: they nourish the cells of the large intestine, stimulating healing and reducing the development of cancer. When absorbed from the intestine, they travel to the liver and decrease the liver’s production of cholesterol, lowering blood cholesterol levels. Oat bran, for example, contains fibres of moderate solubility; eating oat bran can lower cholesterol levels. Within the intestinal canal, SCFAs inhibit the growth of yeasts and disease-causing bacteria. The effects of soluble fibre are not always beneficial, however. Feeding high levels of soluble fibre supplements like guar gum encourages an overgrowth of the normal intestinal bacteria which deprives the body of vitamin B12 and produces an increase in the concentration of bacterial toxins. Excessive consumption of soluble fibre from supplements can may create changes in the intestinal milieu that actually enhance the development of stomach or bowel cancer.
Insoluble fibre does not feed bacteria well and is not readily fermented to SCFAs. Eating wheat bran, which is largely insoluble fibre, has no effect on blood cholesterol levels. Insoluble fibre inactivates intestinal toxins, however, and high intake of insoluble fibre is associated with a decreased risk of colon and breast cancer. Supplements of insoluble fibre as wheat bran or pure cellulose appear to decrease the risk of bowel cancer. Insoluble fibres also inhibit the ability of disease-causing bacteria and parasites to attach themselves to the intestinal wall. Insoluble fibre plays an important role in preventing excess intestinal permeability.
It should be obvious that humans need a mixture of soluble and insoluble fibres in the diet and that food, not supplements, is the best source. Eating high fibre foods protects against the development of the major degenerative diseases of the modern world–heart disease and cancer–increases longevity and protects against the development of parasitic infection. The best sources of mixed fibres are unrefined cereal grains (oats, brown rice, whole wheat), peas, beans and squash. Among fruits, one gets the most fibre per serving from apples and berries.
If you become constipated when increasing dietary fibre, you may need more fluid. Drink eight glasses of liquid a day, between meals, not with meals.