Insoluble fiber is the kind of fiber that we can see. It is the "sawdust-like"
material found covering grains. Wheat bran for example, is nearly all insoluble
fiber. The tough stringy material in leafy vegetables, especially celery, is
another example. Generally, insoluble fiber is composed of cellulose, a
carbohydrate, that is the main structural molecule in plants. Wood gets its
strength from long strands of cellulose that are "glued" together by chemicals
known as lignins. Paper is made by purifying from wood chips all the cellulose
while getting rid of the lignins. When people say that bran tastes "like
cardboard", in fact it almost is.
Be that as it may, insoluble fiber is very important to human health and to
weight control. Insoluble fiber improves bowel regularity and decreases the
time that stool is insisde the colon before elimination which appears to reduce
risk of cancer of the colon. Because insoluble fiber has "bulk" without calories,
it can mechanically fill the stomach for "free" so that a person's desire to stop
eating kicks in sooner. Finally, insoluble fiber delays entry of sugar into the
blood stream after eating so that fiber lowers the "glycemic index" of foods.
Foods that are high in fiber are generally "whole" plant foods; grains that retain
their outer husk, whole fruits (and NOT fruit juices) and whole (but necessarily
raw) vegetables. Interestingly, shellfish and insect carapaces also are
considered fiber, in this case a fiber called "chitin", but generally Americans
don't eat a lot of insects and like to peel their shrimp.