Soluble vs. Insoluble fiber.
Water-soluble fiber is viscous and composed mainly of plant pectin,
gum, and mucilage. The first two are parts of the cell structure of
plant walls and naturally coat seeds; they have gelling properties when
mixed with water. Mucilage is a gelatinous glycoprotein produced by
plants. Insoluble fiber is composed of cellulose and its less
crystalline counterpart hemicellulose. Because it does not dissolve in
water, this latter type of fiber bulks up as it absorbs water. Soluble
fiber, on the other hand, becomes gelatinous and viscous. It too bulks
up, but its primary role in bodily health is to bind to the bile acids
in the small intestine. Soluble fiber takes these acids with it as it
passes through the body, which in turn lowers LDL cholesterol and blood
glucose levels by slowing glucose absorption in the small intestine.
Researchers are still discovering the precise mechanism at work here,
but when people with hyperlipidemia were given a medication that binds
to and removes bile acids, their bad cholesterol improved and their
blood glucose level lowered; soluble fiber has the same binding ability.
Insoluble fiber mainly eases the passage of foods through the
digestive system and cleanses intestinal walls, whereas soluble fiber
promotes actual changes in intestinal pH. Also, because soluble fiber
ferments, it stimulates the growth of "friendly" bacteria in
the gut. It is therefore known as pre-biotic. Insoluble fiber is by
contrast inert. Both types increase food volume without increasing
caloric content, providing satiety, and so they are widely associated
with weight management.