grains and in much smaller amounts in fruits, can inhibit the absorp-
tion of calcium, iron, zinc and copper, and possibly other nutrients.
For those who may have problems getting enough of these minerals,
limiting grains can be helpful.
In addition to their relationship to mineral absorption, some
fibers may have an adverse effect on digestive enzymes. Wheat bran,
for example, can inhibit the production of pancreatic enzymes
responsible for digesting carbohydrates, proteins and fats. The fiber
in legumes may inhibit the enzyme amylase, which is important for
carbohydrate digestion. Other studies show that the fiber in many
cereals contains pancreatic inhibitors that can diminish protein diges-
tion. In addition, the fiber in unprocessed soybeans can induce an
allergy-type reaction in some people, accounting for the intestinal
discomfort some may have with soy.
Energy from Fiber
Since fiber is not absorbed, it does not directly count as an energy
source. For this reason, if you eat a slice of whole-grain bread that
contains 15 grams of carbohydrate, you really can’t count it as 15
grams of usable energy, since some of that carbohydrate is fiber. If
that slice of bread contains four grams of fiber, subtract four grams
from the 15 grams of total carbohydrate, giving a total of 11 grams of
usable carbohydrate.
While the fiber grams are not directly counted as energy calories,
the body does indirectly obtain energy from fiber through fermenta-
tion by bacteria in the large intestine. As mentioned previously, fiber
provides the environment for this bacterial activity. The bacteria pro-
duce short-chain fatty acids, typically butyric, acetic and propionic,
which are absorbed and used by the body as fuel. Some fibers, such
as pectin, are more capable of producing energy than others, such as
fiber from grains. Approximately two calories of energy can be pro-
duced per gram of fiber. This is compared to four calories for other
dietary carbohydrates, four for protein and nine for dietary fats.
Prebiotics — the Other ‘Fiber’
Most of us have heard about the many benefits of dietary fiber, but
certain fiber-like foods called prebiotics can even more dramatically
THE FULL SPECTRUM OF FIBER • 115