Insufficient fiber intake has been postulated as a cause
for constipation and supplementation of fiber as a cure
for over 120 years (3). Fiber is defined as plant wall
constituents that resist digestion in the human small
intestine. Cellulose, hemicellulose, pectins, fructooligosaccharides,
and resistant starches are the major
carbohydrate components. Lignin is the main non-carbohydrate
constituent of fiber. Chemical analysis of foodstuffs
introduced the concept of “crude fiber;” residue
left after extraction with boiling sulfuric acid, sodium
hydroxide, water, alcohol and ether. Since this processing
is significantly more capable of degrading foodstuffs
than the secretions of the gastrointestinal tract, the grams
of crude fiber substantially underestimates total dietary
fiber and leads to confusion when fiber intake is quantitated.
Fiber experts also distinguish “soluble” and “insoluble”
fiber, which have different abilities to form gels
and be fermented, but which have not been distinguished
in most epidemiological studies (6). Each food contains
a different spectrum of substances categorized as fiber in
addition to any differences in the total amount of fiber
present. Thus, fiber sources are not necessarily interchangeable
in terms of potential biological effects.