The various farm animals use different
routes or combination of routes to obtain
blood glucose. Even within a species, the
route of obtaining glucose may vary with
physiological state or husbandry system.
Poultry, growing swine and newborn
ruminants typically will use primarily
route 1. Functional ruminants will use
route 2. Horses and grazing, mature swine
will use a combination of routes 1 and 2.
Carnivores will use primarily route 3.
Different routes of obtaining glucose
are also related to different types of dietary
carbohydrates. Some carbohydrates are
present in the diet in an absorbable form,
but most require at least some digestion for
conversion to an absorbable form. Some
carbohydrates can be digested only with
microbial assistance. Carbohydrates of this
sort are of minimal nutritional value to
animals that rely on route 1 for obtaining
blood glucose.
Classification of Dietary
Carbohydrate
There are four major categories in classifica-
tion of carbohydrates by dietary form (see
Van Soest, 1994): free; intracellular; cell
wall carbohydrates; and chitin (Table 6.1).
From the standpoint of the plant or animal
forming the food, free and intracellular
carbohydrates can be considered together as
non-structural carbohydrates. Cell wall
carbohydrates would only be found in plant
tissue and often are referred to as structural
carbohydrates. Free carbohydrates are those
carbohydrates that are not associated with
the cellular structure of the food. They are
in an absorbable form or require minimal
digestion for absorption. Generally these
are minor components of the diet, but there
are important exceptions. Lactose in milk is
included in this category and is the sole
dietary carbohydrate for many newborn
animals. Trehalose is a non-reducing
disaccharide of glucose found in the
haemolymph of insects and is a readily
available form of dietary carbohydrate of
insect-eating animals. Fructose in honey is
another example in a natural food.
Intracellular carbohydrate is divided
into two subcategories: soluble and storage
polysaccharide. Soluble intracellular
carbohydrate includes those carbohydrates
that are soluble in the cytoplasm of cells.
Most common feed ingredients contain
<5% soluble sugars (Webster and Hoover,
1998), but some ingredients contain much
more. Citrus pulp contains 300 g kg^1 ,
and soybean meal and hays harvested in an
early vegetative stage contain 100 g kg^1
of the dry matter (DM) as free sugars. Maize
plants harvested for silage are rich in
soluble sugars but these sugars serve as
substrates for fermentation and very few
remain after ensiling. Care must be taken in
interpretation of these data because some
analyses may include fructans in the
soluble sugars.
Storage polysaccharides are also
divided into two major subcategories:
starches and fructans. Starches include122 R.W. Russell and S.A. Gahr
Table 6.1.Classification of carbohydrate by dietary
form.I. Free – not associated with the cellular structure
of food
A. Lactose – milk
B. Fructose – honey
C. Trehalose – haemolymph
II. Intracellular – inside the cell
A. Soluble – dissolved in the cytosol of cell
B. Storage polysaccharide
a. Starches- Amylose, 1–4 glucose polymer
- Amylopectin 1–4 and 1–6 glucose
polymer - Glycogen, 1–4 and 1–6 glucose
polymer
b. Fructans - Levans, 2–6 fructose polymer
- Inulins, 2–1 fructose polymer
III. Cell wall
A. Cellulose, 1–4 glucose polymer
B. Hemicellulose, 1–4 xylose polymer
C. Pectin, 1–4 galacturonic acid
D. Gums, 1–4 and 1–3 polymers of various
sugars
E. Lignin, phenylpropenoid polymers (not
carbohydrate)
IV. Chitin, 1–4 N-acetylglucosamine polymer
A. Exoskeleton
B. Cell wall