Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

III. Fat-Soluble Vitamins
697
TABLE 23-1 Requirements for Selected Water Soluble Vitamins Expressed as
mg/1000kcal or 4.2Mjoules a
Vitamin
Animal
Cat Rat Mouse Chick Human
Thiamin 2–3 2 2 1 1–2
Riboflavin 1–2 1 1 0.5 1
Niacin b 20–30 8 8 6–8 5
Pyridoxine b 2–4 2 2 1–2 1
Folate 3–4 0.5 0.5 0.5 0.3
a
Taken from the National Research Council publications dealing with requirements for animals or humans. Values were obtained by dividing the
recommended safe and adequate intake by the recommended energy intake.
b
Cats do not effectively convert tryptophan to niacin; thus, there is absolute need for niacin. In this regard, 10 mg of niacin is produced per
4.2MJ of typical diets containing high-quality protein when utilized by the rat, mouse, chick, or human. The higher pyridoxine need in the cat
is due to the higher protein requirements of carnivores and higher concentrations of enzymes dedicated to nitrogen metabolism. If expressed on a
unit protein basis rather than energy basis, the pyridoxine requirements of most homeothermic animals are similar.
cofactor; vitamin E can act as an agent that scavenges freeradical
containing lipids and oxidants, independent of a
direct association with an enzyme, although recent information
indicates possible roles in cell signaling. Regarding
the water-soluble vitamins, most serve as cofactors or cosubstrates
for enzymes or in cell signaling.
These varied functions of vitamins have also complicated
the development of a simple system of classification
or nomenclature. When the vitamins were originally discovered,
they were isolated as fractions from selected foods,
and as their exact chemical composition was seldom known,
a system of letter designations was developed. However, this
system became complicated when it was discovered that
some functions originally ascribed to vitamins were due to
other substances, such as one of the essential amino acids.
Consequently, the designation of vitamins by letters was not
systematically pursued. Similarly, the lack of chemical composition
data resulted in a complex system of expressing
dosages as arbitrarily defined units. Regarding requirements,
when expressed on an energy basis, vitamin requirements
are often of the same order from one species to the next.
Some examples are given in Table 23-1 . Differences in
dietary requirements between species for given vitamins
(in contrast to physiological or metabolic requirements) are
usually due to presence of unique pathways for their production,
degradation, or disposal. Good examples are ascorbic
acid and niacin, which cannot be made in some animals
and therefore are true vitamins for such animals. Taurine is
another example of a nutrient (although not a true vitamin
as classically defined) where continual disposal or loss from
the body results in a nutritional need, even though taurine
can be synthesized. Further, young and growing animals
may have a relatively higher nutritional need for some nutrients.
Many species during neonatal periods have requirements
for certain compounds, which later in life may be
sufficiently produced (e.g., choline, carnitine, or inositol).
There are also numerous possibilities for deleterious interactions
that can have physiological consequences and
affect given requirements ( Committee on Animal Nutrition,
2001a, 2001b ; McDonell, 2001 ; Rucker and Steinberg,
2002 ; Subcommittee on Laboratory Animal Nutrition,
Board on Agriculture, National Research Council, 1995).
III . FAT-SOLUBLE VITAMINS
A . Vitamin A
1 . Introduction
Observations by Hopkins, Stepp, and others that a growthstimulating
factor could be extracted from milk by means
of lipid solvents, concentrated, and tested in experimental
animal models were seminal steps that eventually led
to the identification of vitamin A ( Goldblith and Joslyn,
1964 ). This growth-promoting factor was also described as
being present in egg yolk, butter, and cod liver oil. In later
studies, “ Factor A ” or vitamin A was shown to be present
as lipid esters in animal tissues and in the form of a “ provitamin
A ” in plants (e.g., compounds in the carotenoid
family). The structures and recommended names of naturally
occurring and commercial forms of vitamin A and
carotenoids are shown in Figure 23-2 . Once chemical features
for the carotenoids and retinoids were resolved in the
1940s and 1950s, studies of their biological function were
undertaken and commercial synthesis of vitamin A and
vitamin A-like molecules proceeded rapidly.
2 . Proforms of Vitamin A: The Carotenoids
Carotenoids comprise a group of more than 700 compounds
(most often red, yellow, and orange pigments in