Vitamin C can be synthesized by most animal species,
except when they are young or under stress conditions.
Likewise, in most species, niacin can be synthesized from
the amino acid tryptophan and vitamin D from the action of
ultraviolet light on precursor compounds in the skin. Thus,
under certain conditions and for specific species, vitamin C,
niacin and vitamin D do not always fit the classic definition
of a vitamin. Vitamin D nonetheless fits the vitamin definition
for cats and dogs, which lack skin receptors for this vitamin,
as well as for poultry and swine that are raised under
management systems that preclude exposure to ultraviolet
light (McDowell 2000).
The major functions of the B-vitamins have been known
for many years. Now we have a clearer picture of the metab-not limited to blood clotting and bone formation,
respectively. Vitamin C, which is synthesized by
most species, may nonetheless be beneficial when provided
to animals under stress. Animals with a functional rumen
were once thought to synthesize B-vitamins and vitamin K
and therefore to have no need for supplementation.
However, deficiencies do occur under special conditions
and supplementation has proven beneficial in the case of thiamin,
niacin, vitamin B12, choline, biotin and vitamin K.
Similarly, supplemental carnitine has been shown to be beneficial
for several species.
Classical deficiency symptoms and non-specific parameters
(e.g., lower production and reproduction rates) are associated
with vitamin deficiencies or excesses. Vitamin
nutrition should no longer be considered important solely
for preventing deficiency signs, as vitamins can play a role
in optimizing animal health (e.g. immune function), productivity
and product quality. Given the huge scope of this
topic, this review will focus on vitamin requirements, vitamin
occurrence and the history of vitamins. The concept of
optimum vitamin allowances and the supplemental vitamins
most needed by livestock will also be covered.