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

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Chapter | 23 Vitamins
FOOD
RELEASED COFACTORS
VITAMINS
Digestive Enzymes
NUCLEOSIDASES
PEPTIDASES
PHOSPHATASES
Vitamin B 12
A
Receptor-Mediated Transport
B
C
E
Active Transport
D
Facilitated Transport
Pericellular Transport Passive Transport
High Luminal Concentrations
Low to Physiological
Luminal Concentrations
FIGURE 23-1 Vitamin absorption. Vitamins in foods are often present as cofactors or in highly modified forms. Pancreatic and intestinal cell-derived
enzymes are required to initiate normal uptake in absorption. Nucleosidases, phosphatases, and peptidases are key factors in processing cofactors to
vitamins. Transport of given vitamins can be receptor mediated and occur via pericellular-related processes, passive transport (usually at high luminal
concentrations), active transport (requires energy), or facilitated processes (requiring a transporter or chaperone).
be essential in the diet of animals. McCollum and Davis
at Wisconsin confirmed that butter or egg yolk, but not
lard, supplied a lipid soluble factor that was necessary
for growth in rats. As a consequence, the first fat-soluble
substance with growth promoting properties (designated
as vitamin A) was reported in the early 1900s, a time that
most considered the beginning of the “ age ” of vitamin
exploration ( Goldblith and Joslyn, 1964 ).
Now there is constant awareness and sensitivity to the
possibility of dietary vitamin deficiencies (and excesses).
Nutritional deficiencies are not uncommon in animals, particularly
animals fed diets of a limited (or restricted) number
of dietary ingredients. A number of subsidiary and contributory
factors may also lead to vitamin-related diseases. These
factors include interference with normal food intake, loss
of appetite (anorexia), impaired absorption or utilization,
increased excretion, and the presence of antagonists. Stressful
physiological states that increase nutrient demands (e.g.,
lactation) may also perturb the vitamin status of animals.
II . DEFINITION, GENERAL PROPERTIES,
AND OVERVIEW OF FUNCTIONS
No definition for vitamins is totally satisfactory. Vitamins
have been defined as organic substances present in minute
amounts in natural foodstuffs that are essential to normal
metabolism, the lack of which causes deficiency diseases.
This definition, however, is not specific and can apply
to a number of compounds derived from the secondary
metabolism of amino acids, simple sugars, and fatty acids.
Suffice to say that in most mammals they represent essential
organic compounds, not easily classified with the macronutrients.
Some may be synthesized, but in insufficient
amounts to meet normal needs during critical developmental
periods.
Vitamins can be further classified according to chemical
and physical properties, such as whether they are soluble
in aqueous solution or lipid solvents. Those vitamins that
are soluble in lipid solvents (vitamins A, D, E, and K) are
absorbed and transported by conventional lipid transport
processes. For water-soluble vitamins, respective solubility
coefficients are major factors that dictate the availability
and ease of absorption. Within physiological ranges of
intake, active processes are usually involved in the absorption
of water-soluble vitamins. Although for some, at high
concentrations (10 times or more the typical requirements),
passive processes may also be involved. In this regard,
the diversity and complexity of vitamin metabolism and
processing should be appreciated at the onset. Vitamins
in foods are often present as cofactors or in highly modified
forms. Pancreatic and intestinal cell-derived enzymes
are required to initiate normal uptake in absorption.
Nucleosidases, phosphatases, and peptidases are key factors
in processing cofactors to vitamins ( Fig. 23-1 ).
Vitamins serve a broad range of functions. For example,
some of the actions of vitamin A and vitamin D are
consistent with the actions of steroid hormones; derivatives
of vitamin A and also vitamin E can act as signal transduction
mediators; vitamin K acts principally as an enzymatic