Page 122 - Small Animal Clinical Nutrition 5th Edition

Minerals and Vitamins 123

have been recorded since about 1150 B.C. (Combs, 1998). and in chemical structure, vitamins are absorbed in the body
A vitamin can be defined by its physical and physiologic
VetBooks.ir characteristics. For a substance to be classified as a vitamin, it through a variety of means. Fat-soluble vitamins require bile
salts and fat to form micelles for absorption.They are then pas-
sively absorbed, usually in the duodenum and ileum, and trans-
must have five basic characteristics: 1) it must be an organic
compound different from fat, protein and carbohydrate, 2) it ported in conjunction with chylomicrons to the liver via the
must be a component of the diet, 3) it must be essential in lymphatic system. In contrast, most of the water-soluble vita-
minute amounts for normal physiologic function, 4) its mins are absorbed via active transport. Some vitamins (e.g.,
absence must cause a deficiency syndrome and 5) it must not cobalamin) require a carrier protein called “intrinsic factor”
be synthesized in quantities sufficient to support normal phys- whereas others need a sodium-dependent, carrier-mediated
iologic function. absorption pump.
These definitions are important to note because not all vita-
mins are essential for every species. For example, vitamin C is Deficiency/Adequacy/Toxicity
essential for primates, guinea pigs and some species of fish, but Similar to other essential trace or micronutrients, differences in
not for most other animal species. Lack of the enzyme L- ingestion levels of vitamins create deficiency, adequacy or toxi-
gulonolactone oxidase prevents those species from synthesizing city.The biologic dose-response curve (Figure 5-2) is appropri-
vitamin C from glucose, thereby, making vitamin C a required ate for vitamins (Box 5-5). A deficiency is a lack of the vitamin
vitamin. Under certain conditions of disease or increased in quantities required for normal physiologic function. In gen-
metabolism, however, vitamin C may be “conditionally essen- eral, fat-soluble vitamins are stored in the lipid depots of all tis-
tial” in those species that have de novo synthesis. sues, making them more resistant to deficiency, but also more
Two other terms warrant definition: vitamers and provita- likely to cause toxicity. Conversely, water-soluble vitamins are
mins. A vitamer is chemically the same compound as a vitamin, depleted at a faster rate because of limited storage; therefore,
but may exert varying physiologic effects because it is an isomer. they are less likely to cause toxicity and more likely to be acute-
Vitamin E is a good example of vitamers, because of its many ly deficient.
forms. α-tocopherol is the most biologically active form, Within the range of adequate intake, requirements are met
whereas γ-tocopherol has little biologic function, but acts as an for each lifestage and tissue stores are maximized. Consuming
in vitro antioxidant. A provitamin is a compound that requires more vitamins than is required to maximize stores can, in many
an activation step before it becomes biologically active. β- cases, lead to clinical signs of toxicity if the ingestion period is
carotene, for example, is cleaved by enzymatic processes to prolonged and the body cannot excrete excesses. It is, therefore,
release two molecules of retinol (vitamin A). prudent to provide vitamins in the appropriate balance for each
The two main categories of vitamins are distinguished by lifestage to meet requirements and build tissue stores, but not
their miscibility in either lipids (fat soluble) or water (water to over-supplement in the pharmaco-toxicologic range.
soluble). There are four fat-soluble vitamins (A, D, E and K)
and nine generally recognized water-soluble vitamins (thi- Factors Affecting Requirements
amin [B ], riboflavin [B ], niacin, pyridoxine [B ], pan- Different lifestages of animals affect vitamin requirements.
1
6
2
tothenic acid, folic acid, cobalamin [B ], biotin and vitamin Growing and reproducing animals accrete tissues and therefore
12
C). Though not a true vitamin in the classic sense, choline is require higher levels of vitamins, minerals, protein and energy
generally added to commercial dog and cat foods and treated for optimal performance. Over-supplementation, however, is
as a vitamin in this chapter. The AAFCO dog food nutrient still contraindicated because these animals are also more sus-
profiles list three fat-soluble and eight water-soluble vitamins ceptible to toxicity. As animals age, metabolic and physiologic
including choline (vitamin K, biotin and vitamin C are not changes may increase the requirement for some vitamins.
listed). The AAFCO cat food nutrient profiles list four fat- Various disease conditions also affect vitamin status.
soluble and nine water-soluble vitamins including choline Prolonged anorexia deprives animals of vitamins and other
(vitamin C is not listed) (2007). There are also a number of nutrients and depletes vitamin stores. Polyuric diseases such as
compounds that are classified as “vitamin-like compounds” diabetes mellitus and kidney disease may increase excretion of
or “quasi-vitamins,” which will also be discussed later in this water-soluble vitamins. Kidney disease can also lead to a sec-
section. ondary vitamin D deficiency by reducing the final hydroxyla-
tion step converting 25-OH-D to 1,25-(OH) -D , which
3
2
3
Function occurs in the proximal tubules of the kidneys.
Vitamins have incredibly diverse physiologic functions. In addition, certain drugs (e.g., antibiotics) may decrease the
Vitamins act as potentiators or cofactors in enzymatic reactions intestinal microflora responsible for vitamin K synthesis. Also,
(Figure 6-4). They also play a significant role in DNA synthe- diuretic therapy may increase excretion of water-soluble vita-
sis, energy release from nutrient substrates, bone development, mins. Some vitamin requirements depend on other nutrient
calcium homeostasis, normal eye function, cell membrane levels. The amount of cobalamin required is related to the
integrity, blood clotting, free radical scavenging, amino acid and amount of folic acid, choline and methionine present because
protein metabolism and nerve impulse transduction (Table 6-3). these nutrients interact metabolically and are dependent on
Because of the differences between fat and water solubility each other. In addition, the amount of tryptophan influences