Page 125 - Small Animal Clinical Nutrition 5th Edition

126 Small Animal Clinical Nutrition

mal’s vitamin requirement for a given lifestage, overcome pro-
Table 6-4. Examples of vitamin-vitamin interactions.* cessing and storage losses and avoid toxicity. Because commer-
VetBooks.ir One vitamin needed for optimal absorption of another cial pet foods are fortified with vitamins, it is usually unneces-
Vitamin B for vitamin B
6
12
sary, and perhaps contraindicated, to concurrently give multi-
Folate for thiamin
A high level of one vitamin may interfere with absorption purpose vitamin-mineral supplements. Supplementation may
or metabolism of another be warranted in the management of diseases that affect vitamin
Vitamin E interferes with vitamin K
Vitamin B interferes with niacin metabolism, but should be monitored if long-term treatment is
6
Thiamin interferes with riboflavin planned.
One vitamin needed for metabolism of another
Riboflavin needed for vitamin B and niacin Fat-Soluble Vitamins
6
Vitamin B needed for niacin
6
One vitamin protects against excess catabolism or urinary Vitamin A
losses of another Vitamin A is a general term describing a group of compounds
Vitamin C spares vitamin B 6
One vitamin protects against oxidative destruction of with the biologic activity of retinol. Carotenoids are provitamin
another A. There are about 600 carotenoids known in nature, but only
Vitamin E spares vitamin A about 10% (e.g., α-carotene, β-carotene and β-cryptoxanthin)
Vitamin C spares vitamin E
A high level of one vitamin can obscure the diagnosis of are provitamin A (Yeum and Russell, 2002). The international
deficiency of another unit (IU) is still used by the pet food industry to measure the
Folate deficiency obscures vitamin B 12 deficiency biologic activity of vitamin A. One IU of vitamin A activity can
*Adapted from Machlin LJ, Langseth L. Vitamin-vitamin interac-
tions. In: Bodwell LE, Erdman JW Jr, eds. Nutrient Interactions. be provided by 0.3 µg of all-trans-retinol. In people, 12 µg β-
New York, NY: Marcel Dekker Inc, 1988; 287-306. carotene, 24 µg α-carotene or 24 µg β-cryptoxanthin is equal to
one retinol activity equivalent (1 µg retinol) (DRI, 2001). Dogs,
but not cats, can use provitamin A as a source of vitamin A.The
retinol equivalent of β-carotene in dogs has not been defined.
The interactions between vitamins may involve the process- Most of the preformed vitamin A in food is in the form of
es of absorption, metabolism, catabolism and excretion. Some retinyl esters, whereas the source of vitamin A from plants is in
vitamins may spare the requirements of others, whereas others the form of provitamin A carotenoids.
may have potentially adverse effects. Even the marginal defi-
ciency of one vitamin can exacerbate a deficiency or increase the FUNCTION
requirement of another vitamin. Some examples of vitamin- Vitamin A is essential for a number of distinct biologic func-
vitamin interactions appear in Table 6-4. tions. It is necessary for normal vision, growth, reproduction,
immune function and maintenance of healthy epithelial tissue.
Availability Vitamin A is also involved in the expression and regulation of
Estimating the vitamin content of foods and foodstuffs and many genes (McClintick et al, 2005).
ultimately the adequacy of a given food is difficult at best
because of cumulative errors made in estimating vitamin con- METABOLISM
tent and availability.These errors include: 1) analytical errors in Retinyl esters in food are hydrolyzed by hydrolases from the
sampling and determination of the vitamin, 2) variation in the pancreas and the mucosal brush border to yield retinol. Retinyl
actual amount of the vitamin (e.g., lot-to-lot variation, season- esters and carotenoids are hydrophobic, thus their dispersion
al effects, demographics, different cultivars), 3) the presence of into the aqueous environment of the small intestinal lumen
vitamins in bound forms in many foodstuffs and foods, 4) stor- requires bile salts for micellar solubilization.This process allows
age losses and 5) processing losses. All of these factors make it access of hydrolytic enzymes to retinyl esters and exposes
difficult to define what vitamin level is optimal for a given retinol to the mucosal surface, allowing free retinol and intact
lifestage and food. To account for potential errors, references β-carotene to diffuse passively into mucosal epithelial cells.
recommend that analytical values in databases be discounted by Absorption of vitamin A esters appears to be high (80 to 90%),
10 to 25% (Combs, 1998). but absorption may vary depending on the level and type of
dietary fat and protein, which exert surfactant effects (Combs,
Supplementation 1998).
Nearly all commercial pet foods contain added vitamins. Dietary carotenoids are only absorbed half as well as pre-
Formulating a ration to meet vitamin requirements entirely formed dietary vitamin A. As the amount of carotenoids in the
from ingredient sources is extremely difficult and poses risks for food increases, however, the absorption efficiency decreases.
the animal. The body uses synthetic and naturally formed vita- Intestinal absorption of carotenoids is much more critically
mins in the same way, although they may have different avail- dependent on the presence of bile salts than is absorption of
abilities. The effects of processing on vitamin stability and the vitamin A. Cats can absorb β-carotene but are unable to con-
availability in conjunction with disputed requirement levels in vert it to retinol (Schweigert el al, 2002).
complex foods make fortification necessary (Chapter 8). In the body, enzymes convert provitamin A carotenoids to
Commercial pet foods, therefore, are fortified to meet an ani- retinols. β,β-carotene 15,15’-monooxygenase is a central cleav-