Page 126 - Small Animal Clinical Nutrition 5th Edition

Minerals and Vitamins 127

age enzyme that cleaves β,β-carotene into two molecules of during gestation and lactation. Unlike dogs, cats cannot meet
VetBooks.ir retinol at the 15,15’ double bond. The gene encoding for the their vitamin A requirement from carotenoids.
enzyme has been cloned from several species including people,
DEFICIENCY AND TOXICITY
mice and chicks (Wyss, 2004). This central cleavage enzyme is
found in the intestinal mucosa, liver, lungs, kidney, testes and The appreciable stores of vitamin A in the body are mobi-
brain. Recently, a second type of cleavage enzyme, β,β-carotene lized as needed to mitigate against the effects of low dietary
9’,10’-dioxygenase, was identified and cloned in mice (Kiefer et intakes of the vitamin.The only unequivocal signs of vitamin A
al, 2001). This enzyme exclusively catalyzes the asymmetric deficiency are the ocular lesions nyctalopia (night blindness)
oxidative cleavage of β-carotene at the 9’,10’ double bond, and xerophthalmia (extreme dryness of the conjunctiva). Other
resulting in the formation of β-apo-10’-carotenal and β- signs include anorexia, weight loss, ataxia, skin lesions,
ionone. increased susceptibility to infection, retinal degeneration, poor
Vitamin A is absorbed almost exclusively as the free alcohol coat, weakness, increased cerebrospinal fluid pressure, nephritis,
retinol. Within mucosal cells, retinol is re-esterified mostly to skeletal defects (periosteal overgrowth and narrowing of foram-
palmitate and incorporated into the chylomicrons of the ina) and impaired reproduction (NRC, 2006).
mucosa. Afterwards, it diffuses into lymph. A small amount of Vitamin A toxicities have been encountered in numerous
retinol may be oxidized first to retinal and then to retinoic acid, species.The most characteristic signs of hypervitaminosis A are
which may form a compound (glucuronide) that passes into the skeletal malformation, spontaneous fractures and internal hem-
portal blood. Vitamin A is transported through the lymphatic orrhage (Case 6-5). Other signs include anorexia, slow growth,
system with low-density lipoprotein (LDL) to the liver, where weight loss, skin thickening, suppressed keratinization,
it is deposited mainly in hepatocytes and stellate and parenchy- increased blood clotting time, reduced erythrocyte count,
mal cells. enteritis, congenital abnormalities, conjunctivitis, fatty infiltra-
Some vitamin A derivatives are re-excreted into the intestin- tion of the liver and reduced function of liver and kidneys.
al lumen via the bile. This is true for much of retinoic acid and Queens fed a diet with 606,000 RE/kg food had an increased
some retinol.The major vitamin A components of bile are vita- number of kittens born with defects such as cleft palate, cran-
min A glucuronides, many of which are reabsorbed. Thus, ioschisis, foreshortened mandible, stenotic colon, enlarged
enterohepatic circulation may provide an important means of heart and agenesis of the spinal cord and small intestine
conserving vitamin A. Although dogs and cats excrete vitamin (Freytag et al, 2003). Dogs seem less sensitive to excess dietary
A in urine, cats excrete a lesser amount. vitamin A than some other mammals (Cline et al, 1997).
When vitamin A is mobilized from the liver, stored vitamin The dietary maximum of vitamin A in the AAFCO (2007)
A ester is hydrolyzed before it is released into the bloodstream. dog and cat nutrient profiles is 250,000 IU/kg DM for dogs
Vitamin A retinol is transported to tissues in the bloodstream and 750,000 IU/kg DM for cats. NRC (2006) proposed a safe
by a specific transport protein called retinol-binding protein upper limit of 15,000 RE (50,000 IU)/kg DM for growing
(RBP). RBP is synthesized and secreted by hepatic parenchy- puppies and gestating and lactating bitches, and 64,000 RE
mal cells. (213,333 IU)/kg DM for adult dogs. The safe upper limit of
In contrast to most other species, dogs and cats have a unique vitamin A for cats is 80,000 RE (266,667 IU)/kg DM for
way of metabolizing vitamin A.Cats require preformed vitamin growth and 100,000 RE (333,333 IU)/kg DM for mainte-
A because they lack the oxygenase enzyme necessary for β- nance, gestation and lactation.
carotene cleavage. In addition, studies have shown that cats and
dogs do not depend on RBP to transport vitamin A in plasma SOURCES
(Schweigert, 1988; Wilson et al, 1987; Schweigert et al, 1990; Naturally rich sources of vitamin A include fish oil, liver, egg
Schweigert et al, 1990a). Cats and dogs transport vitamin A as and dairy products.The most common vitamin A supplements
retinyl esters (mostly retinyl stearate) bound to LDL and very used in pet foods include vitamin A esters (all trans retinyl
low-density lipoprotein in amounts 10 to 50 times those of palmitate, acetate or propionate) or vitamin A provided as fish
other mammals (Schweigert, 1988). This is of interest because oils. Because of stability issues, vitamin A sources are often
free circulating retinyl esters are a sign of hypervitaminosis A in coated, beaded, prilled or spray dried with antioxidants and
almost all other animal species, including people. emulsifying agents.
Concentrations of carotenoids in plants vary widely accord-
REQUIREMENTS ing to geographic location, maturity, method of harvest,
The AAFCO (2007) recommended allowance for vitamin A amount and type of processing, length and conditions of stor-
is 5,000 IU/kg DM for dogs for all lifestages (growth, repro- age and exposure to high temperature, sunlight and air. As a
duction and maintenance) and 9,000 IU/kg DM for cats for result, vitamin A is among the most variable nutrients in the
growth and reproduction and 5,000 IU/kg DM for mainte- diet. The vitamin A content in animal tissues can also be vari-
nance. NRC (2006) recommends a vitamin A allowance of able; concentrations can be very high in certain tissues such as
1,515 RE (retinal equivalent) (5,050 IU)/kg DM for dogs for liver. Levels of vitamin A in animal tissue vary depending on
all lifestages, 1,000 RE (3,333 IU)/kg DM for cats for growth either the level of vitamin A or carotenoid present in the dog’s
and maintenance and 2,000 RE (6,667 IU)/kg DM for cats or cat’s diet.