Page 137 - Small Animal Clinical Nutrition 5th Edition

138 Small Animal Clinical Nutrition

addition, choline is required in the body in substantially greater Vitamin B 12 and folate are required for the synthesis of
VetBooks.ir amounts (>1,000 mg/kg) than the other B vitamins (<100 methyl groups and metabolism of the one-carbon unit.
Biosynthesis of labile methyl from a formate carbon requires
mg/kg). Furthermore, choline does not function as a coenzyme
plays a role in regulated transfer of
folate, whereas vitamin B
or cofactor as do most other B vitamins.
12
Choline, 2-hydroxy-N, N, N-trimethylethanaminium, has the methyl group to tetrahydrofolic acid. Therefore, a deficien-
three methyl groups that enable choline to serve as a methyl cy of one or both of these vitamins increases the requirement
donor in the body.It is an integral component of lecithin (phos- for choline.
phatidylcholine). Choline is a strong base and decomposes in Excess dietary protein and/or high-fat foods increase the
alkaline solution. choline requirement. In most species, the choline requirement
is greater for younger animals than for adults. Some adult
FUNCTION species may not require choline.
Choline plays several important roles in the body. It is an The AAFCO (2007) recommended allowance for choline is
integrated component of phosphatidylcholine, a structural ele- 1,200 mg/kg DM for dogs and 2,400 mg/kg DM for cats for
ment of biologic membranes. Phosphatidylcholine also pro- all lifestages. The NRC (2006) recommended allowance for
motes lipid transport. Diminished synthesis of phosphatidyl- choline is 1,700 mg/kg DM for dogs and 2,550 mg/kg DM for
choline in the liver due to choline deficiency results in accumu- cats regardless of lifestage. Table 6-5 lists AAFCO and NRC
lation of lipids in the liver. Choline, as acetylcholine, is a neu- allowances for dogs and cats.
rotransmitter. Choline, as a component of platelet-activating
factor (1-O-alkyl-2acetyl-sm-glycero-3-phophocholine), is DEFICIENCY AND TOXICITY
important in clotting and inflammation. After oxidation to Choline deficiency in most animal species is characterized by
betaine, choline is a source of labile methyl groups for trans- depressed growth, hepatic steatosis and hemorrhagic renal
methylation reactions (e.g., the formation of methionine from degeneration (Combs, 1998). Additional signs of choline defi-
homocysteine and creatine from guanidoacetic acid). ciency in dogs include thymic atrophy and elevated plasma phos-
phatase values and increased blood prothrombin time. Table 6-5
METABOLISM lists normal plasma levels of choline for cats and dogs (Baker et
Choline is present in food predominantly as phosphatidyl- al, 1986). Choline and phosphatidylcholine levels in blood may
choline; less than 10% is present as either the free base or sphin- be measured to confirm deficiency suggested by clinical signs.
gomyelin (Combs,1998).Choline is released from phosphatidyl- Studies with dogs suggested a low tolerance to lecithin
choline and sphingomyelin by digestive enzymes and absorbed (phosphatidylcholine). Reduced erythrocytes resulted from
from the jejunum and ileum mainly by a carrier-mediated daily oral administration of lecithin (equivalent to 150 mg of
process. Intestinal microorganisms metabolize most free choline choline) (NRC, 1987). However, neither AAFCO (2007) nor
ingested to trimethylamine, which is absorbed and excreted in NRC (2006) has recommended a maximum or safe upper limit
urine. Phosphatidylcholine is not subject to such extensive for dietary choline for dogs and cats.
microbial metabolism; therefore, metabolism of phosphatidyl-
choline results in less urinary trimethylamine. Once absorbed, SOURCES
choline is transported in the lymphatic circulation primarily in All natural fats contain some choline; therefore, choline is
the form of phosphatidylcholine bound to chylomicrons. widely distributed in foods and foodstuffs. Lecithin has also
Most species can synthesize choline, as phosphatidylcholine, been shown to be an effective emulsifying agent in foods and is
by the sequential methylation of phosphatidylethanolamine. the form of choline ingested in most foods. Egg yolks, glandu-
The activity is greatest in the liver, but is also found in many lar meals and fish are the richest animal sources and cereal
other tissues. germs, legumes and oilseed meals are the best plant sources.
Choline is added to most pet foods as choline chloride and is
REQUIREMENTS added separately from the vitamin premix because of its hygro-
The requirement for choline is affected greatly by dietary fac- scopic nature and propensity to reduce the stability of other
tors such as methionine, betaine, myoinositol, folate and vita- vitamins if added in the premix.
min B , as well as the combination of different levels and
12
composition of fat, carbohydrate and protein in the diet. In Vitamin C
addition, age, gender, caloric intake and growth rate influence Because of de novo synthesis, vitamin C is not technically a
the lipotrophic action of choline and thereby its requirement. vitamin for healthy dogs and cats. (See vitamin definition.)
Choline and methionine are the two principal methyl donors However, it is included here because of its biochemical func-
in transmethylation.Therefore, dietary adequacy of methionine tions, including in vivo and in vitro antioxidant properties
and choline directly affects the requirement of the other. (Chapter 7).
Methionine can completely replace choline as a methyl donor.
For example, in cat foods, if dietary methionine exceeds 0.62% FUNCTION
DM, 3.75 parts of methionine can be substituted for 1 part Vitamin C, ascorbic acid or specifically L-ascorbic acid, is a
choline (AAFCO, 2007). very labile compound that is readily oxidized to dehy-