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Beynen AC, 2021. Choline in dog food

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Abstract

Choline in dog food In 1933, some 70 years after the identification of choline (Note 1), Best and co-workers asserted that addition of choline to the diet prevented and cured accumulation of fat in the liver of insulin-treated dogs with experimentally-induced diabetes. The observation indicates that dietary choline can affect lipid metabolism, but is no proof that choline is an essential nutrient for dogs (Note 2). In fact, it is still uncertain whether choline is indispensable in the canine diet (cf. Note 2). For the mammalian body, choline is vital since it is a component of phosphatidylcholine and acetylcholine. Phosphatidylcholine, or lecithin, is a phospholipid and a major constituent of cell membranes and lung surfactant. Acetylcholine is a neurotransmitter: it is released by nerve cells to send signals to other cells, including brain, muscle and gland cells. In relation to dog nutrition, choline is traditionally included with the B vitamins (1). Choline is seen as a conditionally required nutrient. Dogs can synthesize choline, but the rate is thought to fall short under some, but undefined conditions (1). The recommended allowance of choline for both puppies and adult dogs has been set (1) at 1,524 mg per kg dry food (= 15 MJ metabolizable energy). Only selected petfood ingredients are high in choline. To secure choline supply, compared to the recommended allowance, most commercial dog foods are supplemented with choline chloride as is shown by their ingredient lists. The recommended choline allowance for dogs has a weak basis. The allowance is inferred from three, small-scale studies with growing dogs. The studies were carried out in the early 1940s. The purified diets used in those studies possibly did not facilitate optimum bodily choline synthesis, thus inflating the dietary choline requirement. In two experiments it was quite clear that addition of choline to the diet did not respond to the dogs' growth potential, pointing to some kind of nutritional inadequacy. More than half of 282 home-made diets for dogs had a calculated choline content that was considerably lower than the recommended choline allowance. A recipe for a home-made ration to raise puppies and maintain working dogs also had a much lower calculated choline content than the recommended allowance (Note 3). The recipe, which was published in 1942, had been tested and was selected as giving the best results. There are no indications that choline supply within its range of amounts in commercial and home-made dog foods have the potential to cause health problems (but see Note 4). On the other hand, the ideal level of choline supplementation is uncertain, including its necessity. Research that revisits the requirement of dietary choline in dogs would be of both practical and scientific use. Function and synthesis Choline (2-hydroxy-ethyl trimethyl ammonium, C5H14NO +) is absolutely necessary for mammalian cells. The dog acquires choline from its diet and via de-novo biosynthesis. Both exogenous and endogenous choline ends up in phosphatidyl choline (PC), which accounts for almost the entire body-choline pool. PC is a major lipid component of cell membranes. Some cellular choline occurs in phospholipids other than PC, in free form and in acetylcholine.
Bonny Canteen 2021; 2: 129-140.
Anton C. Beynen
Choline in dog food
In 1933, some 70 years after the identification of choline (Note 1), Best and co-workers asserted
that addition of choline to the diet prevented and cured accumulation of fat in the liver of insulin-
treated dogs with experimentally-induced diabetes. The observation indicates that dietary choline
can affect lipid metabolism, but is no proof that choline is an essential nutrient for dogs (Note 2).
In fact, it is still uncertain whether choline is indispensable in the canine diet (cf. Note 2).
For the mammalian body, choline is vital since it is a component of phosphatidylcholine and
acetylcholine. Phosphatidylcholine, or lecithin, is a phospholipid and a major constituent of cell
membranes and lung surfactant. Acetylcholine is a neurotransmitter: it is released by nerve cells to
send signals to other cells, including brain, muscle and gland cells.
In relation to dog nutrition, choline is traditionally included with the B vitamins (1). Choline is seen
as a conditionally required nutrient. Dogs can synthesize choline, but the rate is thought to fall
short under some, but undefined conditions (1).
The recommended allowance of choline for both puppies and adult dogs has been set (1) at 1,524
mg per kg dry food (= 15 MJ metabolizable energy). Only selected petfood ingredients are high in
choline. To secure choline supply, compared to the recommended allowance, most commercial dog
foods are supplemented with choline chloride as is shown by their ingredient lists.
The recommended choline allowance for dogs has a weak basis. The allowance is inferred from
three, small-scale studies with growing dogs. The studies were carried out in the early 1940s. The
purified diets used in those studies possibly did not facilitate optimum bodily choline synthesis,
thus inflating the dietary choline requirement. In two experiments it was quite clear that addition
of choline to the diet did not respond to the dogsgrowth potential, pointing to some kind of
nutritional inadequacy.
More than half of 282 home-made diets for dogs had a calculated choline content that was
considerably lower than the recommended choline allowance. A recipe for a home-made ration to
raise puppies and maintain working dogs also had a much lower calculated choline content than
the recommended allowance (Note 3). The recipe, which was published in 1942, had been tested
and was selected as giving the best results.
There are no indications that choline supply within its range of amounts in commercial and home-
made dog foods have the potential to cause health problems (but see Note 4). On the other hand,
the ideal level of choline supplementation is uncertain, including its necessity. Research that
revisits the requirement of dietary choline in dogs would be of both practical and scientific use.
Function and synthesis
Choline (2-hydroxy-ethyl trimethyl ammonium, C5H14NO+) is absolutely necessary for mammalian
cells. The dog acquires choline from its diet and via de-novo biosynthesis. Both exogenous and
endogenous choline ends up in phosphatidyl choline (PC), which accounts for almost the entire
body-choline pool. PC is a major lipid component of cell membranes. Some cellular choline occurs in
phospholipids other than PC, in free form and in acetylcholine.
Choline is synthesized by the liver through methylation of the ethanolamine moiety of phosphatidyl
ethanolamine (PE). The reaction yields PC and is catalysed by phosphatidyl ethanolamine N-
methyltransferase (PEMT). Choline can be released from PC via the action of phospholipases. The
PEMT and phospholipase reactions form the pathway for choline biosynthesis in animals. Another
metabolic route of PC production uses free choline, either of metabolic or dietary origin (Note 5).
In dogs fed a choline-deficient, high-fat, low-protein diet for 14 days, compared with a commercial
chow or the choline-deficient diet with daily choline supplement, phospholipid synthesis in liver was
markedly increased (2). Total tissue phospholipid synthesis was determined by incorporation of
intravenously injected, radioactive phosphate (P32). It is not possible to link increased phospholipid
synthesis to increased choline synthesis (Note 6, 7).
Theoretical considerations
The PEMT reaction, or de-novo choline synthesis, utilizes methyl groups from S-adenosyl methionine
(AdoMet). Availability of AdoMet is determined by both the dietary supply of methionine and its
enzymatic recovery from homocysteine, which requires vitamin B12 and folic acid for the methyl
transfers (Note 8, 9). It follows that inadequate intake of either methionine, folic acid or vitamin B12
impairs de-novo choline biosynthesis, causing choline deficiency when dietary choline is insufficient.
The condition of diminished de-novo synthesis due to insufficient dietary methionine, folic acid
and/or vitamin B12, raises the dietary requirement of choline. It is noteworthy that a similar
statement was made in 1985 by the US National Research Council (3) when reconfirming the choline
requirement of dogs that was set in 1974 (Note 10). In fact, the statement exposed an important
caveat of the proposed choline requirement.
Allowance
In 1974, the US National Research Council advanced the choline requirement of dogs (4, Note 11).
The requirement was reconfirmed in the 1985 (3, Note 12) and 2006 (1, Note 13) editions of the
Council’s reports.
The 2006 edition of the report (1) has put the recommended allowance of choline at 102 mg
choline/MJ metabolizable energy or 1,700 mg/kg dietary dry matter (ddm) containing 4000 kcal
(16.736 MJ). In addition, an intake level of 81 mg choline/MJ has been defined as adequate (1, Note
14). The amounts set as allowance and adequate intake hold for all life stages: growing puppies after
weaning, adult dogs during maintenance and for bitches in late gestation and peak lactation (1).
The choline allowance is based on three small-scale studies (5-7, Note 11). In those studies,
published in 1941, 1943 and 1944, growing dogs were fed on choline-free diets (Note 15). In two
studies (5, 6), the diets were likely deficient in folic acid and vitamin B12, this qualification remaining
undecided for the third study (Note 16). Thus, choline biosynthesis by the growing dogs was possibly
impaired (see under Theoretical considerations). In that case, the choline requirement, as based on
the three studies, would be inflated.
There are additional indications that the proposed adequate choline intake (1, Note 13) is an
overestimated standard. It is 50% higher than the calculated choline content of an efficacy-proven
recipe of a home-made, maintenance ration for dogs (see under Home-made dog foods). Likewise,
the standard is not met by many recipes of other home-made dog diets. A theoretical approach to
choline turnover in dogs points to excessiveness of the proposed adequate intake (Note 17).
Simplified recipes
Poultry (74% moisture) and (white) rice (10% moisture) may contain 3,028 and 2.3 mg total
choline/kg dm (8). Thus, a mixture composed of 30% poultry, 50% rice and 20% choline-free
ingredients would hold 910 mg choline/kg ddm, which is 33% lower than proposed adequate intake
of 81 mg choline/MJ, or 1,360 mg/kg ddm (1). After replacement of about 4.5% rice by dry matter
from chicken egg (75% moisture) or liver (70% moisture) the mixture meets the allowance. The two
high-choline foods have 10,040 and 9,668 mg total choline/kg dm, the PC component providing 95
and 74% of total choline (8).
The choline content of the abstract poultry-and-rice recipe could be lower when using poultry by-
product meal as petfood ingredient instead of poultry as human foodstuff. Dry, rendered poultry
meal with 12% crude fat may contain 18,480 mg PC/kg dm (9), equaling 2,587 mg choline/kg dm (PC
contains 14% choline). Thus, its use in the recipe decreases the mixture’s choline content to 777 mg
choline/kg ddm, which is 43 % lower than the choline standard.
Commercial dog foods
It is possible to produce dry dog foods that meet the recommended allowance of choline without
the use of choline supplements. Such formulas require relatively high levels of specific chicken
products, namely whole egg, egg yolk or liver powder. Krill meal will do also (see below). Perhaps
there are less expensive, applicable high-choline ingredients. Nevertheless, most complete, dry dog
foods contain added choline-chloride preparations. This practice may be prompted by cost control
and/or by pursuing certainty about meeting the choline standard.
By using brochures of 10 dog-food brands, the amounts of added choline, expressed as mg
choline/MJ metabolizable energy, were calculated for complete puppy and adult foods. The
calculations were based on the declared, added amounts of choline chloride and the analysis panels.
For 10 puppy and 10 adult foods, the means/ranges were 80/29-135 and 75/32-119 mg/MJ. The
proposed adequate intake and recommended allowance are 81 and 102 mg choline/MJ (1).
Home-made dog foods
The so-called Auburn I ration is a home-made diet that was especially recommended for
reproduction and the raising of pups (10). The calculated choline content of the efficacy-proven
recipe is 86 mg choline/MJ (Note 3). Thus, the Auburn I ration, which was published in 1942,
contained 6% more choline than the proposed adequate intake and 16% less than the recommended
allowance.
The Auburn Ration II, which is simpler and less expensive than Ration I, was recommended for
raising pups and maintaining working dogs (10). The diet comprised 58% yellow corn meal, 20%
wheat shorts, 20% meat scrap, 1% salt and 1% sardine oil. The calculated content of choline in
Ration II is 54 mg/MJ (Note 3), which is 33 and 47% lower than the proposed adequate intake and
recommended allowance.
In two studies, the amount of choline in recipes of home-made, maintenance dog foods was
calculated on the basis of food tables. For 200 (11) and 82 (12) recipes, the median choline contents
were 61 and 52 mg/MJ. Thus, 50% of all recipes had a choline content that was more than 25%
below the proposed adequate intake. For 21 home-made diets for dogs with cancer, the calculated
median choline content was 100 mg/MJ (13), likely reflecting the relatively high usage of choline-
containing vitamin supplements.
Choline and fatty liver
In growing dogs fed a diet that is deficient in choline and does not adequately support choline
biosynthesis (7, 14, Note 16), fat may accumulate in the liver. Shortage of dietary choline and
insufficient choline synthesis impair PC formation, which in turn diminishes hepatic synthesis and
secretion of very-low density lipoproteins. These lipoproteins, which require PC as structural
component, act as vehicles for triglycerides. Reduced synthesis of the lipoproteins causes hepatic
lipidosis in the form of triglycerides.
Blood choline
The PC and lyso-PC in krill meal represent about 17,000 mg choline/kg (15). Isonitrogenous
substitution of krill meal for fish and chicken meal in dry food raised plasma choline concentration in
dogs from about 8 to 13 μmol/l (16).
Note 1
In 1862, Strecker identified choline in ox bile. The Greek word for bile is chole. Bilineurine is a
synonym or trivial name for choline (17).
Note 2
In 1933, Best et al. (18) wrote that choline prevented or cured fatty liver in depancreatized, diabetic
dogs treated with insulin. The protective effect of choline had been shown earlier in rats fed a high-
fat diet consisting of mixed grain and 40% beef fat (19). The small-scale dog study (18) was not
controlled and the authors used indirect reasoning. The depancreatized dogs were daily fed
approximately 300 g lean beef and 100 g sucrose with small amounts of a wheat-germ preparation,
orange juice and cod-liver oil.
The lean beef may have contained 82 mg total choline/100 g (8), 74% moisture and 502 kJ
metabolizable energy/100 g. The 100 g sucrose may have provided 1,600 kJ. Thus, the whole diet
supplied about 178 g dry matter (dm) and 3.11 MJ per day, which corresponds with 1,382 mg
choline/kg dm or 79 mg choline/MJ. The unaccounted wheat-germ is high in choline (8). The
proposed adequate choline intake is 81 mg choline/MJ (see under Allowance).
The diet of the depancreatized dogs was not choline deficient, when compared with the proposed
adequate choline intake of normal dogs. On the other hand, dietary choline in the form of
phosphatidyl choline (PC) in lean meat likely was essentially unavailable due to the absence of
pancreatic lipase. Thus, the depancreatized dogs were possibly dependent on choline biosynthesis.
Per 100 g, lean beef may contain 23 g protein and 0.69 g methionine so that the diet contained 0.67
g methionine/MJ, which can be considered abundant for adult dogs (1). The wheat germ and lean
beef might have provided sufficient folic acid and vitamin B12. In that case, choline biosynthesis
would have been unimpeded in normal dogs (see under Theoretical considerations), but might be
depressed in insulin-treated, depancreatized dogs.
More than 10 years after Best et al. (18) published their study, it was reported (7, 14) that feeding
choline reversed fatty liver in young growing, choline-deficient dogs. The dogs were fed choline-free
diets, but it is not known whether or not the diets were deficient in folic acid and/or vitamin B12
(Note 16).
An essential nutrient is required for normal body function and cannot be synthesized in adequate
amounts. Choline is required for normal body function (see under Function and synthesis), including
normal liver function by preventing fat accumulation. However, it remains to be demonstrated that
dietary choline is required for normal body function of dogs when fed a choline-deficient diet that is
otherwise optimal with regard to nutrient composition, particularly methionine, folic acid and
vitamin B12 (cf. Note 16).
It is noteworthy that dogs with experimentally induced exocrine pancreatic insufficiency developed
hepatic lipidosis (20). The dogs were fed on a mixture of commercial canned and dry food, indicating
that the intakes of choline, methionine, folic acid and vitamin B12 were sufficient. However, digestion
of PC and protein were incomplete, which reduced the absorption of choline and peptides, including
methionine.
Note 3
In 1942, Koehn published a report entitled “Practical dog feeding” (10). The Auburn Rations I and II
contained 86 and 54 mg choline/MJ metabolizable energy. The recommended allowance and
proposed adequate intake are 102 and 81 mg choline/MJ (1).
The recommended home-mixed ration for reproduction (Auburn Ration I) consisted of yellow corn
meal (35 lbs.), wheat bran (10 lbs.), wheat shorts (20 lbs.), meat scrap (10 lbs.), fish meal (10 lbs.),
skim-milk powder or dried buttermilk (10 lbs.), alfalfa leaf meal (2 lbs.), bone meal (2 lbs.) and salt (1
lb.).
Auburn Ration I had been fed to 12 brood bitches (fox hounds) as the sole ration for 3.5 years.
During that period, over 500 dogs were raised. The ration was especially recommended for
reproduction and the raising of pups.
The Auburn Ration I contained 23.5% protein, 5.0% fat and 47.2% carbohydrates (10). The calculated
energy density is 1,340 kJ/100 g when assuming that protein, fat and carbohydrates provide 17, 37
and 16 kJ metabolizable energy/g. The following contributions, expressed as mg, to the content of
total choline per kg ration were adopted*: yellow corn meal (350 g/kg)/77, wheat bran (100
g/kg)/74, wheat shorts (200 g/kg)/148, meat scrap (100 g/kg)/237, fish meal (100 g/kg)/440, skim-
milk powder (100 g/kg)/167, alfalfa leaf meal (20 g/kg)/9, bone meal (20 g/kg)/0, salt 10 g/kg/0.
Thus, the choline content of the food mixture is 1,152 mg/kg, or 86 mg/MJ.
*yellow corn meal yellow corn = 22 mg total choline/100 g food (8); wheat bran = 74 mg/100 g
food (8); wheat shorts wheat bran in terms of fat content (10) = 74 mg/100 g food (8); meat scrap
ground dry-rendered residue from animal tissues = 11% fat (10) 15.4% PC (9) = 237 mg
choline/100 g product; fish meal = 440 mg/100 g (21); skim-milk powder = skim milk with dry matter
content increased from 9 to 96% = 167 mg/100 g (8); alfalfa leaf meal 2.5% total fat = 12.6% PC of
total lipids (22) = 44 mg choline/100 g; bone meal (without fat) = 0 mg choline; salt = 0 mg choline.
A simpler, less expensive, nutritionally complete ration was proposed for raising pups and
maintaining working dogs (10). That ration (Auburn Ration II) contained yellow corn meal (58 lbs.),
wheat shorts (20 lbs.), meat scrap (20 lbs.), salt (1 lb.) and sardine oil (1 lb.).
The Auburn Ration II contained 20.0% protein, 6.3% fat and 51.4% carbohydrates (10). The
calculated energy density is 1,396 kJ/100 g. The following contributions, expressed as mg, to the
content of total choline per kg ration were adopted*: yellow corn meal (580 g/kg)/128, wheat shorts
(200 g/kg)/148, meat scrap (200 g/kg)/474, salt (10 g/kg)/0 and sardine oil (10 g/kg)/0. Thus, the
choline content of the food mixture is 750 mg/kg, or 54 mg/MJ.
Note 4
Administration by stomach tube of 8 or 10 mg dissolved choline hydrochloride/kg body weight.day
reduced erythrocyte numbers in dogs (23, 24). It was concluded that choline-induced anemia is
hyperchromic and probably the result of depression of erythropoiesis (24). The oral dose of 6 mg
choline/kg body weight.day corresponds with 400 mg choline/kg ddm, when assuming a food intake
of 15 g dry matter/kg body weight.day (choline chloride contains 75% choline).
For 10 puppy and 10 adult dry foods, the range of supplemental choline (in the form of choline
chloride) was 488-2,250 mg/kg diet, or about 542-2,500 mg/kg ddm. Thus, it is relevant to know
whether the observed choline-induced anemia is reproducible and/or depends on specific
(experimental) conditions.
The same author of the two studies mentioned (23, 24) had shown earlier that oral choline chloride
(8 mg/kg body weight.day) markedly reduced the red-blood cell counts in dogs that were
polycythemic as caused by daily exposure to reduced atmospheric pressure (25).
Note 5
Choline may be derived from the diet as free choline or in the form of phosphatidyl choline (PC),
glycerophosphocholine, phosphocholine or sphingomyelin. Choline may also be obtained from
PEMT-catalysed PC synthesis in the body. However, another metabolic route of PC production uses
free choline.
For the other route, free(d) choline is first phosphorylated by choline kinase and ATP as phosphate
source. Phosphocholine, thus formed, is converted to CDP-choline at the expense of CTP (cytidine
triphosphate), by phosphatidyl cytidyltransferase. The phosphocholine moiety of CDP-choline is then
connected (by electron uptake) with the hydroxyl group at position 3 of a 1,2-diacylglycerol molecule
so that PC and cytidine monophosphate are formed. This pathway of PC is principal for the synthesis
of dipalmitoyl PC in lung, but it also takes place in the liver and other organs.
Note 6
It could be suggested that an increase in phospholipid synthesis in liver, as induced by choline
deficiency, coincides with an accelerated hepatic PEMT reaction, pointing to enhanced de-novo
choline synthesis. Alternatively, it would be acceptable that the synthesis of phospholipids other
than PC was stimulated so as to act as PC replacers. It should be stressed that total tissue
phospholipid synthesis was measured rather than PC and/or choline synthesis. Furthermore, it is
possible that the choline-deficient diet could not support choline biosynthesis due to a methionine
deficiency (Note 7).
Note 7
The experimental dogs were fed 15 g/kg body weight.day of a choline-deficient diet for a 14-days
period (2). The diet contained 8% casein, 38% lard, 44% sucrose, 2% cod-liver oil, 3% brewer’s yeast
and 5% salt mixture. Based on the ingredients, the diet was essentially choline-free. The estimated,
metabolizable energy content of the diet is 1,726 kJ/100 g.
Dietary protein and methionine supply equaled about 4.6 and 0.12 g/MJ. The amount of protein
corresponds with the minimum requirement for adult dogs (1). However, the amount of methionine
was 25% lower than the minimum requirement (1). Possibly, the brewer’s yeast provided sufficient
folic acid and the dog’s liver released vitamin B12 from its stores. Beyond that, dietary intake of
methionine probably was too low to support choline biosynthesis.
The dogs fed the choline-deficient diet were able to upregulate total hepatic phospholipid synthesis
when compared with their counterparts fed a commercial chow or given a daily choline supplement.
The latter dogs received the choline-deficient diet to which 1% choline chloride was added daily,
which was equivalent to 435 mg choline/MJ (choline chloride contains 75% choline).
Note 8
S-adenosyl methionine (AdoMet) is the universal donor of methyl-groups in various biological
reactions (Note 9). PEMT catalyses the transfer of three methyl groups, one at a time, from AdoMet
to PE in order to generate PC. Phospholipase may release the tri-methylated choline from PC (cf.
Note 5) for the synthesis of phospholipids other than PC, acetylcholine or betaine. There may be an
ongoing process of PEMT-catalysed PC synthesis and catabolism, as part of cellular turnover.
The PEMT reaction involves irreversible methyl-group transfer. Apart from PC, the reaction also
yields S-adenosyl homocysteine. The latter is hydrolysed and the freed homocysteine is converted
into methionine by methyl transfer effectuated by methionine synthase, which requires processed
vitamin B12 as co-factor and uses 5-methyltetrahydrofolate as methyl donor. Thus, shortage of either
folic acid or vitamin B12 impairs methionine remethylation within the methionine-homocysteine
cycle.
De-novo choline synthesis depends on the availability of AdoMet, which in turn is determined by
both the dietary supply of methionine and its recovery from homocysteine, which involves vitamin
B12 and folic acid. Thus, if the diet provides an inadequate amount of choline and also is deficient in
either methionine, folic acid or vitamin B12, choline deficiency would develop.
Note 9
Within mammalian cells, DNA methylation involves the transfer of a methyl group onto C-5 of
cytosine. The degree of methylation of cytosines in DNA regulates gene expression by recruiting
proteins involved in gene repression or by inhibiting the binding of transcription factors to DNA (26).
Histones, proteins that condense and structure DNA, may be methylated on their lysine and arginine
residues, causing either activation or repression of transcription (27).
Note 10
Text published by the 1985 issue of the Nutrient Requirements of Dogs:
“Complex interactions occur involving single carbon transfer by choline, methionine, folate, and
vitamin B12. Choline requirements can only be determined when the diet contains a minimal, but
adequate level of methionine and adequate levels of folate and vitamin B12. When previous
experiments were undertaken, the minimal requirement of the dog for methionine was not known,
and isolated sources of vitamin B12 and folic acid were not available. Furthermore, for rats the
dietary requirement of choline is influenced by the lipid content of the diet, the chain length and
degree of saturation of the fatty acids, and the total caloric content of the diet (Best et al., 1954;
Salmon and Newberne, 1962; Zachi et al., 1965; Patek et al., 1966).” The report does not provide the
retrievals of the four references.
Note 11
In 1974, the US National Research Council published a document entitled “Nutrient Requirement of
Dogs” (4). The document was the result of review and revision of the 1972 edition of the report
bearing the same title. The full text under the heading Choline in the revised 1974 report reads as
follows.
“The dietary requirement for choline is markedly affected by the dietary protein concentration and,
more specifically, by the dietary concentration of methionine. Since both choline and methionine
may serve as labile methyl donors in metabolism, the dietary supply of one tends to spare the need
for the other.
Schaefer et al. (1941) pointed out that a number of workers have found that dietary casein
concentrations of 40 percent or more tend to obviate the need for dietary choline. In their own
studies, puppies receiving a 19-percent casein diet became choline deficient. Controls receiving 50
mg of choline per kilogram of body weight per day grew satisfactorily over the 37-day experimental
period.
On a 15-percent casein diet, Fouts (1943) found that 10 or 20 mg of choline per kilogram of body
weight would not prevent or cure the deficiency state in puppies, while a 100-mg level would. When
41 percent casein was provided, no choline deficiency nor any response to supplemental choline
could be shown.
McKibbin et al. (1944) fed a diet containing 10 percent protein from peanut flour plus 10 percent
casein to puppies and concluded that choline requirements were probably not greater than 1000 mg
per kilogram of diet or 50 mg per kilogram of body weight per day.
It is concluded that the choline requirements for adult maintenance may be met by 26 mg per
kilogram of body weight per day and those for growth of puppies by 52 mg per kilogram of body
weight per day. These amounts will be supplied by 1200 mg of choline per kilogram of diet.”
Note 12
Text published by the 1985 issue of the Nutrient Requirements of Dogs:
“It is concluded that the choline requirement for adult maintenance may be met by 25 mg per
kilogram of body weight per day and those for growth of puppies by 50 mg per kilogram of body
weight per day. Diets containing 340 mg choline per 1,000 kcal ME will supply these quantities of
choline when fed to adult or growing dogs.”
Note 13
Text published by the 2006 issue of the Nutrient Requirements of Dogs (and Cats):
“There appears to be no new published information on choline requirements of dogs since the NRC
(1985) that proposed a choline allowance of 340 mg per 1,000 kcal ME. It is recommended that this
dietary concentration be used for all physiological life stages. This dietary concentration of choline
will supply approximately 45 mg.kg BW-0.75.d-1 for an adult dog at maintenance.”
The choline amount of 340 mg/1,000 kcal metabolizable energy (= 340 mg/4.184 MJ = 81 mg/MJ)
corresponds with the proposed adequate intake.
Note 14
The “recommended allowance” is defined as the concentration or amount of a nutrient in a diet
formulated to support a given physiological state. The “adequate intake” is defined as the
concentration in the diet or amount required by the animal of a nutrient that is presumed to sustain
a given life stage when no “minimal requirement” has been demonstrated. The “minimal
requirement” is defined as the minimal concentration or amount of a bioavailable nutrient that will
support a defined physiological state.
Note 15
Schaefer et al. (5) used a diet containing 66% sucrose, 19% casein, 8% cottonseed oil, 3% cod-liver
oil, 4% salt mixture and added thiamine, riboflavin, nicotinic acid, calcium panthothenate and
pyridoxine. Dosing puppies with choline (50 mg/kg body weight.day) supported growth, whereas
their untreated counterparts showed a quick growth plateau followed by anorexia and weight loss.
The authors stressed that the growth rate of the choline-fed puppies was very suboptimal.
Fouts (6) showed that a puppy lost weight rapidly after reduction of the amount of casein in the diet
to 15%. The animal responded definitely to administration of 100 mg choline/kg body weight.day,
but recovery was not complete. The dog’s diet contained 51% sucrose, 15% casein, 28%
hydrogenated cotton-seed oil, 4% salt mixture and 2% cod-liver oil. The following vitamins were fed
by dropper daily: thiamine, riboflavin, pyridoxine, calcium panthothenate, nicotinic acid and vitamins
A and D in the form of fish-liver oil.
McKibbin et al. (7) developed a diet for the production of acute choline deficiency in weanling
puppies, the prerequisite being good growth when supplemented with choline. In one puppy fed the
basal ration with 0.1% choline, growth was qualified as excellent. The diet comprised 59.5% sucrose,
10% casein, 15% extracted peanut flour, 7% cotton-seed oil, 2% cod-liver oil, 4% salts, 0.5% K2HPO4
and 2% liver fraction.
Note 16
The diets in the three studies (5-7, Note 15) had calculated energy contents of 1,786, 2,181 and
1,735 kJ metabolizable energy/100 g. Based on the ingredients, the diets of Schaefer et al. (5) and
Fouts (6) can be considered choline-free. The liver fraction in the diet of McKibbin et al. (7) was
designated as low in choline and the extracted peanut flour was reported to contain 64% protein.
The estimated methionine concentrations of the three diets (5-7) were 0.27, 0.18 and 0.24 g/MJ. For
growing puppies, the minimal requirement and recommended allowance of methionine have been
set at 0.13 and 0.16 g/MJ (1) so that the methionine supply by the three diets would support choline
biosynthesis. The diets used by Schaefer et al. (5) and Fouts (6) likely were deficient in folic acid and
vitamin B12. Liver is a good source of the two vitamins, but whether that holds for the liver fraction
used by McKibbin et al. (7) is unknown.
In conclusion, the three choline-deficient diets provided sufficient methionine. Two diets (5, 6)
probably were deficient in folic acid and vitamin B12. The third diet (7) may or may not have
furnished adequate folic acid and/or vitamin B12.
Note 17
Within the dog’s body, there will be an ongoing process of PC synthesis and catabolism (cf. Note 8).
Synthesis reflects the sum of the PEMT-dependent and -independent pathways. Choline is de-novo
synthesized during the PEMT reaction, while PC is also synthesized with the use of available, free
choline. The diet provides free choline, PC and other phospholipids (cf. Note 5). In the body, free
choline is furnished by phospholipase-mediated hydrolysis of phospholipids.
The turnover rate of plasma PC in a 20-kg dog may amount to 5.5 g/day (9). PC turnover includes
(re)synthetized PC within epithelial cells and hepatic PC secretion. Two arbitrary assumptions are
made: plasma-PC turnover equals whole-body PC turnover and 10% of the choline released by
catabolism cannot be re-used by the anabolic component of turnover.
A turnover rate of 5.5 g plasma PC/day equals 778 mg choline/day (PC contains 14% choline) and
corresponds with choline loss of 78 mg/day. For a 20-kg dog consuming 300 g dry matter per day,
and 100% efficacy of choline absorption, the loss is compensated for by 260 mg choline/kg ddm (78
x 1000/300).
Data on the absorption of ingested choline by dogs were untraceable. Studies on in-situ (28) and
isolated (29) intestinal loops of dogs point to an efficiency of choline absorption of about 33% of
intake. However, apparent absorption of choline in the form of dietary PC may be higher than 95%
of intake.
If the absorption efficiency of total dietary choline is 50%, then the inevitable loss of choline from PC
turnover is replaced by a dietary choline level of 520 mg choline/kg ddm, which is 62 and 69% lower
than the proposed adequate intake (1360 mg/kg ddm) and recommended allowance of choline
(1700 mg/kg ddm).
Note 18
Zeisel et al. (8) reported that “chicken roasted (w/skin” and “chicken roasted (no skin)” contain
65.83 and 78.74 mg total choline/100 g, indicating that subcutaneous adipose tissue does not
contain more choline than the rest of the body. Chicken with skin (62% moisture) or without skin
(74% moisture) may contain 19 and 2 g fat/100 g. Total lipid in chicken thigh contains about 15% PC
(9), or 2% choline.
Note 19
In dogs (n=26) subjected to caloric restriction, group-mean plasma concentration of free choline1
was 69 μmol/l before weight loss and 58 μmol/l after weight loss (30). The weight-reduction diet
contained 206 mg choline/MJ. The dogs that were included in the study were switched from their
habitual diet to the experimental diet and were fed restricted amounts of energy.
1Plasma choline concentrations were about 10 times higher than those reported by reference 16.
Note 20
Intravenously administered choline has been affirmed to protect dogs from endotoxin-induced
multiple organ injury and platelet dysfunctions (31-33).
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