Introduction

Choline is a trimethylated hydroxide that is found in biological tissues in a free form and as a component of lecithin, acetylcholine, certain plasmalogens, and sphingomyelins, the components of nervous tissue (Figure 1). By strict definition, choline is not a vitamin; however, it is an essential nutrient. Despite the fact that most animals synthesize choline, it must be consumed in the diet because de novo synthesis is inadequate to maintain health. Choline is mainly found as a component of specialized fat molecules known as phospholipids, the most common of which is called phosphatidylcholine or lecithin (Figure 1).

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Figure 1. Chemical structure of choline (left) and phosphophtidyl choline (right). The boxed area indicates the portion of phosphophtidyl choline that is derived from choline. The synthesis of choline potentially consumes 3 methionine as a donor for methyl (CH₃) groups.

Choline is crucial to brain, neuromuscular signaling, and normal nerve transmission. Choline is required for synthesis of phospholipids which are essential components of all membranes and is an important source of labile methyl groups. Choline deficiency in nonruminants is not common except under the most severe circumstances because choline is widely distributed in plant and animal tissues. However, choline deficiency induced experimentally is manifested as fatty liver, hemorrhaging kidneys, elevated blood pressure, and impaired neurological function. In nonruminants, choline deficiency can be avoided by supplying dietary sources of other methyl donors, such as betaine, methionine, and folic acid, in conjunction with adequate vitamin B-12.

One of the earliest signs of choline deficiency in nonruminants is a reduction in lipoprotein assembly and secretion of triglycerides from liver to plasma. Addition of other methyl donors such as methionine serves to prevent the accumulation of liver lipid in rats, perhaps as substrates for choline synthesis. Currently, there is considerable interest in use of choline and related compounds to reduce fatty liver associated with the onset of calving in transition dairy cattle.

Dietary Need for Choline in Dairy Cows

One of the primary roles of choline is in synthesis of phosphatidylcholine, an essential component of cell membranes. In addition to the structural component of cell membranes, phosphatidylcholine is required for the secretion of very low density lipoprotein (VLDL) from liver. It is well established that the rate of VLDL synthesis in ruminants is low compared to other species and that fatty liver associated with calving is not uncommon. Choline-deficient rats show three-fold increases in hepatic triglyceride concentrations and reduced plasma methionine as well as phosphatidylcholine concentrations compared to rats fed a choline adequate diet (Pomfret et al., 1990; Yao and Vance, 1988). Choline status therefore has been suggested as a factor in alleviating the severity and incidence of fatty liver and may have some application in the transition dairy cow.

There is an estimated requirement for gram quantities of choline for normal tissue metabolism and milk production in lactating dairy cattle (Erdman, 1992), yet very little dietary choline escapes ruminal degradation (Dawson et al., 1981). Therefore, choline supply may potentially also limit milk production. Increasing the postruminal supply of choline by infusion of choline into the abomasum has increased milk production and milk fat yield in some (Erdman and Sharma, 1991) but not all experiments. Part of the lack of consistency in response to rumen-protected choline may be due to the supply of other methyl donor sources, including methionine and folic acid.

It is interesting to note that the Food and Nutrition Board of the Institute of Medicine established a dietary reference intake for choline rather than a recommended daily allowance (RDA) because scientific evidence was insufficient to calculate an RDA (Pitkin et al., 2000).The main criterion for establishing an adequate intake level (AI) is for the prevention of liver damage due to insufficiency. The AI for adult men age 19 and over is 550 milligrams (mg)/day and for adult women age 19 and over is 425 mg/day. Likewise, choline requirements have not been established for the lactating cow. The recommended concentration of choline in milk replacer diets is 1000 mg/kg at a feeding rate of 0.53 kg per 45 kg calf or ~ 530 mg/day (NRC, 2001). It is interesting to note that the requirement for choline in the calf and the AI for humans is very similar on a body weight basis. These data confirm that an estimate of the minimum choline needed in dairy cows for maintenance functions (based on metabolic body size) is approximately 4 to 6 g/day.

Relationship between Methionine and Choline

Inadequacy of choline supply is manifested by decreased concentrations of choline, betaine, phosphatidylcholine, methionine, and S-adenosyl ethionine and increased triglyceride concentrations in liver (Pomfret et al., 1990). Deficiencies lead to reductions in circulating lipoproteins as a direct result of impaired secretion by liver (Lombardi et al., 1968). Choline-deficient rats show three-fold increases in hepatic triglyceride concentrations and reduced plasma methionine and phosphatidylcholine concentrations compared to choline-adequate rats (Pomfret et al., 1990; Yao and Vance, 1988).

Choline and methionine metabolism are closely associated, and as much as 28% of absorbed methionine is used for choline synthesis (Emmanuel and Kennelly, 1984). Methionine plays a direct role in VLDL synthesis in bovine (Auboiron et al., 1995) and acts to reduce plasma ketones during early lactation (Durand et al., 1992). Active synthesis of phosphatidylcholine is necessary for VLDL secretion from rat hepatocytes (Yao and Vance, 1988). Thus, supplying choline directly may enhance synthesis of phosphatidylcholine and increase VLDL synthesis or serve to increase methionine availability for lipoprotein synthesis to indirectly alter liver triglyceride clearance as VLDL.

It is well documented that methionine supplemented in the rumen-protected form increases milk protein production (Donkin et al.,1989; Rulquin and Delaby, 1994) and often increases milk fat coincidentally (Rulquin and Verite, 1993), although the latter response is variable. The maximal quantity of amino acids mobilized during early lactation is between 15 and 21 kg of body protein (Botts et al., 1979; Komaragiri and Erdman, 1997), which amounts to approximately 1 kg of lysine and 0.22 kg of methionine over 5 weeks or 28 g of lysine and 6 g of methionine each day. More closely matching the quantity and pattern of amino acids supplied in relation to the animal’s needs (specifically, methionine and lysine) during the transition period may retard the rate of breakdown of labile protein. The potential for choline to spare methionine catabolism may depend on the supply and profile of amino acids absorbed from the small intestine of the dairy cow. Feeding soy protein diets to growing rats leads to lipid accumulation that is reduced by the addition of either choline or methionine (Aoyama et al., 1992) and suggests potential for controlling the severity of fatty liver in transition dairy cows by modulating postruminal amino acid supply.

Methionine, Choline, and Folic Acid

As much as 50% of the methionine required by ruminants must be synthesized through the remethylation of homocysteine to methionine (Figure 2), and this need may be greater during lactation (Xue and Snoswell, 1985ab). It has been estimated that as much as 30% of the methionine absorbed by dairy cows is used for choline synthesis (Erdman, 1992); therefore, a potential also exists to improve amino acid nutrition of the transition cows through changes in choline status. Methionine remethylation requires either betaine or 5-methyl tetrahydrofolate (5-THF) and is dependent on vitamin B-12 (Figure 2). In sheep, the primary transmethylating partner in this reaction is 5-methyltetrahydrofolate, a form of folic acid. The adequacy of folic acid in lactating and transition dairy cows should be questioned. There is a 40% decrease in serum folate observed during the late gestational and immediate prepartum periods (Girard et al., 1989; Girard et al., 1994). The greatest demand for folic acid in dairy cattle appears to be during gestation (Girard and Matte,1995), and serum levels are responsive to dietary supplementation (Girard et al., 1994). Folic acid may also play a role in modulating methionine status in the transition dairy cow. Calculations relative to folic acid use and supply indicate a slight deficit at DM intakes approximating those of the transition dairy cow (Donkin, 1997).

Figure 2. The relationship between methionine, choline, folate, and betaine. The methyl donor (SAM) is synthesized from methionine and is used to transfer a methyl group, in the formation of phosphatidylcholine. Once SAM donates a methyl group, it becomes S-adenosyl homocysteine, which is metabolized to homocysteine. Homocysteine can be converted to methionine in a reaction that requires methyltetrahydrofolate (THF) and vitamin B-12. Alternatively, betaine (a metabolite of choline) may be used as the methyl donor for the conversion of homocysteine to methionine. The primary methyl donor for the regeneration of methionine from homocysteine in ruminants is 5-methyl THF (Xue and Snoswell, 1985ab).

Choline and Carnitine and Fatty Acid Oxidation

Decreased fatty acid oxidation and carnitine in liver have been reported due to choline deficiency (Carter and Frenkel, 1978). Choline serves as a methyl donor in the synthesis of carnitine from methionine and lysine (Griffith, 1987). Carnitine is necessary for the translocation of long-chain acyl moieties across the inner mitochondrial membrane of liver cells. The addition of carnitine to bovine liver slice incubations increased the rate of palmitate oxidation (Drackley et al., 1991), and infusing carnitine into the abomasum of lactating dairy cows numerically decreased (P = 0.11) plasma non-esterified fatty acid concentrations (LaCount et al., 1996). Therefore, choline indirectly may act to reduce the accumulation of liver lipid by providing carnitine to enhance hepatic fatty acid oxidation.

When Should Rumen-Protected Choline Be Fed?

In addition to its role as a methyl donor for choline synthesis, methionine may play a direct role in lipoprotein metabolism. The L-methionine added to milk fed to calves stimulates VLDL synthesis (Auboiron et al., 1995), and feeding the hydroxy analog form of methionine increases circulating lipoproteins and milk fat percentage in lactating dairy cattle. Furthermore, methionine and lysine infusions in lactating dairy cows reduced plasma ketones during the second week of lactation (Durand et al., 1992). Providing choline may act to spare methionine catabolism in transition cows. Dietary choline must be protected from rumen degradation to be effective. The supply of methionine from the diet, or rumen bacterial synthesis, folic acid status, vitamin B-12 status, and potential for fatty liver developments all play a role in determining the effectiveness of choline supplementation in the transition cow.

Early Studies to Evaluate the Potential Benefits of Choline Supplementation

A series of studies performed using rumen-protected forms of choline or duodenal choline infusions indicate an increase in milk production with increased postruminal choline supply (Erdman and Sharma, 1991). Early studies examined the degradation of choline in the rumen and noted almost complete catabolism of methionine in the rumen (Neill et al., 1979). One of the early experiments using unprotected choline chloride indicated as much as an 8 lb increase in fat-corrected milk production when 50 g/day of choline were fed (Erdman et al., 1984). These data are surprising in light of subsequent experiments that indicated the complete degradation of choline chloride using in vitro incubations (Sharma and Erdman, 1989). However, animal differences, differences in basal diets, level of intake, and experimental design may have influenced the outcome of these early trials. More consistent response to choline is observed when supplied postruminally via infusions or the rumen-protected form although the effect(s) are not always consistent or repeatable. A summary of the effects of choline on milk production and composition is presented in Table 1. Choline increased milk yield in four of seven studies when choline was infused abomasally or fed in the rumen-protected form. The maximum response in milk production was 7 lb/day (from 47.3 to 54.3 lb/day) and 8.4 lb day for fat-corrected milk yield (Sharma and Erdman, 1989).

Effects of Rumen-Protected Choline in Transition Cows

Four separate studies that have addressed the potential for using rumen-protected choline to improve health and productivity of transition dairy cows have been reported as either peer-reviewed publications (Hartwell et al., 2000; Hartwell et al., 2001) or in abstract form (Piepenbrink and Overton, 2000; Siciliano-Jones and Putnam, 2000; Vazquez et al., 1999). All studies used Reashure™, a rumen-stable choline manufactured by Balchem Corp. (Slate Hill, NY). At least one treatment for each study included 60 g/day of the product. A summary of the highlights of these data is presented in Table 2.

Milk production was improved with rumen-protected choline feeding in two of the three studies reported. One of the trials (FI and BC) was a field study, and rumen-protected choline increased milk production in one-half of the six herds used in the trial (Putnam, 2001). While these data suggest a benefit to the inclusion of rumen-protected choline, information is not yet complete on the mode of action of choline or feeding conditions and management factors that complement its use. It is noteworthy that the percentage increase in milk production during the first 56 to 60 days of lactation is similar for the Purdue and FARME Institute/Balchem study (106% of control) and is of a similar magnitude of response to the early choline feeding studies (Table 1).

Rumen-protected choline is beneficial for transition cows fed 10% rumen degradable protein (RDP) and 4.0% rumen undegradable protein (RUP) (% of dietary DM), but it decreased milk production in cows fed 10% RDP and 6.2% RUP during the prepartum period (Hartwell et al., 2000). Liver fatty acid oxidation is not altered by rumen-protected choline, although liver triglycerides may be reduced with rumen-protected choline in some instances (Piepenbrink and Overton, 2000). The latter suggests an increase in triglyceride export to reduce fatty liver in transition cows fed rumen-protected choline.

Studies at Purdue University have demonstrated the negative effects of feeding increased protein to transition cows and the carryover effects on feed intake postcalving (Greenfield et al., 2000; Hartwell et al., 2000). It is well established that overconditioning at calving leads to decreased production and reduced postpartum intakes and increased severity of fatty liver (Reid et al., 1986). Rumen-protected choline served to reduce liver lipid in dairy cows when prepartum body condition score was 3.75 or greater and high-protein diets (10% RDP and 6.2% RUP) were fed prepartum (Hartwell et al., 2000). On this basis, targeted supplementation with rumen-protected choline is recommended even for moderately overconditioned cows (3.8 body condition score) during the transition period (-28 to +28 days relative to calving). The response to rumen-protected choline may vary depending on protein sources of the basal diet, energy concentration in the transition diet, yield of microbial protein, methyl donors for the remethylation of methionine, and supply of vitamin B-12 and folic acid.

Summary

Rumen-protected choline holds promise for modulating metabolism in transition cows to reduce incidence and severity of fatty liver at calving (Figure 3). The milk production response to rumen-protected choline is 5 to 7 lb day during the first 56 to 60 days of lactation. The frequency of a significant positive milk production response to rumen-protected choline is observed in 50% of the studies conducted. Metabolic responses to rumen-protected choline have been equivocal. A predictable response to rumen-protected choline feeding may depend on the basal diet, supply of other B vitamins and related factors, and other management factors, including the body condition score of cows entering the transition period.

Figure 3. Proposed mechanism of choline action in lactating dairy cows. Adipose tissue lipolysis results in the release of nonesterified fatty acids (NEFA) into blood. The NEFA extracted by liver are either esterified to triglyclycerides (TG) or partially oxidized to ketones to provide energy (ATP) for liver metabolism. Ketones are released into blood and further oxidized by muscle. Alternatively, liver TG can be stored as droplets (fatty liver) or packaged into very low density lipoproteins (VLDL) and exported into blood. Choline may affect the synthesis of the apolipoprotein components of VLDL to increase TG export from liver or the metabolism of ketones by peripheral tissues. The solid lines indicate locations where choline may act to modify lipid metabolism.

Author Information

Shawn S. Donkin
Department of Animal Sciences
Purdue University

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