J. Anim. Sci. 2005. 83:1625-1632
© 2005 American Society of Animal Science
Effects of choline on blood metabolites associated with lipid metabolism and digestion by steers fed corn-based diets1
D. J. Bindel,
E. C. Titgemeyer2,
J. S. Drouillard and
S. E. Ives
Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600
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Abstract
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Ruminally cannulated steers (281 ± 18 kg) were used to evaluate effects of choline on digestion and metabolism. Four steers were implanted with 24 mg of estradiol and 120 mg of trenbolone acetate, and four steers were not implanted. Cattle were assigned to concurrent 4 x 4 Latin squares. Dietary treatments were a 2 x 2 factorial: 0 or 4% tallow (DM basis) in corn-based diets, and 0 or 5 g/d supplemental choline administered abomasally. Blood collected before and 6 h after the initial choline infusion was used to assess acute responses to choline. Digestibility and blood metabolites were measured after adaptation to choline, as well as after an abomasal dose of 100 g of lipid. Digestibilities of dietary DM (P = 0.29) and of dietary total fatty acids (P = 0.42) were not affected by choline. Apparent digestibilities of C18:0 and C18:1 fatty acids were greater (P < 0.05) when diets contained 4% tallow. Digestibilities of fatty acids in the lipid dose were less than those in the diet, and no biologically important differences in fatty acid disappearance resulted from the treatments. No significant acute responses to choline were detected. After adaptation to choline, no important differences in plasma metabolites occurred in response to choline infusion. Plasma urea was less (P < 0.05) for implanted cattle, reflecting increased deposition of protein. Plasma cholesterol was greater (P < 0.05) for steers fed 4% tallow. Changes in plasma triglycerides in response to an abomasal lipid dose were less (P < 0.05) for steers fed 4% tallow, probably due to greater triglyceride concentrations at the time of lipid dosing. In summary, few responses to abomasally infused choline were observed in either digestion or plasma metabolites.
Key Words: Cattle • Choline • Digestion • Implant • Metabolism
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Introduction
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Ruminally protected choline has improved growth performance of finishing cattle without negatively affecting carcass characteristics (Drouillard et al., 1998
; Bryant et al., 1999; Bindel et al., 2000). Drouillard et al. (1998) observed an interaction between dietary fat and supplemental choline, but results of Bindel et al. (2000) contradicted this finding.
The mechanism by which choline improves growth performance is unknown. Improvements may be due to alterations in lipid metabolism and/or transport (Bryant et al., 1999). In sheep, Bryant et al. (1999) observed increases in plasma NEFA after 28 d of choline administration, but plasma NEFA tended to decrease after 56 d. Bindel et al. (2000) observed numerically decreased plasma NEFA after supplementing choline for 90 d to finishing heifers.
In dairy cattle, choline supplementation has improved lactational performance (Sharma and Erdman, 1989; Erdman and Sharma, 1991; Pinotti et al., 2003). In response to supplementing dairy cows with choline, Pinotti et al. (2003) observed a transient decrease in plasma NEFA at calving; others have observed no changes in plasma NEFA (Piepenbrink and Overton, 2003) or only numerical increases (Sharma and Erdman, 1989).
Estrogen decreases bile flow in rats (Crocenzi et al., 2001), which could affect phospholipid metabolism, as well as lipid digestion. In addition, in rats, estrogen can prevent the hepatic triglyceride accumulation that occurs when a choline-deficient diet is fed by increasing methylation of phosphatidylethanolamine to form choline (Young, 1971), making the animal less dependent on dietary choline. Thus, implantation of cattle with estradiol-containing products could alter choline utilization.
Our objective was to determine whether choline supplementation affected lipid digestion and blood metabolites in finishing cattle and to determine whether responses to choline supplementation were affected by implanting cattle with steroidal hormones.
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Materials and Methods
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Eight Angus-cross, ruminally cannulated steers (281 ± 18 kg initial BW) were used in two concurrent 4 x 4 Latin square designs. Four steers were implanted with Revalor-S (24 mg of estradiol and 120 mg of trenbolone acetate; Intervet, Millsboro, DE) 1 wk before the study, and four steers were not implanted. These two groups were used in separate Latin squares. Steers housed in individual tiestalls were adapted to a finishing diet (92% concentrate, 8% alfalfa hay; DM basis) before the start of the experiment. Dietary treatments were arranged as a 2 x 2 factorial and included dietary tallow concentrations of 0 and 4% (DM basis; Table 1), and supplemental choline amounts of 0 and 5 g/d. Choline was provided as choline chloride in an aqueous solution, which was administered twice daily at feeding via infusion of 100 mL of solution into the abomasum. The infusions were provided through flexible polyvinylchloride tubing (5 mm i.d., 8 mm o.d.) that passed through the ruminal cannula and reticuloomasal orifice and was anchored in the abomasum with a rubber flange. Steers had ad libitum access to their diets. Fresh feed was provided twice daily at 12-h intervals, and representative samples of feed and orts were collected daily.
Experimental periods were 14 d long. The first 5 d were for dietary adaptation; choline infusions began on d 6 and continued through the end of each period. Blood was collected by jugular venipuncture before and 6 h after the initial choline infusion on d 6. These samples were collected to measure acute responses to administration of choline. On d 10 through 14, total fecal collections were conducted for all steers by use of fecal collection bags. Representative fecal samples were collected from total output, dried (55°C), ground (1-mm screen), and analyzed for DM (105°C for 24 h) and long-chain fatty acids (gas chromatographic analysis of methyl esters after derivatization in methanolic HCl). The chromatographic separation of the methyl esters of fatty acids used a 2-mm i.d. glass column with a length of 2 m and packed with 10% SP2330 on 100/120 Chromosorb W AW (Supelco, Bellefonte, PA), N2 at 20 mL/min as the carrier gas, and an initial oven temperature of 130°C that was increased at a rate of 3.5°C/min until a final temperature of 210°C was reached.
On the evening of d 12, a catheter was placed into the jugular vein of each steer and used for blood collection on d 13 and 14. After placement and sample collections, catheters were flushed with a 3.5% sodium citrate solution. On d 13, blood was collected before feeding and 3 and 6 h after feeding.
On d 14, all steers received a pulse dose into the abomasum of a lipid mixture (50 g of stearic acid, 20 g of palmitic acid, 20 g of oleic acid, and 10 g of tallow), which was analyzed to contain 0.9 g of C14:0, 27.3 g of C16:0, 1.3 g of C16:1, 45.3 g of C18:0, 15.7 g of C18:1, 0.8 g of C18:2, and 0.3 g of C18:3. The dose was given 2 h after feeding through the infusion line described previously. One gram of ytterbium (as YbCl3) was added to the fat dose to verify that the subsequently collected fecal samples contained the undigested portions of the lipid mixture. Before dosing of the lipid mixture, and at 1, 2, 4, and 8 h after dosing, blood was collected via the jugular catheter.
At all sampling times, blood samples were collected into heparinized tubes for analysis of plasma glucose, total 
-amino N, urea, and cholesterol, whereas samples collected into EDTA-containing tubes were used for analysis of plasma triglycerides and NEFA. Plasma was analyzed for glucose (Gochman and Schmitz, 1972), total -amino N (by automated trinitrobenzenesulphonic acid analysis; Palmer and Peters, 1969), and urea (Marsh et al., 1965), all with a Technicon Auto Analyzer II (Tarrytown, NJ). Plasma concentrations of NEFA (Wako Chemicals, Richmond, VA, No. 994-75409E, as modified by Eisemann et al., 1988), cholesterol (Sigma Diagnostics, No. 352, St. Louis, MO; Allain et al., 1974), and triglycerides (Sigma Diagnostics, No. 337; McGowan et al., 1983) were all measured with commercially available kits.
Fecal collections from the final day were sampled, dried, ground, and analyzed for DM and long-chain fatty acids. Ytterbium content was measured by atomic absorption spectrophotometry with an NO2/acetylene flame after ashed residues were solubilized in a solution containing 3 M HCl and 3 M HNO3 (Ellis et al., 1980).
Digestibilities of fatty acids from the lipid mixture administered on d 14 were calculated by subtracting the average daily fecal output of fatty acids on d 10 to 13 from the fecal output of fatty acids on d 14. This value was corrected for Yb recovery in the feces and was considered to be the increase in fecal fatty acid output in response to the pulse dose of the lipid mixture. This value was divided by the quantity of the fatty acids in the infusate to calculate digestibility of the infused lipid.
Data were analyzed statistically using the MIXED procedure of SAS (SAS for Windows 6.12; SAS Inst., Inc., Cary, NC). The model was for concurrent Latin squares, with the effect of implant equal to square. For variables with only a single measure (e.g., digestibility), the model included effects of implant, period, tallow, choline, tallow x choline, implant x tallow, implant x choline, and implant x tallow x choline. Steer within implant (error term for testing effects of implant) was included as a random effect. For variables with repeated measures, a split-plot analysis was used with the previously described model with time and its interactions with all other terms also included in the model. Period x tallow x choline x steer within implant also was included as a random effect to serve as an error term for testing effects of tallow, choline, their interaction, and their interactions with implant. Two observations were completely missing, and both of these were implanted steers fed the no-tallow diet without choline supplementation. Individual blood samples from other steers (three on d 6, one on d 13, and three on d 14) also were missing. None of the missing observations seemed to be related to treatment. Treatment differences were considered significant at = 0.05.
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Results and Discussion
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Digestibilities
Intakes of DM averaged 1.98% of initial BW and were not affected by treatment (Table 2). Digestibility of DM and OM from the diet was not affected by treatment either. By design, intakes of total fatty acids and most individual fatty acids were greater for steers fed diets containing tallow. Digestibilities of individual fatty acids were affected by treatment, with the greatest response being due to the addition of tallow to the diet. Several factors must be considered when evaluating treatment responses in fatty acid digestion, including the source of fatty acids, the total amount of fat in the diet, and the potential for saturation of the unsaturated fatty acids within the rumen. Digestibilities presented in Table 2 are apparent digestibilities, so the negative effect of endogenous losses of fatty acids on calculated digestibilities will decrease as the fat concentration in the diet increases. In contrast, intestinal digestibility of fat typically decreases as intake of fat increases (Zinn, 1994). Among the predominant fatty acids in tallow (C16:0, C18:0, and C18:1), digestibility of C16:0 was not affected by tallow addition, whereas digestibilities of C18:0 and C18:1 were greater (P < 0.05) when tallow was added to the diets. Although the digestibilities of many of the individual fatty acids were greater in the tallow-containing diets, the digestibility of total fatty acids was numerically less for the tallow-containing diets because the digestibility of C18:0 was much less than for the other predominant fatty acids, and the tallow contained a greater proportion of C18:0 than did the fat in the basal diet. The range in fecal fat excretion across the dietary treatments was small (28 to 73 g/d).
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Table 2. Intakes and digestibilities of steers fed diets with 0 or 4% tallow, without or with 5 g/d supplemental abomasally infused choline
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Choline and its interaction with tallow affected the digestibility of C18:0 fatty acids, but this response was predominantly a result of large negative digestibilities for the control diet, which contained relatively little C18:0, when choline was supplemented. The increase in fecal C18:0 excretion in response to supplemental choline was 9 g/d for cattle fed diets without tallow. In contrast, fecal C18:0 excretion was decreased 7 g/d by choline supplementation when cattle were fed diets with 4% tallow. Although digestibilities of total fatty acids did not differ among treatments, fecal excretion of total fatty acids followed a pattern similar to that for C18:0, with an 11 g/d increase in fecal fatty acids in response to choline supplementation for cattle fed diets without tallow and a 12 g/d decrease in response to choline supplementation for cattle fed diets containing 4% tallow.
To further test the effect of supplemental choline on lipid digestion, cattle were dosed with 100 g of a lipid mixture to challenge the capacity of the intestine to absorb greater amounts of fatty acids. The lipid dose, which was abomasally infused as a single pulse dose, provided an amount of fat equal to approximately half the daily fat intake by steers fed diets without tallow and equal to approximately one-fourth the daily fat intake by steers fed diets with 4% tallow. The digestibilities presented in Table 3 are those calculated for the supplemental lipid dose alone. Digestibilities of total fatty acids were much less for the supplemental dose than for those from the diet (Table 2
). This result would be expected because efficiency of fat digestion declines as intake of lipid increases (Zinn, 1994). Often times, it is difficult to measure the effects of treatments on digestion because digestion is nearly complete and little room exists for improvement. Given the poor digestibility of the supplemental dose, the potential existed for choline supplementation to positively affect digestion; however, digestibilities of the supplemental lipid were variable, which made it difficult to draw specific conclusions. Regardless, a few points should be noted. For C16:0 fatty acids, the digestibilities were calculated to be negative, which implies that the increase in fecal C16:0 in response to the lipid dose was greater than the amount dosed to the abomasum, which probably reflects a stimulation of endogenous losses or microbial synthesis in response to the dosing. In contrast, fecal losses of C18:2 were less after the lipid dose than before (data not shown). Reasons for this response are unclear, but the magnitude of change in fecal excretion of C18:2 was not large when expressed as a daily amount.
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Table 3. Fecal recovery of abomasally dosed Yb and calculated digestibilities of fatty acids from abomasally infused fat of steers fed diets with 0 or 4% tallow, without or with 5 g/d abomasally infused cholinea
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Treatment effects (P < 0.05) on digestion of the lipid dose included effects of tallow x choline for C16:0 and total fatty acids, of implant x choline for C16:0, and of implant x tallow x choline for C16:0 (Table 3). In all instances, these effects were a result of much lower digestion by the implanted steers consuming the diet without tallow and receiving no supplemental choline (data not shown). Excluding the response for the implanted cattle fed the diet without tallow, supplemental choline did not appreciably affect digestion of the lipid dose. The inability of supplemental choline to improve digestion of either the total diet or of fatty acids suggests that the benefits of supplemental choline in finishing cattle are not a result of increased digestion.
Plasma Metabolites
On d 6, steers were given their initial dose of choline at feeding. Changes in concentrations of plasma metabolites between this initial sample and subsequent samples collected 6 h after this initial choline dose are presented in Table 4. In response to this initial choline supplement, there were no significant treatment changes in the plasma concentrations of triglycerides, cholesterol, NEFA, glucose, urea, or total -amino N (Table 4).
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Table 4. Changes in plasma metabolite concentrations between feeding and 6 h after feeding in cattle abomasally dosed or not dosed with 2.5 g choline at feeding (d 6)a
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On d 13, when steers had been adapted to choline supplementation for 7 d, no significant differences in the measured plasma metabolites occurred in response to choline (Table 5). Plasma glucose concentration was greater at 6 h after feeding for nonimplanted steers, but this difference was not observed before feeding or at 3 h after feeding (implant x hour; P < 0.05; data not shown). Plasma urea (Table 5) concentrations on d 13 averaged 21% less (P < 0.05) for implanted cattle (1.68 vs. 2.12 mM), reflecting increased use of AA for protein deposition rather than oxidation. Dietary tallow additions led to 22% greater (P < 0.05) plasma urea concentrations, probably a reflection of less microbial use of ammonia and greater ruminal N availability; diets were formulated to be isonitrogenous, but tallow-containing diets had less ruminally fermented carbohydrate, and protein from soybean meal replaced some of the protein from corn. Plasma cholesterol on d 13 was 38% greater (P < 0.01) for steers fed tallow-supplemented diets (Table 5). This would be expected because lipoprotein transport of absorbed fatty acids should be greater in response to the increased dietary lipid content. An implant x choline x time interaction for plasma cholesterol occurred (P < 0.05) because plasma concentrations increased over time after feeding for implanted steers supplemented with choline, whereas cholesterol concentrations decreased over time after feeding for nonim-planted steers supplemented with choline (data not shown). The biological importance of this interaction is probably minimal. On d 13, tallow increased plasma NEFA by 59% (P = 0.10) and plasma triglycerides by 55% (P = 0.11; Table 5); the lack of significance can be attributed, in part, to the relatively large standard errors.
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Table 5. Plasma metabolite concentrations of steers fed diets with 0 or 4% tallow and without or with 5 g/d abomasally infused cholinea
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Figure 1 demonstrates the patterns for the measured metabolites (averaged across treatments) after the abomasal lipid infusion. In response to the lipid challenge on d 14, plasma glucose, total -amino N, and NEFA all demonstrated differences (P < 0.05) over time, and these reflect response to the abomasal lipid dose. Responses for the other measured metabolites also were present, but they did not reach statistical significance. Regardless of the magnitude of changes over time, the model would still be capable of measuring different responses among treatments if they existed.
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Figure 1. Changes in plasma metabolites of steers after abomasal infusion of a lipid dose. At 2 h after feeding, steers were dosed abomasally with 100 g of a lipid mixture. Values are concentrations at the listed time after dosing minus concentrations at the time of dosing, and they represent averages across all treatments. Plasma concentrations of glucose, NEFA, and -amino N all demonstrated changes over time, P < 0.05.
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Table 6 provides the changes in plasma metabolites over time after the lipid dose, with values presented as differences between the value at the listed time and the value observed at the time of lipid dosing. These differences were calculated to remove the confounding of differences in baseline concentrations with the changes that occurred in response to the lipid dose. Changes in plasma triglycerides in response to the lipid dosing were less (P < 0.05) for steers receiving tallow-containing diets than for steers fed the control diet, although much of this response can be attributed to lower plasma triglyceride concentrations at the time of lipid dosing for the steers fed the control diet (5.9 mg/ 100 mL) than for the steers fed the tallow-containing diet (9.0 mg/100 mL). Thus, steers fed the control diet were more responsive to the abomasal lipid dose than were steers fed 4% tallow. This increased responsiveness cannot be attributed to greater digestion of the lipid dose because digestion of its fatty acids was numerically less for steers fed the control diet than for those fed 4% tallow (30 vs. 48%; Table 3), suggesting that the dietary lipid concentration effected metabolic changes in the cattle. Changes in plasma concentrations of -amino N demonstrated a tallow x choline x time interaction (P < 0.05), with the responses over time being different among all of the tallow/choline treatment combinations. This response is unlikely to be biologically important.
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Table 6. Changes in plasma metabolites of steers after abomasal infusion of a lipid dose to steers fed diets with 0 or 4% tallow and without or with 5 g/d abomasally infused cholinea
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Comparison with Previous Research
Relatively few studies have considered the effects of supplemental choline on digestion and metabolism in growing cattle. In response to supplementation of finishing heifers with ruminally protected choline, Bindel et al. (2000) observed that plasma triglyceride concentrations increased with small amounts of choline supplementation when a diet without supplemental tallow was fed, but for heifers fed a diet containing 4% tallow there were no changes in triglycerides in response to choline supplementation. This observation agrees with our observations in response to lipid dosing, in which cattle fed the 4% tallow diet were relatively non-responsive to the abomasally supplemented lipid. In their study, Bindel et al. (2000) observed numerical decreases in plasma NEFA in response to choline supplementation, but no response in plasma cholesterol, glucose, or insulin. Addition of 4% tallow to diets increased plasma triglycerides, NEFA, and cholesterol (Bindel et al., 2000), which is similar to our findings.
In growing lambs (Bryant et al., 1999), supplemental choline increased plasma triglyceride concentrations and, after 28 d, increased plasma NEFA and insulin concentrations. However, the responses in NEFA and insulin disappeared after lambs had been supplemented with choline for 56 d. These same researchers observed that diet DM digestibility in lambs was increased by one of the intermediate amounts of supplemental choline, although this was attributed to an analytical issue, and it was suggested subsequently that choline had little effect on diet digestibility. Digestibility of ether extract was decreased by supplementation with ruminally protected choline in the lambs, but this was attributed to the poor digestibility of the lipid carrier for the rumen protection of the choline. Thus, in sheep, it was concluded that supplemental choline had little effect on diet digestion; however, it should be noted that the growth and feed intake responses in lambs (decreases with intermediate amounts of choline, followed by an increase at the greatest amount; Bryant et al., 1999) were different than those observed in cattle (increases with intermediate amounts of choline, with gain returning to near that of controls for the greatest amount; Bryant et al., 1999; Bindel et al., 2000), suggesting that metabolic responses to choline also might differ between the two species.
More studies have been conducted with choline supplementation to dairy cattle than with beef cattle or sheep. In general, choline supplementation either has not affected milk production or has led to modest increases in milk production, with increases in milk fat secretion in some instances. In one study, responses to choline supplementation were observed for dairy cows fed a diet supplying a small amount of metabolizable methionine, but responses were negative in cows fed a diet supplying a large amount of metabolizable methionine (Hartwell et al., 2000). These data suggest that the responsiveness of cattle to choline may depend on the availability of methyl donors; however, responsiveness of beef cattle to choline supplementation may not be similarly dependent on dietary protein supply; growing beef cattle respond to supplemental choline with increases in growth under conditions in which both the methionine and metabolizable protein supply exceeded the animal’s requirements (Bryant et al., 1999; Bindel et al., 2000).
The response of dairy cattle to supplemental choline has been attributed to its role as a lipotropic agent that can play a valuable part in decreasing liver adiposity, which is frequently observed in the periparturient period. Indeed, liver fat content has been shown to decrease numerically in response to choline supplementation in periparturient cows (Piepenbrink and Overton, 2003). For finishing beef cattle, hepatic lipid infiltration is unlikely to occur.
Changes in the concentrations of plasma metabolites in response to choline supplementation in dairy cows have been variable. Pinotti et al. (2003) observed a striking decrease in plasma NEFA at parturition but not at times as close as 7 d before calving or at 10 d after calving. In contrast, Sharma and Erdman (1989) observed numerical increases in serum NEFA in response to abomasal choline supplementation.
Estrogen decreases bile flow in rats (Crocenzi et al., 2001), which could affect phospholipid metabolism as well as lipid digestion. In rats, estradiol also can prevent the hepatic triglyceride accumulation that occurs when a choline-deficient diet is fed, presumably due to the ability of estradiol to increase methylation of phosphatidylethanolamine to form choline (Young, 1971). Thus, implantation with an estradiol-containing product might increase endogenous choline synthesis and make the animal less responsive to supplemental choline; however, it is unknown whether the relationship between estradiol and choline metabolism occurs in cattle, or whether the concentrations of estradiol in the implanted cattle were large enough to elicit a response. Regardless, in our study, few interactions between implantation with an estradiol-containing product and choline supplementation were detected. Moreover, growth assays that have demonstrated the benefits of choline supplementation for finishing cattle (Drouillard et al., 1998; Bryant et al., 1999; Bindel et al., 2000) all have involved cattle that received an estradiol-containing implant, suggesting that the exogenous estradiol did not obviate the benefit of supplemental choline.
General Conclusions
Relatively few responses to choline supplementation were observed in either digestion or plasma metabolite concentrations. The lack of response to choline was consistent for acute responses (d 6), response after adaptation (d 13), and response to a lipid challenge (d 14). It is possible that our research model was not sufficiently sensitive to detect differences in either digestive or metabolic characteristics or that choline does not greatly affect lipid digestion or metabolism in growing cattle. Alternatively, the beneficial effects of choline supplementation in growing cattle are mediated through changes that are not manifested in blood characteristics or not related closely enough to lipid metabolism to change the measured responses. However, plasma metabolites related to protein metabolism (urea and -amino N) were not greatly affected by choline supplementation either, suggesting that protein status of cattle is not affected by choline supplementation.
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Footnotes
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1 Contribution No. 05-35-J, Kansas Agric. Exp. Stn., Manhattan. 
2 Correspondence: 132 Call Hall (e-mail: etitgeme{at}oznet.ksu.edu).
Received for publication September 21, 2004.
Accepted for publication April 12, 2005.
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Abstract
Introduction
