Niacin Absorption from the Rumen1 and A. M. TRUSK Deparbwnt of Animal Sciences University of lllinols
P. S. ERICKSON? M. R. MURPHY? C. S. MCSWEENEY!
Urbana 61801 ABSTRACT
Attempts also have been made to determine whether or not niacin can be directly absorbed Abmrption of niacin from the rumen from the rumen (14, 17), although the results was tested in vivo using a buffer solution were equivocal. Rkrat et al. (17) isolated the that contained either nicotinic acid or rumens of two anesthetized sheep by ligating nicotinamide. The experimental design the cardia and reticulo-omasal orifice. After was a 3 x 3 Latin square with three digesta were removed and the organ was midlactation, ruminally cannulated Holwashed, it was refilled with a similar volume stein cows. Orthogonal comparisons of Tyrode's solution (20) containing a mixture were control versus niacin and nicotinic of B vitamins (three to four times the amounts acid versus nicotinamide. Nicotinamide normally found in the organ) and acetate. The was more rapidly absorbed from the Nniacin source used was identified as pellagramen than nicotinic acid; this may have preventive factor (PPF), an early designation been caused by differences in the dissofor this vitamin. Porter (14) interpreted this to ciation constants of the compounds. The mean NA, although no basis for this assump effects that diffemtial absorption may tion was given; it is possible that nicotinamide have on ruminal fermentation and animal (NM), or pehaps some mixture, may have metabolism require further study. been used. In any case, RQat et al. (17) (Key words: niacin, absorption, rumen) showed that an average of 14% of the PPF in the solution introduced into the rumen disapAbbreviation key: NA = nicotinic acid, N M = peared after 2 h. They also reported that the nicotinamide, PPF = pellagra-preventive facconcentrations of PPF in blood from rumen tor. veins increased during the infusion an average of 20% in one sheep and 40% in the other. A INTRODUCTION rumen perfusion study also suggested that PPF Synthesis of niacin (form unspecified) in was absorbed from the rumen. Porter (14) reported results of experiments the rumen of sheep (15, 16) and nicotinic acid (Cowie and Porter, unpublished data) in which (NA) in the rumen of cattle (14) has been B vitamin solutions were placed in a pouch demonstrated. This synthesis also has been formed from the rumen of a goat. Absorption suggested to be under metabolic control; i.e., of NA varied with the amount placed in the more is synthesized when small amounts are rumen pouch. When physiological saline was provided in the ration and vice versa (1, 14). placed in the pouch, 10 pg of NA were recovered from the pouch after 1 h. More NA also was recovered after 1 h (35 pg) when a solution containing 25 pg was placed in the Received February 6, 1991. Accepted April 29, 1991. pouch. The opposite was found when 250 pg 'Supported by Illinois Agricultural ExpBiment Statim were placed in the pouch; only 180 pg were (Hatch 35-037 1). recovered after 1 h. Continual sloughing of i o ~ 2Present address: ~nimal~ealthand ~ ~ t r i t HO~Tcells from the lining of the pouch, even though manu-LaRoche Inc., Nutley, NJ 07110. 3T0whom correspondence should be addressed. Uni- they were separated from the solution, conversity of Illinois, Department of Animal Sciences, 315H founded absorption estimates. Animal Sciences Lab, 1207 W. Gregory Drive, Urbana Although direct absorption from the rumen 61801. is possible, it normally is thought to be limited 4Present address: CSIRO. Division of Tropical Animal because only a small portion (3 to 7%) of the Reduction, Long Pocket Laboratories,Private Bag Numvitamin is in the Supernatant fraction of rumen ber 3, P.O.Indooroopilly, Queeosland 4068, Australia. 1991 J Dairy Sci 74:3492-3495
3492
3493
NIACIN ABSORPTION FROM THE RUMEN
TABLE I. Buffer solutions used to determine nicotinic acid or nicotinamide absorption across ruminal epithelium.1 Trmmellt component
Control
Nicotinic acid
Nicotinamide
Naco3, g NazHp04.7H20, g KCL g NaCI. 8
205.8 147 11.97 9.87
205.8 147 11.97 9.87 2.52 15.75 15.75 3.15
205.8 147 11.97 9.87 2.52 15.75 15.75
~%SO~.~Z gO, Co-EDTA,' g
Polycthyleoe glycol' g Nicotinic acid, g Nicotinamide, g ~ a c d
PH
252
15.75 15.75
...
...
...
.84 6.98
.84 6.95
...
3.15 .84 6.98
'Added to 21 L of distilled water and adapted from MCDougall (11). 'Not added to rinsing buffer. 3Added last, followed by m~hgwith CO2
fluid; most is bound within the microbes themselves. More than two-thirds of the niacin is in the supematant fraction of abomasal, duodenal, and ileal digesta (15). Absorption from the small intestine seems to be the major route by which niacin is made available to the host (15). Both sources of niacin currently are marketed for use in ruminant diets; however, the subject of differential absorption from the rumen and the effects it might have on fermentation in that organ do not appear to have been studied adequately. Supplementation of dajr cow diets with 6 to 12 g/d of niacin (4) would be expected to increase the fiaktion of the vitamin in rumen fluid supernatant, at least temporarily. If one form of the vitamin were absorbed directly from the rumen more quickly than the other, source effects on rumen fermentation and perhaps microbial protein flow could occur despite the fact that total niacin absorption (the sum of ruminal and postruminal absorption) might not change. The objective of this experiment was to examine the potential effect of vitamin source on niacin absorption from the rumen.
The rumens of the cows were emptied manually for determination of niacin absorption. Then they were filled with tap water, washed, and emptied twice. This was folIowed by a final filling with the control buffer (Table l), washing, and evacuation. Buffer solutions (21 L), containing CO-EDTA and polyethylene glycol (molecular weight 4OOO) as nonabsorbable fluid markers, and a niacin source (if appropriate) were placed in the ventral and caudoventral blind sacs of the rumen (Table 1). The cranial pillar was held manually between contractions to inhibit liquid flow Out of the rumen and salivary dilution. Water and feed were unavailable during the 1-h sampling peri-
od.
Solutions were sampled (50 ml) just prior to placement in the rumen and at 20-min intervals for 1 h (four samples per period). The solutions were at ambient temperature (average maximum, minimum, and mean temperahues on experimental days were 30, 17, and 24T, respectively) and had similar hydrogen ion concentrations (Table 1). Samples were frozen for later analysis of fluid markers and vitamins. Subsamples were diluted in .5% acetic acid and analyzed for Co (7) by atomic absorpMATERIALS AND METHODS tion spectrophotometry (Model 2380, PerkinThree ruminally cannulated, midlactation Elmer, Norwalk, Cl"). Polyethylene glycol Holstein cows were used in a 3 x 3 Latin concentrations were not determined. High persquare design with 2d periods. Treatments formance liquid chromatography on a Beckwere control, NA, or NM. All cows were fed man Ultrasphere-ODS C18 column (Model alfalfa grass haylage, corn silage, high mois- 421 controller, Model lOOA pump, Beckman ture shelled corn, soybean meal, and a trace Instruments, Fullerton, CA) using a Hitachi mineral-vitamin mix. specnophotometer with a multiple wavelength J o d of Dairy Science Vol. 74, No. 10, 1991
3494
BRICKSON ET AL.
W absorbance detector (Model 100-10, Hitachi Ltd, Tokyo, Japan) set at 254 nm was used to analyze NA and NM concentrations. Standard solutions containing analytical grade NA and NM (Sigma Chemical Co., St. Louis, MO) were eluted using a solvent system of .2% (wt/vol) orthophosphoric acid in doubledistilled water at a flow rate of .85 d m i n . There was a linear relationship (r2 > .99)between the concentration of each standard (0 to .2 &) and the area of the chromatogram peak Absorption was determined using the equation of Martens and Stassel (10):
where A = absorption; V = volume (21 L); C, = NA or NM concentration at time = 0 min, C,
= NA or NM concentration at t = 20,40, or 60 min; Co-EMTAi = marker concentration at t = 0 min, and Co-EDTAf = marker concentration at t = 20, 40,or 60 min. Statistical analysis was by general linear models procedure of SAS (Release 5.18, 1986, SAS Institute Inc., Cary, NC). The effects of time and treatment x time on absorption and Co-EDTA concentration were not significant (P > .lo); therefore, mean absorption and CO-EDTA concentration within cow, period, and treatment (n = 9) were the dependent variables. Orthogonal comparisons were control versus niacin and NA versus NM. Significance was determined at P < .10 unless otherwise noted. RESULTS AND DISCUSSION
Nicotinamide was absorbed at a rate of .98 g/h, but NA did not appear to be absorbed from the rumen over the 141 period (Table 2). Lack of NA absorption lowered the overall niacin mean; therefore, the difference between
control and niacin treatments was not significant. Mean Co-EDTA concentration was 5 % greater for the niacin than for the control treatment; however, the reason this occurred is not apparent. The absorption equation would have compensated for such differences. Nicotinic acid may not have been absorbed because its pKa is 4.85 (12). Most of the NA would be ionized at the pH (-7) of the buffer in the rumen, although inclusion of acetate or other VFA in the buffer or rapid fermentation may enhance NA absorption by decreasing ruminal pH. In contrast, N M is weakly acidic. Its pKa is -14 (13). At the buffer pH, most N M would have been in the undissociated form, which may have increased the likelihood of its absorption from the m e n . Supplementation of niacin to the diets of dajr cows was not commonly practiced until it was shown to reduce blood ketone concentrations (6). Earlier work had indicated that niacin reduced blood plasma NEFA concentrations in cows and goats (21, 22, 23). Niacin probably elicits its ketonedepressing effect by reducing lipolysis and NEFA concentrations, possibly by reducing adenyl cyclase activity or stimulating phosphodiesterase activity (5). Niacin increased microbial protein synthesis in some (18, 19), but not other (1, 2), in v i m experiments. Three studies also have suggested that Nmen protozoa numbers are inmased with niacin supplementation (3, 5, 8). AU ciliates that can be cultivated axenically require niacin; both NA and NM are effective for most of the organisms tested (9). These results support the results of other research conducted in our laboratory in which blood plasma NEFA, measured as area beneath an epinephrine-stimulated NJFA response curve, was less (NS) for N M than for NA. However, cows fed NA had increased (NS) protozoa numbers in their ruminal fluid (5).
TABLE 2. Least squares means of nicotinic acid and nicotinamide absorption across the ruminal epithelium and CoEDTA concentration (diluted).
Variable vitamin absorbed,' CO-EDTA,~ppm
a
Coni101
Nicotinic acid
Nicotinamide
SE
-.07 1.66
-.17 1.80
.98 1.72
.26 .02
'Nicotinamide absorption was greater than nicotinic acid absorption (P e .lo). the niacin than the conmi treatmeat (P < .IO).
2The C-EDTA concentration was greater for Journal of Dairy Science Vol. 74, No. 10, 1991
NIACIN ABSORPTION FROM THE RUMBN
Schussler et al. (19) reported that sheep consuming a high concentrate diet and 250 ppm of NA produced ruminal fluid capable of digesting more cellulose in vitro (P < .05) than that from sheep fed a high concentrate diet but not receiving NA. Our results suggest that N M may be more effective in reducing lipolysis, and NA may be more effective in increasing protozoa numbers. These data also suggest that NA may have to be converted to NM before significant absorption from the rumen occurs. Increasing protozoa numbers, specifically entodinia, may result in increased bacterial numbers because entodinia serve to regulate the rumen environment by consuming starch. An increased microbial population may lead to increased microbial protein flows and increased intestinal amino acid supply. This could account for the increased milk protein content and yield observed when niacin was given as a supplement to cows in some studies (4, 18). Additional studies are required to address the effects that differential ruminal absorption of niacin sources may have on ruminal fermentation and animal metabolism. Supplementation strategies might be discovered that will capitalize on this phenomenon. ACKNOWLEDGMENTS
The assistance of Jill Cline, Wellington Nombekela, and Dan Schauff in sampling is gratefully acknowledged. REFERENCES
1 Abdouli, H., and D. M. ScWer. 1986. Effect of two dietary niacin concentrations on ruminal fluid free niacin concentration and of supplemental niacii and source of inocohun on in vitro microbial Browtfi fermentative activity and nicotinamide adenine dinucleotide pool size. J. Anim. Sci. 62254. ZAbdouli, H., and D. M. Schaefa. 1986. Impact of niacin and length of incabation on protein synthesis, soluble to total protein ratio and fermentative activity of rominal microOrganipmJ. J. Anim. Sci 62244. 3Dennis, S. M.,M. J. Arambel, E. E. Bartley, D. 0. Riddell, and A. D. Dayton. 1982. Effect of heated or unheated soybean d with or WithOUl niacin on rumen protozoa. J. Dairy Sci. 65:1643. 4Erickso11,P. S. 1989. Nia~i~-lipid inteIactim in lactating dairy cows. Ph.D. Diss., Univ. Jllinois, Urbana 5Ericks011, P. S., A. M. Trusk, and M. R Murphy. 1990. Effects of niacin source on epinephrine. stimnlation of plasma wnestcrified fatty acid and glucose concentrations, on diet digestiiility aml on rumen
3495
protozoal numbers in lactating dairy cows. J. Nutr. 120: 1648. 6FronL, T. J., and L. H. Schultz. 1979. Oral nicotinic acid as a treatment for ketosis. J. Dairy Sci. 62:1804. 7Hart, S. P., and C. E. Polan. 1984. Simultaneous extraction and determination of ytterbium and cobaltethylenediaminetetraacetatecomplex in feces. J. Dairy Sci. 672388. 8Honrer. J. L., C. E. Coppock, J. R. Moya, J. M. Labore, and J. K. Lanham. 1988. Effects of niacin and whole cottonseed on ruminal fermentation, protein digestibility, and nutrient digestibility. J. Dairy Sci. 71:1239. 9 LiUy, D. M. 1967. Growth facton in protozoa. Page 275 in chemical zoology. M. Florkin and B. T. Scheer, ed. Vol. 1. Rotozoa G. W. Kidder. ed. Academic Press. New York, NY. IO-, H,and E.-M. WSSCL 1988. M@WD ebsorption from the tealporarily isolated Illmen of sbacp: Iy) &CC! of hyper- hypomagntsatmla. Q. I. Bxp. Physiol. 73:217. 11 McDougaU, E. I. 1948. Studies on nunhint saliva. I. Thc CompoSitiOn and output of sheep’s saliva Biochrm. J. 4399. 12 Mack Index. 1983. 10th ed. hi. Windholz, ed. Merck and Co., Inc., Rahway, NJ. 13 Morrison, R. T., and R N. Boyd. 1966. Orgunic chemistry. 2nd ed.Allyn and Bacon Inc., Boston, MA. 14 Porter, J.W.G. 1961. Vitamin synthesis in the rumen. Page 226 in Digestive physiology and nutrition of the nlminant. D. Lewis, ed. Butterworths, London, Engl. 15-4 A., 0. Champigny, and R. Jacquot. 1959. ModaliUs de I’absorption vitaminique chez les ruminants: forme et disponiitd des vitamines B du bo1 alimentaire aux Whts niveaux digestifs. C. R. HeM. Seances Acad. Sci. 249:1274. 16 RQaz A., and R. Jacqaot. 1954. J?volution des teneurs en vitamines B dam le bo1 alimentake des ruminants. C. R. Hebd. Seances Acad. Sci. 239:1693. 1 7 R h 4 A., J. Molle, and H. Le Bars. 1958. Mise en 6vidence chez le mouton de la permhbilitk du rumen aux vitamints B et conditions de letu absorption ce nivesu C. R Hebd. Seances A d . Sci. 246:2051. 18 Riddell, D. O., E. E. Bartley, and A. D. Dayton. 1981. Effed of nicotinic acid on microbial protein synthesis in vitro and on dairy cattle and milk production, J. Dairy Sci. 64:782. 19 Schussler, S. L., G. C. Pahey, Jr., J. B. Robinson, S. S. Masters, S. C. Loerch, and I. W. Spears. 1978. The effect of supplemental niacin on in vitro cellulose digestion and protein synthesis.Int. J. Vit. Nutr. Res. 48:359. 2OTyrode, M. V. 1910. lle mode of action of some purgative salts. Arch. Int. pharmacodyn. Ther. 20205. 21 Waterman, R, and L. H. Schultz. 1972. Nicotinic acid loading of normal cows: effects on blood metabolites and excretory forms. J. Dairy Sci. 55:1511. 22Watuman, R, and L. H. Schultz. 1973. 1Carbon14labeled palmitic acid metabolism in fasted, lactating goats following nicotinic acid administration.J. Dairy Sci. 561569. 23Waterman, R, J. W. Schwah, and L. H. Schultz. 1972. Nicotinic acid treabnent of bovine ketosis. 1. FBects on circulating metabolites and interrelationships. J. Dairy Sci. 551447. Journal of Dairy Science Vol. 74, No. 10, 1991
P. S. ERICKSON? M. R. MURPHY? C. S. MCSWEENEY!
Urbana 61801 ABSTRACT
Attempts also have been made to determine whether or not niacin can be directly absorbed Abmrption of niacin from the rumen from the rumen (14, 17), although the results was tested in vivo using a buffer solution were equivocal. Rkrat et al. (17) isolated the that contained either nicotinic acid or rumens of two anesthetized sheep by ligating nicotinamide. The experimental design the cardia and reticulo-omasal orifice. After was a 3 x 3 Latin square with three digesta were removed and the organ was midlactation, ruminally cannulated Holwashed, it was refilled with a similar volume stein cows. Orthogonal comparisons of Tyrode's solution (20) containing a mixture were control versus niacin and nicotinic of B vitamins (three to four times the amounts acid versus nicotinamide. Nicotinamide normally found in the organ) and acetate. The was more rapidly absorbed from the Nniacin source used was identified as pellagramen than nicotinic acid; this may have preventive factor (PPF), an early designation been caused by differences in the dissofor this vitamin. Porter (14) interpreted this to ciation constants of the compounds. The mean NA, although no basis for this assump effects that diffemtial absorption may tion was given; it is possible that nicotinamide have on ruminal fermentation and animal (NM), or pehaps some mixture, may have metabolism require further study. been used. In any case, RQat et al. (17) (Key words: niacin, absorption, rumen) showed that an average of 14% of the PPF in the solution introduced into the rumen disapAbbreviation key: NA = nicotinic acid, N M = peared after 2 h. They also reported that the nicotinamide, PPF = pellagra-preventive facconcentrations of PPF in blood from rumen tor. veins increased during the infusion an average of 20% in one sheep and 40% in the other. A INTRODUCTION rumen perfusion study also suggested that PPF Synthesis of niacin (form unspecified) in was absorbed from the rumen. Porter (14) reported results of experiments the rumen of sheep (15, 16) and nicotinic acid (Cowie and Porter, unpublished data) in which (NA) in the rumen of cattle (14) has been B vitamin solutions were placed in a pouch demonstrated. This synthesis also has been formed from the rumen of a goat. Absorption suggested to be under metabolic control; i.e., of NA varied with the amount placed in the more is synthesized when small amounts are rumen pouch. When physiological saline was provided in the ration and vice versa (1, 14). placed in the pouch, 10 pg of NA were recovered from the pouch after 1 h. More NA also was recovered after 1 h (35 pg) when a solution containing 25 pg was placed in the Received February 6, 1991. Accepted April 29, 1991. pouch. The opposite was found when 250 pg 'Supported by Illinois Agricultural ExpBiment Statim were placed in the pouch; only 180 pg were (Hatch 35-037 1). recovered after 1 h. Continual sloughing of i o ~ 2Present address: ~nimal~ealthand ~ ~ t r i t HO~Tcells from the lining of the pouch, even though manu-LaRoche Inc., Nutley, NJ 07110. 3T0whom correspondence should be addressed. Uni- they were separated from the solution, conversity of Illinois, Department of Animal Sciences, 315H founded absorption estimates. Animal Sciences Lab, 1207 W. Gregory Drive, Urbana Although direct absorption from the rumen 61801. is possible, it normally is thought to be limited 4Present address: CSIRO. Division of Tropical Animal because only a small portion (3 to 7%) of the Reduction, Long Pocket Laboratories,Private Bag Numvitamin is in the Supernatant fraction of rumen ber 3, P.O.Indooroopilly, Queeosland 4068, Australia. 1991 J Dairy Sci 74:3492-3495
3492
3493
NIACIN ABSORPTION FROM THE RUMEN
TABLE I. Buffer solutions used to determine nicotinic acid or nicotinamide absorption across ruminal epithelium.1 Trmmellt component
Control
Nicotinic acid
Nicotinamide
Naco3, g NazHp04.7H20, g KCL g NaCI. 8
205.8 147 11.97 9.87
205.8 147 11.97 9.87 2.52 15.75 15.75 3.15
205.8 147 11.97 9.87 2.52 15.75 15.75
~%SO~.~Z gO, Co-EDTA,' g
Polycthyleoe glycol' g Nicotinic acid, g Nicotinamide, g ~ a c d
PH
252
15.75 15.75
...
...
...
.84 6.98
.84 6.95
...
3.15 .84 6.98
'Added to 21 L of distilled water and adapted from MCDougall (11). 'Not added to rinsing buffer. 3Added last, followed by m~hgwith CO2
fluid; most is bound within the microbes themselves. More than two-thirds of the niacin is in the supematant fraction of abomasal, duodenal, and ileal digesta (15). Absorption from the small intestine seems to be the major route by which niacin is made available to the host (15). Both sources of niacin currently are marketed for use in ruminant diets; however, the subject of differential absorption from the rumen and the effects it might have on fermentation in that organ do not appear to have been studied adequately. Supplementation of dajr cow diets with 6 to 12 g/d of niacin (4) would be expected to increase the fiaktion of the vitamin in rumen fluid supernatant, at least temporarily. If one form of the vitamin were absorbed directly from the rumen more quickly than the other, source effects on rumen fermentation and perhaps microbial protein flow could occur despite the fact that total niacin absorption (the sum of ruminal and postruminal absorption) might not change. The objective of this experiment was to examine the potential effect of vitamin source on niacin absorption from the rumen.
The rumens of the cows were emptied manually for determination of niacin absorption. Then they were filled with tap water, washed, and emptied twice. This was folIowed by a final filling with the control buffer (Table l), washing, and evacuation. Buffer solutions (21 L), containing CO-EDTA and polyethylene glycol (molecular weight 4OOO) as nonabsorbable fluid markers, and a niacin source (if appropriate) were placed in the ventral and caudoventral blind sacs of the rumen (Table 1). The cranial pillar was held manually between contractions to inhibit liquid flow Out of the rumen and salivary dilution. Water and feed were unavailable during the 1-h sampling peri-
od.
Solutions were sampled (50 ml) just prior to placement in the rumen and at 20-min intervals for 1 h (four samples per period). The solutions were at ambient temperature (average maximum, minimum, and mean temperahues on experimental days were 30, 17, and 24T, respectively) and had similar hydrogen ion concentrations (Table 1). Samples were frozen for later analysis of fluid markers and vitamins. Subsamples were diluted in .5% acetic acid and analyzed for Co (7) by atomic absorpMATERIALS AND METHODS tion spectrophotometry (Model 2380, PerkinThree ruminally cannulated, midlactation Elmer, Norwalk, Cl"). Polyethylene glycol Holstein cows were used in a 3 x 3 Latin concentrations were not determined. High persquare design with 2d periods. Treatments formance liquid chromatography on a Beckwere control, NA, or NM. All cows were fed man Ultrasphere-ODS C18 column (Model alfalfa grass haylage, corn silage, high mois- 421 controller, Model lOOA pump, Beckman ture shelled corn, soybean meal, and a trace Instruments, Fullerton, CA) using a Hitachi mineral-vitamin mix. specnophotometer with a multiple wavelength J o d of Dairy Science Vol. 74, No. 10, 1991
3494
BRICKSON ET AL.
W absorbance detector (Model 100-10, Hitachi Ltd, Tokyo, Japan) set at 254 nm was used to analyze NA and NM concentrations. Standard solutions containing analytical grade NA and NM (Sigma Chemical Co., St. Louis, MO) were eluted using a solvent system of .2% (wt/vol) orthophosphoric acid in doubledistilled water at a flow rate of .85 d m i n . There was a linear relationship (r2 > .99)between the concentration of each standard (0 to .2 &) and the area of the chromatogram peak Absorption was determined using the equation of Martens and Stassel (10):
where A = absorption; V = volume (21 L); C, = NA or NM concentration at time = 0 min, C,
= NA or NM concentration at t = 20,40, or 60 min; Co-EMTAi = marker concentration at t = 0 min, and Co-EDTAf = marker concentration at t = 20, 40,or 60 min. Statistical analysis was by general linear models procedure of SAS (Release 5.18, 1986, SAS Institute Inc., Cary, NC). The effects of time and treatment x time on absorption and Co-EDTA concentration were not significant (P > .lo); therefore, mean absorption and CO-EDTA concentration within cow, period, and treatment (n = 9) were the dependent variables. Orthogonal comparisons were control versus niacin and NA versus NM. Significance was determined at P < .10 unless otherwise noted. RESULTS AND DISCUSSION
Nicotinamide was absorbed at a rate of .98 g/h, but NA did not appear to be absorbed from the rumen over the 141 period (Table 2). Lack of NA absorption lowered the overall niacin mean; therefore, the difference between
control and niacin treatments was not significant. Mean Co-EDTA concentration was 5 % greater for the niacin than for the control treatment; however, the reason this occurred is not apparent. The absorption equation would have compensated for such differences. Nicotinic acid may not have been absorbed because its pKa is 4.85 (12). Most of the NA would be ionized at the pH (-7) of the buffer in the rumen, although inclusion of acetate or other VFA in the buffer or rapid fermentation may enhance NA absorption by decreasing ruminal pH. In contrast, N M is weakly acidic. Its pKa is -14 (13). At the buffer pH, most N M would have been in the undissociated form, which may have increased the likelihood of its absorption from the m e n . Supplementation of niacin to the diets of dajr cows was not commonly practiced until it was shown to reduce blood ketone concentrations (6). Earlier work had indicated that niacin reduced blood plasma NEFA concentrations in cows and goats (21, 22, 23). Niacin probably elicits its ketonedepressing effect by reducing lipolysis and NEFA concentrations, possibly by reducing adenyl cyclase activity or stimulating phosphodiesterase activity (5). Niacin increased microbial protein synthesis in some (18, 19), but not other (1, 2), in v i m experiments. Three studies also have suggested that Nmen protozoa numbers are inmased with niacin supplementation (3, 5, 8). AU ciliates that can be cultivated axenically require niacin; both NA and NM are effective for most of the organisms tested (9). These results support the results of other research conducted in our laboratory in which blood plasma NEFA, measured as area beneath an epinephrine-stimulated NJFA response curve, was less (NS) for N M than for NA. However, cows fed NA had increased (NS) protozoa numbers in their ruminal fluid (5).
TABLE 2. Least squares means of nicotinic acid and nicotinamide absorption across the ruminal epithelium and CoEDTA concentration (diluted).
Variable vitamin absorbed,' CO-EDTA,~ppm
a
Coni101
Nicotinic acid
Nicotinamide
SE
-.07 1.66
-.17 1.80
.98 1.72
.26 .02
'Nicotinamide absorption was greater than nicotinic acid absorption (P e .lo). the niacin than the conmi treatmeat (P < .IO).
2The C-EDTA concentration was greater for Journal of Dairy Science Vol. 74, No. 10, 1991
NIACIN ABSORPTION FROM THE RUMBN
Schussler et al. (19) reported that sheep consuming a high concentrate diet and 250 ppm of NA produced ruminal fluid capable of digesting more cellulose in vitro (P < .05) than that from sheep fed a high concentrate diet but not receiving NA. Our results suggest that N M may be more effective in reducing lipolysis, and NA may be more effective in increasing protozoa numbers. These data also suggest that NA may have to be converted to NM before significant absorption from the rumen occurs. Increasing protozoa numbers, specifically entodinia, may result in increased bacterial numbers because entodinia serve to regulate the rumen environment by consuming starch. An increased microbial population may lead to increased microbial protein flows and increased intestinal amino acid supply. This could account for the increased milk protein content and yield observed when niacin was given as a supplement to cows in some studies (4, 18). Additional studies are required to address the effects that differential ruminal absorption of niacin sources may have on ruminal fermentation and animal metabolism. Supplementation strategies might be discovered that will capitalize on this phenomenon. ACKNOWLEDGMENTS
The assistance of Jill Cline, Wellington Nombekela, and Dan Schauff in sampling is gratefully acknowledged. REFERENCES
1 Abdouli, H., and D. M. ScWer. 1986. Effect of two dietary niacin concentrations on ruminal fluid free niacin concentration and of supplemental niacii and source of inocohun on in vitro microbial Browtfi fermentative activity and nicotinamide adenine dinucleotide pool size. J. Anim. Sci. 62254. ZAbdouli, H., and D. M. Schaefa. 1986. Impact of niacin and length of incabation on protein synthesis, soluble to total protein ratio and fermentative activity of rominal microOrganipmJ. J. Anim. Sci 62244. 3Dennis, S. M.,M. J. Arambel, E. E. Bartley, D. 0. Riddell, and A. D. Dayton. 1982. Effect of heated or unheated soybean d with or WithOUl niacin on rumen protozoa. J. Dairy Sci. 65:1643. 4Erickso11,P. S. 1989. Nia~i~-lipid inteIactim in lactating dairy cows. Ph.D. Diss., Univ. Jllinois, Urbana 5Ericks011, P. S., A. M. Trusk, and M. R Murphy. 1990. Effects of niacin source on epinephrine. stimnlation of plasma wnestcrified fatty acid and glucose concentrations, on diet digestiiility aml on rumen
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