Digestion and Absorption of Lipids and Bile Acids in Sheep Fed Stearic Acid, Oleic Acid, or Tristearin

Digestion and Absorption of Lipids and Bile Acids in Sheep Fed Stearic Acid, Oleic Acid, or Tristearin D. SKLAN, A. ARIELI,W. CHALUPA, and D. S. KRONFELD Faculty of Agriculture Hebrew University Rehovot 76-100, Israel and School of Veterinary Medicine

University of Pennsylvania Kennett Square 19348 ABSTRACT Sheep were fed diets containing 7.5% added stearic acid, oleic acid, or tristearin for 21 days. In addition, 50 /aCi/kg cerium-141 was included for the last 10 days on experimental diets as an unabsorbed reference substance. In the rumen dietary triglycerides were approximately 50% hydrolyzed, and hydrogenation resulted in saturation of the free fatty acid fraction. Some net synthesis of phospholipids, presumably microbial phospholipids, occurred in the rumen. In the intestine immediately distal to the pylorus, extensive secretion of bile acids, cholesterol, phospholipids, triglycerides, free fatty acids, and lipase occurred. This resulted in doubling of fatty acid fluxes through the duodenum. These endogenous secretions were reabsorbed rapidly however, with the major site of lipid and bile acid absorption in the region .6 to 4 m distal to the pylorus. Additional but less absorption occurred in the more distal segments of the small intestine. Overall absorption of stearic acid, oleic acid, and tristearin supplements was in the range 60 to 70%, and no differences were apparent between fats. Unsaturated fatty acids were over 90% absorbed as compared with 55 to 65% for saturated fatty acids. No significant effect of any of the supplements was observed on ruminal total volatile fatty acids,

Received February 29, 1984. 1985 J Dairy Sci 68:1667-1675

ratios of volatile fatty acids, or on overall cellulose or caloric digestion.

INTRODUCTION Ruminant diets generally contain up to 4% lipids, which are mainly esterified fatty acids. These are hydrolyzed rapidly in the rumen by lipolytic bacteria (9,23). Biohydrogenation rapidly follows and appears to depend on a free carbonyl group (12); thus, lipolysis may be the rate-limiting step in biohydrogenation (21). Dietary fatty acids are incorporated readily into intracellular microbial lipids, mainly as phospholipids and nonesterified fatty acids. Bacteria and protozoa are capable of both de novo synthesis and of catabolism of fatty acids, and net effects on fatty acids have been variable in the rumen (23). Addition of fat to ruminant diets depresses fiber digestibility (4,33). This effect has been attributed to several factors: physical coating of the fibers with fat preventing microbial attack, reduced microbial activity due to adsorption of fat onto the microbial surface, or to reduced cation availability due to binding to fatty acids (6,7,20). Unsaturated fatty acids appear to be more toxic than saturated fatty acids to ruminal microbes (13). Digestion and absorption in the intestine of ruminants differ from nonruminants both in the nature and physical form of the lipids entering the small intestine. Also, pH of the proximal small intestine is lower than for nonruminants (16). Moreover, the relative importance of phospholipids, and in particular of lysolecithin, in intestinal micellarization at low pH appears to be greater than in non1667

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SKLAN ET AL.

ruminants (17,18,29). Overall fat absorption from conventional ruminant diets (3 to 5% fat) is generally in the region of 80%, but it is reduced to 55 to 60% on adding up to 10% fat. Overall digestibility of individual fatty acids ranges from 60% for stearic acid to over 90% for linoleic and linolenic acids (1,2,19,20,22). Increasing the fat content of diets for ruminants could enhance energy intake (22) and improve the balance of lipogenic to glucogenic nutrients for specific purposes (14). These potential advantages must be weighed against the adverse effect on fermentative cellulose digestion (4, 33). Supplementary fats preferably should not be metabolized in the rumen and should not impair fermentation (15). These inert fats also should be digested efficiently and absorbed further along the digestive tract. The present report concerns estimates of digestion and absorption of certain fats in sheep with radioactive cerium (141Ce) as an unabsorbed reference substance.

MATERIALS AND METHODS

Twelve crossbred sheep were transferred to experimental diets at the age of 12 wk and 15 to 17 kg body weight. These diets consisted of a basal portion of 20% soybean meal (45% crude protein), 11% wheat bran, 35.7% ground corn, 25% vetch hay (25% fiber, 16% crude protein), .5% calcium carbonate, .1% sodium chloride, and .2% commercial salt and vitamin mix. The latter provided (mg/kg): zinc 75, manganese 30, iron 30, copper .5, I .1, cobalt .1, as well as 13.4 mg tocopherol equivalents, 21.1 gg cholecalciferol, and 50 /~g retinol equivalent. Addition of 7.5% of three Iipids were the experimental treatments. Treatment SA was stearic acid (technical grade, Aldrich Chemical Co., WI), treatment OA was oleic acid (technical grade, BDH Chemical Co., Poole), and treatment TS was tristearin (Fluka AG, Buchs). Experimental diets were fed for 21 days and during the last 10 days included 50 /~C/kg 141Ce (Radiochemical Centre, Amersham). Feed consumption averaged 690 to 720 g/day. At the end of the experimental period animals were killed with an intravenous overdose of sodium pentobarbital; the gastrointestinal tract was exposed rapidly, and contents of Journal of Dairy Science Vol. 68, No. 7, 1985

intestinal segments and gall bladder were extruded gently, cooled, stored at 4°C, transferred to the laboratory, and then stored at - 2 0 ° C until analysis. Net weight of intestinal contents was determined, and samples were homogenized in a high speed homogenizer and samples taken for analyses. Chemical Analyses

Counts of 141Ce in intestinal samples were by scintillation. In an additional sample, pH was brought to 8.0 with .5 M sodium carbonate before lyophilizing and adiabatic bomb calorimetry. Samples also were taken for volatile fatty acid (VFA) determination on 20% carbowax 20M with 3% phosphoric acid by gas chromatography. Further samples were lyophilized for bile acid determinations, as described by Singer and Fitschen (25), and for lipid extractions. Total fatty acids were determined by gas chromatography following addition of heptadecanoic acid as internal standard. Neutral lipids, cholesterol, and phospholipids (PL) were separated by thin-layer chromatography after addition of internal standards of heptadecanoic acid, triheptadecanoin, and diheptadecanoyl phosphatidyl choline (Sigma Chem. Co., St. Louis, MO) with petroleum ether:ether:acetic acid (80:20:1 vol/vol) and PL with chloroform:methanol:water:acetic acid (65:25:2:2) as developing solvents and quantitated by gas chromatography as in (29). Lipase activity was determined titrometrically essentially as described by Sarda and Desnuelle (24). Calculations

When an unabsorbed reference substance is fed until steady state, the ratio between any dietary component and the unabsorbed reference substance can be used to calculate net changes between segments. A decrease of ratio is defined as disappearance (digestion, absorption, catabolism) and an increase of ratio as appearance (secretion or production). Daily fluxes can be calculated b y the ratio at any point multiplied by the daily intake of the unabsorbed reference substance (29). Statistical significance between treatments was tested by analysis of variance (31).

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LIPID DIGESTION AND ABSORPTION IN SHEEP RESULTS

I

During the e x p e r i m e n t a l period animals grew at 292 + 49, 221 ± 53, and 249 + 33 g / d a y for the SA, OA, and TS t r e a t m e n t s (mean -+ SD, n=4); no differences b e t w e e n t r e a t m e n t s were significant ( P > . I ) . Details o f recovery and intestinal distribution of 141Ce are in (28). Counts o f the unabsorbed reference substance were used to calculate ratios of bile acids and cholesterol to 141Ce in the small intestine (Figure 1). Major secretion of bile acids and cholesterol occurred in the p r o x i m a l small intestine some 40 to 60 cm distalto the pylorus. Daily secretion of bile salts and cholesterol was 5.6 -+ .6 and .15 -+ .03 g/kg b o d y weight per day. R e s o r p t i o n p r o c e e d e d rapidly with app r o x i m a t e l y 70 to 75% of secreted bile salts and cholesterol taken up within 3 m f r o m the pylorus. A b s o r p t i o n o f both bile salts and cholesterol continued in m o r e distal segments of the intestine but slower. In the large intestine no f u r t h e r changes were observed. Overall absorption (calculated b e t w e e n d u o d e n u m and feces, mean -+ SD, n=12) o f bile acids was 94 -+ 2% and of cholesterol was 75 ± 3%. Lipase activity was d e t e r m i n e d in intestinal contents, and the ratio of activity to 141 Ca was calculated (Figure 2). Maximal activity was in the intestinal area 40 to 60 cm distal to the pylorus. This activity decreased rapidly with increasing distance f r o m the pylorus so that activity in the terminal ileum and large intestine was minimal. No differences in luminal lipase activity were significant b e t w e e n the three dietary t r e a t m e n t s ( P > . I ) . In a similar m a n n e r ratios o f total PL, phosphatidyl choline (PC), p h o s p h a t i d y l etha-

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Figure 2. Lipase activity to x41Ce along the gastrointestinal tract. Means -+ SD from four animals fed the stearic acid diet. C represents cecum, LI mean of two samplings 1 and 3 m from the cecal junction, and F feces.

n o l a m i n e (PE), and lyso phospholipids (lysoPL) to 141Ce were calculated along the gastrointestinal tract. Ratios of PL and PC to 141 Ce are in Figure 3. T o t a l PL increased in the r u m e n by 60 to 80% over that in the feed, and the PL comprised over 85% PC with small a m o u n t s of PE. This net increase o f PL was apparent in rumen, reticulum, omasum, and abomasum. In the d u o d e n u m , secretion o f PL was intense, especially in the segment 20 to 40 cm distal to the pylorus, increasing PL fluxes 6 times. Bile contained 11.2 + 3.7 m g / m l (mean -+ SD) PL, which was over 90% PC, and this resulted in daily secretion o f pbospholipids of .77 ± .23 g/kg b o d y weight per day. The composition of PL in the intestinal segments 20 to 200 cm distal to the pylorus was 64 to 67% PC, 7 to 16% PE, and 16 to 23% lysoPL. In m o r e distal segments the p r o p o r t i o n of PE decreased rapidly to less than 5% of total PL

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Figure 1. Ratios of bile acid (open circles) and cholesterol (filled circles) to 141Ce ratios along the gastrointestinal tract. Means +- SD are from 12 animals. C represents cecum, LI mean of two samplings 1 and 3 m from the cecal junction, and F feces.

FROM

PYLORUS (rn)

Figure 3. Ratios of total phospholipid (open circles) and phosphatidyl choline (filled circles) to ~4zCe along the gastrointestinal tract of the sheep. R represents lumen, RE reticulum, O omasum, A abomasum, C cecum, LI mean of two samplings i and 3 m from the cecal junction, and F feces. Journal of Dairy Science Vol. 68, No. 7, 1985

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SKLAN ET AL.

TABLE 1. Fatty acid composition of the lipid fractions at some sites along the gastrointestinal tract of sheep stearic acid) Site

16:0

Triglycerides Feed Rumen Abomasum Duodenum Jejunum Ileum

X 18.3 17.6 18.5 19.1

18:0

18:1

18:2

(%)

18.0

SD 1.8 2.1 .1 2.4 4.4 2.2

X 18.3 16.0 18.0 55.6 47.8 56.3

SD 2.1 3.0 1.1 1.7 4.8 5.0

X 20.4 23.2 20.7 13.8 17.3 14.0

SD 1.3 3.5 3.0 2.0 1.4 1.0

X 41.0 42.6 41.6 9.2 6.3 6.4

SD 4.3 2.8 1.9 2.0 1.9 1.2

5.3 5.2 4.8 12.1 4.6 5.0

1.2 .9 .8 1.3 .5 .4

85.0 85.2 89.2 66.6 87.0 90.8

2.4 3.0 4.1 6.0 2.3 1.3

4.4 4.0 4.1 10.2 3.6 2.4

.2 1.0 .3 2.0 .9 .6

5.0 1.5 1.3 8.5 1.0 1.1

1.0 .3 .4 1.4 .3 .3

Feed

26.6

2.5

Rumen Abomasum Duodenum Jejunum Ileum

45.9 45.8 30.2 37.2 32.8

1.4 3.3 2.4 2.6 4.3

11.8 16.0 15.6 24.7 33.7 28.4

1.9 1.1 1.1 3.8 3.6 3.1

17.4 15.9 16.2 21.9 12.8 16.8

1.9 1.3 .6 4.3 1.2 1.4

38.5 17.0 17.2 19.7 11.9 8.4

2.1 1.6 1.8 3.6 1.8 1.4

Free fatty acids Feed Rumen Abomasum Duodenum Jejunum Ileum

23.2

Phospholipids

1 Results are means -+ SD from four animals. Duodenum refers to a site 40 to 60 cm distal to the pylorus, jejunum to a site 3 to 4 m distal to the pylorus, and ileum to the intestinal site 50 cm proximal to the ileocecal junction.

within 4 m from the pylorus, whereas that of lysoPL d e c r e a s e d m o r e slowly r e m a i n i n g 10 to 15% t h r o u g h t h e t e r m i n a l ileum. Disappeara n c e o f PL was rapid in t h e p r o x i m a l small i n t e s t i n e w i t h t h e rate decreasing w i t h increasing d i s t a n c e f r o m t h e p y l o r u s ; 90% o f s e c r e t e d PL was r e a b s o r b e d w i t h i n 4 m o f t h e pylorus. F a t t y acid c o m p o s i t i o n s o f PC a n d PE f r a c t i o n s were similar in r u m e n , r e t i c u l u m , o m a s u m , a n d a b o m a s u m , b u t t h e F A profile c h a n g e d in t h e p r o x i m a l small i n t e s t i n e w i t h 16:0 content decreased and 18:0 content increased. Distally t h e F A c o m p o s i t i o n t e n d e d t o w a r d increased 1 6 : 0 a n d d e c r e a s e d 1 8 : 2 (Tables 1 to 3). T h e lysoPL c o n t a i n e d over 70% s a t u r a t e d F A in t h e s t o m a c h s a n d o v e r 85% s a t u r a t e d F A in t h e i n t e s t i n e s ( d a t a n o t s h o w n ) . No d i f f e r e n c e s in PL secretion, absorpt i o n , or c o m p o s i t i o n were d e t e c t e d b e t w e e n t h e three dietary treatments (P>.I). R a t i o s o f t o t a l f a t t y acids, w h i c h i n c l u d e d PL, triglycerides (TG), a n d free f a t t y acids Journal of Dairy Science Vol. 68, No. 7, 1985

( F F A ) , to 141Ce are t o g e t h e r w i t h stearic acid p e r c e n t a g e in Figure 4 as p e r c e n t a g e o f t h e ratio in t h e feed. Little n e t change o c c u r r e d in a m o u n t s o f f a t t y acids p r o x i m a l to t h e d u o d e n u m . S e c r e t i o n t o t h e d u o d e n u m distal to t h e p y l o r u s resulted in 2 to 3 t i m e s increase o f f a t t y acid flux in t h e s e s e g m e n t s ; daily secret i o n o f f a t t y acids was 1.64 + .48 g/kg b o d y weight per day. H o w e v e r , r e a b s o r p t i o n occurred rapidly, r e a c h i n g p r e d u o d e n a l c o n c e n t r a t i o n s w i t h i n 3 m o f t h e pylorus. A b s o r p t i o n c o n t i n u e d at decreasing rates into t h e ileum with additional minor disappearance of fatty acids in t h e large intestine. T h e p r o p o r t i o n of 1 8 : 0 increased in r u m e n , r e t i c u l u m , a n d o m a s u m in all t r e a t m e n t s , d e c r e a s e d in t h e p r o x i m a l small intestine, and t h e n increased reaching over 90% of F A in SA a n d a p p r o x i m a t e l y 70% in b o t h O A and TS t r e a t m e n t s in t h e large intestine. Overall p e r c e n t a g e a b s o r p t i o n o f t o t a l f a t t y acids was 61 t o 69% ( T a b l e 4), and n o d i e t a r y d i f f e r e n c e s were significant. Overall

LIPID DIGESTION AND ABSORPTION IN SHEEP

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TABLE 2. Fatty acid composition of the lipid fractions at some sites along the gastrointestinal tract of sheep oleic acid. 1 Site

16:0

18:0

18:1

18:2

(%) Triglycerides Feed Rumen Abomasum Duodenum Jejunum Ileum

X 14.9 18.6 14.0 27.2 26.3 31.5

SD 2.4 1.4 1.6 3.1 2.1 4.6

X 8.2 15.6 19.9 22.0 25.8 34.3

SD .7 2.4 3.5 3.7 5.0 2.8

X 26.3 28.4 29.1 28.1 26.0 21.8

SD 2.8 2.2 1.6 3.9 1.8 1.3

X 45.9 33.0 32.1 19.5 16.4 10.4

SD 3.2 3.4 2.4 3.1 1.9 .5

Free fatty acids Feed Rumen Abomasum Duodenum Jejunum Ileum

9.1 13.6 10.8 19.0 17.0 15.8

.8 1.4 1.5 2.2 2.3 2.1

1.5 47.5 56.6 35.6 56.0 59.0

.3 4.0 2.7 3.7 3.4 4.8

68.6 29.9 28.9 32.8 20.0 17.3

2.5 2.4 1;4 1.5 2.7 2.5

18.3

1.8

4.3 4.2 9.0 3.7 3.3

.9 .5 1.6 1.0 .7

Pho spholip ids Feed Rumen Abomasum Duodenum Jejunum Ileum

23.3 41.9 37.4 34.0 40.6 36.1

2.7 4.8 2.1 4.4 3.5 3.5

7.2 16.7 18.1 19.9 26.8 28.6

1.8 .9 1.3 1.9 4.1 4.1

23.7 19.3 22.4 27.4 15.5 18.8

3.7 1.2 1.6 1.5 1.8 1.6

41.2

2.2

15.5

1.0

16.9 14.9 11.9 9.4

1.4 2.7 1.8 .8

1 Results are means +- SD from four animals. Duodenum refers to a site 40 to 60 cm distal to the pylorus, jejunum to a site 3 to 4 m distal to the pylorus, and ileum to the intestinal site 50 cm proximal to the ileocecal junction. a b s o r p t i o n o f 16:0 varied b e t w e e n 57 to 70%, w h e r e a s t h a t o f 18:0 i n c l u d e d n e t p r o d u c t i o n f r o m m o r e u n s a t u r a t e d f a t t y acids. B o t h 18:1 and 18:2 s h o w e d a b s o r p t i o n - d i s a p p e a r a n c e o f over 90% (Table 4). F r a c t i o n a t i o n o f lipids into TG and F F A b y thin-layer c h r o m a t o g r a p h y allowed calculation o f ratios o f TG and F F A to 14a Ce, and t h e s e are for SA and TS in Figure 5. The TG and F F A d a t a f r o m O A sheep w e r e similar to t h o s e o f SA and are n o t s h o w n . F i f t y p e r c e n t or m o r e dietary TG was c o n v e r t e d to F F A in t h e r u m e n in b o t h t r e a t m e n t s , and s o m e additional disapp e a r a n c e o f TG o c c u r r e d in r e t i c u l u m and omasum. Distal to t h e pylorus, s e c r e t i o n o f b o t h TG and F F A was f o u n d , and this was similar in b o t h SA-fed and TS-fed sheep. This s e c r e t i o n was f r o m t h e bile, w h i c h c o n t a i n e d , in a d d i t i o n to PL, .67 m g / m l TG and 1.08 m g / m l F F A . The m a j o r site o f a b s o r p t i o n o f F F A was in t h e p r o x i m a l small intestine. A b s o r p t i o n c o n t i n u e d m o r e slowly in the ileum. The TG were a l m o s t c o n s t a n t f r o m 4 m distal to t h e

p y l o r u s in SA sheep b u t c o n t i n u e d to decrease in TS animals until approximately 10 m distal to t h e pylorus. F a t t y acid profiles o f the m a j o r lipid fract i o n s at s o m e sites along the g a s t r o i n t e s t i n a l t r a c t are in Tables 1 to 3. In all t r e a t m e n t s r u m i n a l TG r e f l e c t e d f e e d TG, and t h e secret i o n o f TG into t h e d u o d e n u m c o n t a i n e d less 18:2. Distal to the d u o d e n u m , a small decrease o f u n s a t u r a t i o n was a p p a r e n t . F e e d F F A w e r e m u c h m o r e u n s a t u r a t e d t h a n r u m e n F F A , and 18:2 d i s a p p e a r e d a l m o s t c o m p l e t e l y f r o m F F A b y the a b o m a s u m ; s o m e s e c r e t i o n o f 18:2 and 2 0 : 4 (generally less t h a n 2%) o c c u r r e d in t h e duodenum, but unsaturates decreased with d i s t a n c e f r o m t h e pylorus. R u m i n a l PL possess a d i f f e r e n t F A profile to t h e feed with m o r e 16:0 and less 18:2. In t h e d u o d e n u m , the F A profile again changed with 16:0 and 18:2 decreasing; u n s a t u r a t e s again decreased with d i s t a n c e f r o m t h e pylorus. R u m i n a l V F A and ratio o f acetic plus b u t y r i c acid to p r o p r i o n i c acid are in Table 5. No e f f e c t s o f d i e t a r y lipids o n a m o u n t s o f V F A

slightly

Journal of Dairy Science Vol. 68, No. 7, 1985

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SKLAN ET AL.

TABLE 3. Fatty acid composition of the lipid fractions at some sites along the gastrointestinal tract of sheep tristearin, Site

16:0

18:1

18:0

18:2

(%) Triglycerides Feed Rumen Abomasum Duodenum Jejunum Ileum

X 24.5 26.6 27.7 27.7 25.3 27.7

SD .3 .7 1.0 .8 2.0 .8

X 60.3 60.1 62.0 64.0 63.6 64.6

SD 1.8 2.1 4.3 2.0 1.9 1.0

X 8.0 6.3 5.6 2.6 6.0 5.6

SD .8 .9 .8 .4 2.1 1.3

X 6.6 8.3 4.3 5.3 3.8 2.1

SD 1.3 1.0 .6 .6 .6 .4

Free fatty acids Feed Rumen Abomasum Duodenum Jejunum Ileum

23.2 20.2 17.6 21.5 24.5 21.1

1.8 2.4 1.6 2.7 1.8 2.2

23.1 61.5 63.3 40.8 62.6 74.1

3.1 2.9 1.9 3.6 2.0 3.8

20.2 14.0 13.8 27.2 6.1 3.0

3.0 4.0 1.9 3.8 .9 .3

33.2 4.0 3.3 8.4 2.8 1.4

3.7 .4 .4 1.3 .6 .4

Phospholipids Feed Rumen Abomasum Duodenum Jejunum Ileum

27.0 45.6 39.4 23.4 40.9 33.0

3.2 1.6 3.1 1.6 3.8 5.0

14.2 18.6 16.0 31.7 26.1 27.0

1.8 2.4 1.8 4.2 1.8 2.3

17.8 17.6 18.9 24.8 14.2 21.1

2.7 .9 2.6 3.8 2.3 3.6

31.6 16.0 18.7 13.7 10.8 8.4

3.6 2.8 2.6 2.1 1.5 1.1

1 Results are means -+ SD from four animals. Duodenum refers to a site 40 to 60 cm distal to the pylorus, jejunum to a site 3 to 4 m distal to the pylorus, and ileum to the intestinal site 50 cm proximal to the ileocecal junction. DISCUSSION

or mo l a r p r o p o r t i o ns were significant (P> .I). Overall caloric d i g e s t i o n as d e t e r m i n e d b y c a l o r i m e t r y o f f e e d a n d f e c e s w a s also s i m i l a r in t h e t h r e e t r e a t m e n t s a n d r a n g e d f r o m 72 t o 79%, a n d overall cellulose d i g e s t i o n w a s 58 t o

In o u r o b s e r v a t i o n s a p p r o x i m a t e l y 50% o f d i e t a r y T G w e r e h y d r o l y z e d in t h e r u m e n , b o t h in d i e t s w i t h n o a d d e d T G a n d in T S - f e d s h e e p . The TG that remains u n h y d r o l y z e d m a y be

61% f o r all d i e t s ( T a b l e 5).

w i t h i n f o o d p a r t i c l e s a n d n o t e x p o s e d t o lipase.

TABLE 4. Intake and overall absorption of fatty acids in the gastrointestinal tract of the sheep. 1 Total fatty acids

16:0

18:0

18:1

18:2

(g/kg body w e ~ h t p e r d a y ) Stearic acid Intake Absorption

X 2.99 1.94

SD .20 .12

X .19 .14

SD .02 .01

X 2.31 1.35

SD .06 .03

X .17 .17

SD .01 .00

X .28 .27

SD .01 .00

Oleic acid Intake Absorption

3.11 2.14

.17 .13

.33 .19

.02 .01

.08 --.50

.01 .03

1.82 1,69

.06 .12

.78 .76

.04 .05

Tristearin Intake Absorption

3.17 1.94

.23 .12

.73 .45

.04 .02

1.74 .91

.05 .02

.23 .21

.03 .00

.36 .35

.03 .01

1

Means _+ SD are from 4 animals.

Journal of Dairy Science Vol. 68, No. 7, 1985

LIPID DIGESTION AND ABSORPTION IN SHEEP

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Figure 5. Ratio of triglyceride (open circles, triangles) and free fatty acid (filled circles, triangles) to ~'1Ce in the gastrointestinal tract of sheep fed stearic acid (circles) or tristearin (triangles). F (left) represents feed, R rumen, RE reticulum, O omasum, A abomasum, C cecum, LI mean of two samplings 1 and 3 m from the cecal junction, and F (right) feces.

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]6

C L1 F

Figure 4. Total fatty acid (open circles) to t4~ Ce ratios as a percentage of amount found in the feed and stearic acid (filled circles) as a percentage of total fatty acids in the gastrointestinal tract of sheep. SA represents sheep fed the stearic acid diet, OA oleic acid diet, and TS the tristearin diet, R rumen, RE reticulum, O omasum, A abomasum, C cecum, LI mean of two samplings 1 and 3 m from the cecal junction, and F feces. H y d r o g e n a t i o n o f u n s a t u r a t e d f a t t y acids f o l l o w s h y d r o l y s i s in t h e r u m e n and reticulum, resulting in an F F A f r a c t i o n w i t h less t h a n 5% dienes in the a b o m a s u m . Net s y n t h e s i s o f PL

in the r u m e n p r o b a b l y r e p r e s e n t s microbial p h o s p h o l i p i d s . Little change o f TG and PL f a t t y acids o c c u r r e d b e t w e e n r u m e n and abom a s u m . These o b s e r v a t i o n s are c o n s i s t e n t with views o f ruminal lipid m e t a b o l i s m (8,11, 23) and s u p p o r t t h e c o n j e c t u r e t h a t a free c a r b o x y l g r o u p is a p r e r e q u i s i t e for h y d r o g e n a t i o n in t h e r u m e n (12). The p r o x i m a l d u o d e n u m r e p r e s e n t s t h e site o f extensive s e c r e t i o n o f bile and p a n c r e a t i c juice to the intestine. As e x p e c t e d , a considerable a m o u n t o f bile salts, t o g e t h e r with PL, TG, and F F A are f o u n d w i t h lipase activity. These c o m p o n e n t s were all d e t e c t e d to a small e x t e n t i m m e d i a t e l y distal to t h e pylorus, t h a t is, proximal to bile and p a n c r e a t i c ducts. I~ is p r o b a b l e t h a t s o m e c o u n t e r c u r r e n t m o v e m e n t takes place in this intestinal area as in birds (5,30).

TABLE 5. Rumen volatile fatty acids and energic and cellulose digestion)

Treatment

Volatile fatty acids

(C2+C4)/C3

CaIoric digestion

X 2.30 2.31 2.68

X 74.6 78.6 72.3

--(raM) -Stearic acid Oleic acid Tristearin

X 79.8 85.5 82.8

SD 4.6 5.1 3.9

Cellulose digestion (%)

SD .27 .28 .50

SD 4.5 8.0 3.5

X 58 61 60

SD 7 5 7

1Means _+SD are from four animals, no differences between treatments were significant (P>.05). Journal of Dairy Science VoL 68, No. 7, t985

1674

SKLAN ET AL.

Secretions to the duodenum doubled fatty acid flux through the duodenum, and immediately following these massive duodenal secretions of bile acids and lipids, rate of absorption was rapid in the intestinal area immediately distal to pancreatic and bile ducts and continued more slowly in jejunum and ileum. The major PL of bile was PC. Phosphatidyl ethanolamine and lysoPL were in high concentrations only in the 3 m distal to the pylorus. Lipase activity and disappearance of TG continued, in particular, in TS animals where the substrate concentration was high until 10 m distal to the pylorus. The major site of absorption of bile acids was the proximal small intestine as in rats and birds (26), although birds showed an even higher proportion of total bile salt absorption in the duodenum (26,29). The major absorption site of cholesterol and fatty acids was also in the proximal small intestine. Fatty acid absorption in birds was maximal at a site somewhat more proximal than in the sheep, which was similar to rat (31). Composition of fatty acids in lipid fractions of the gastrointestinal tract suggests that bile PL are reabsorbed selectively more rapidly than the PL reaching the intestine from the stomach. Likewise, the duodenal TG FA profiles reflect the influx of bile, but this is not apparent in the jejunum and ileum. A similar preferential absorption of bile lipids that are secreted in close contact with the factors required for micellarization has been reported in chicks (27). In the distal small intestine and large intestine, unsaturated fatty acids of all fractions including PL and TG were decreased. Hydrogenation of free acids in the lower gastrointestinal tract of sheep has been reported (33), and it may be possible that a free carboxyl group may not be required for hydrogenation in this intestinal segment. Dietary fats had no significant effect on carbohydrate metabolism in the rumen as no changes of total VFA concentration or ratios of acetic plus butyric:propionic were observed. In addition, both cellulose digestion and caloric absorption were not influenced by dietary fat. This is despite the expected more toxic effect of the OA diet. However, it is possible that with higher proportions of fiber in the diet, a significant effect may occur. All three fat supplements were absorbed to a Journal of Dairy Science Vol. 68, No. 7, 1985

similar extent, and overall absorption was approximately 60 to 70% as in (1,2,10). In sheep fed OA, hydrogenation and absorption resulted in disappearance of almost all of the high intake of 18:1 and a relatively low disappearance of 18:0. However, in the diets where 18:0 was the major fatty acid, absorption was not less than that in OA sheep. The rate of hydrolysis of TG did not seem to limit fatty acid absorption in TS-fed animals. These studies demonstrated efficient postruminal digestion and absorption of fats that are inert in the rumen (3,15). This increases the feasibility of using inert fats as components of ruminant rations, especially those of lactating cows (14,15). ACKNOWLEDGMENTS

This research was supported by grant US310-80 from the United States-Israel Binational Agricultural Research and Development Fund. Skilled technical assistance was from O. Kedar.

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Journal of Dairy Science Vol. 68, No. 7, 1985