THE ROLE OF BILE AND PANCREATIC JUICE IN THE ABSORPTION OF FAT IN EWES AND LAMBS

Intestinal digestibility of long-chain fatty acids in lactating dairy cows: A meta-analysis and meta-regression

The objective of this analysis was to examine the intestinal digestibility of individual long-chain fatty acids (FA) in lactating dairy cows. Available data were collated from 15 publications containing 61 treatments, which reported total and individual FA duodenal flows and calculations of intestinal digestibility. All studies involved lactating dairy cows, and estimates of digestibility were based on measurements either between the duodenum and ileum (18 treatments) or between the duodenum and feces (43 treatments). Fatty acid digestibility was calculated for C16:0, C18:0, C18:1 (cis and trans isomers), C18:2, and C18:3. Digestibility of C18:0 was lower than for C18:1 and C18:3, with no difference in digestibility between saturated FA (C16:0 and C18:0). We weighted the studies by the reciprocal of the variance to generate best-fit equations to predict individual FA digestibility based on duodenal flow of FA and dietary independent variables. The flow of C18:0 negatively affected the digestibility of C18:0 and was also included in the best-fit equations for all other 18-carbon FA using duodenal flow characteristics. The type of fat supplemented had an effect on digestibility of individual FA, with whole seeds having reduced digestibility. Our meta-analysis results showed minimal differences in the digestibility of individual FA. However, C18:0 flow through the duodenum had a negative effect on the digestibility of several individual FA, with the largest negative effect on C18:0 digestibility. The mechanisms that reduce C18:0 absorption at high concentrations are unknown and warrant further investigation.

Age-related changes in the biliary bile acid composition of bovids

The biliary bile acid composition of 12 tribes of bovids (66 species, 168 animals) was determined by high-performance liquid chromatography and mass spectrometry. In adult animals, the biliary bile acids were conjugated with taurine or glycine and consisted mostly (> 90%) of three bile acids: cholic acid (CA), chenodeoxycholic acid (CDCA), and deoxycholic acid (DCA). Biliary bile acid composition did not vary among species, and was identical in male and female bovids. Within each species, there were consistent changes in biliary bile acid composition with age. Three steady-state stages could be distinguished: (1) the fetal stage, when bile acid input is from placental transfer from the mother as well as biosynthesis (from cholesterol) by the newborn liver (45 ± 12% CA; 50 ± 11% CDCA; 5 ± 4% DCA (mean ± SD)); (2) the infant stage, when bile acid input is solely from biosynthesis by the infant liver (80 ± 6% CA; 20 ± 6% CDCA; 0.5 ± 0.7% DCA); and (3) the adult stage, when bile acid input is not only from biosynthesis by the adult liver but also from intestinal absorption of DCA, formed by bacterial 7-dehydroxylation of CA (75 ± 12% CA; 6 ± 7% CDCA; 19 ± 9% DCA). The transition from the infant stage to the adult stage, indicating the development of an anerobic cecum, occurred before weaning. These three stages of biliary bile acid composition are likely to be present in other placental vertebrates, including most primates, in whom a cecum containing an anerobic flora develops after birth; the functional implications of these changes are discussed.

Fat in lactation rations: review

Recent research has demonstrated the effectiveness of added fat in diets to maintain milk production and fat percent. Much of the earlier work which indicated that fat affects digestion negatively may not be applicable because of great differences in the nature of diets and fats fed and especially in total feed intake. Nevertheless, much remains to be learned about interactions of fat, fiber, calcium, and rumen microorganisms if feeding of fat is to be maximized. The uniquely high acidity in the duodenum combined with detergent action of bile acids, lysolecithin, and fatty acids causes saturated fatty acids to be more digestible in ruminants than in nonruminants. Large quantities of added dietary fat increase concentrations in plasma of very low density lipoprotein triglyceride which increases their uptake by the mammary gland with inhibition of short chain fatty acid synthesis and consequent changes in milk fatty acid composition. In some cases, secretion of milk fat is increased. Current research and practice demonstrate that 3 to 5% fat may be added to diets for lactation to increase energy intake of high-producing cows and/or to reduce starch feeding, thereby increasing the ratio of forage to concentrate to prevent depression of milk fat.

A kinetic concepto of lipid transport in ruminants

Summarization of the literature shows a strong correlation between dietary fatty acid intake and total lipid concentration in plasma in lactating cows whereas total milk fat secreted is related to neither of these. In the process of plasma triglyceride removal, chylomicra and very low density lipoproteins are converted to low density lipoproteins. Limited kinetic data indicate that the fractional removal rates for chulomicra and very low density lipoproteins are rapid in lactating cows whereas fractional removal of low density lipoproteins is slower, resulting in accumulation of the latter in plasma. Under such conditions, low density lipoprotein concentrations of plasma would not be expected to reflect quantitatively the transfer of plasma triglyceride fatty acids to milk fat. Quantitative analysis or triglyceride fatty acid turnover in density less than 1.006 lipoproteins should delineate the role of plasma lipid transport in milk fat synthesis. High fat diets protected from rumen biohydrogenation have proven to be a useful approach in studying ruminant fat metabolism and may be used more extensively to elucidate the role of cholesterol in plasma lipid transport and the metabolism of essential fatty acids in ruminants.

Influence of the products of phospholipolysis of phosphatidylcholine on micellar solubilization of fatty acids in the presence of bile salts

1. The solubility of fatty acids in aqueous solutions containing bile salts and lysolecithin at pH values between 2-0 and 7-4 was studied. Both the 1-acyl and 2-acyl isomers of lysolecithin increased the solubility of fatty acids to the same extent, the order of solubility being linoleic greater than oleic greater than elaidic greater than palmitic greater than stearic. 2. The influence of the products of phospholipolysis of lecithin on palmitic acid solubility was determined. On a molar basis, lysolecithin was more effective than were bile salts in promoting the solubilization of the fatty acid. 3. In bile salt solutions in which the phospholipid concentration was constant on a molar basis, in solubility of palmitic acid decreased linearly with the progressive replacement of lecithin by lysolecithin. Palmitic acid was solubilized to the same extent on replacing lecithin with lysolecithin on a constant weight basis. 4. In bile salt solution containing lysolecithin and oleic acid in equimolar amounts, the solubility of palmitic acid was similar to that in bile salt solution containing lecithin in equivalent proportion. 5. The results are discussed in relation to the action of phospholipolytic activity on the intestinal absorption of fatty acids in sheep.

Micellar solubilization of fatty acids in aqueous media containing bile salts and phospholipids

1. The solubility of fatty acids in aqueous media containing bile salts alone and in admixture with either lecithin (phosphatidylcholine) or phosphatidylethanolamine was determined. 2. Over the pH range 2-0-7-4, the order of fatty acid solubility in aqueous solutions containing bile salts was linoleic greater than oleic greater than elaidic greater than palmitic greater than stearic. The solubility of each fatty acid increased as the pH of the miceus solutions of bile salts greatly increased the solubility of palmitic acid and stearic acid. 4. In the presence of bile salts and lecithin, the solubility of oleic acid and elaidic acid decreased with increasing pH of the micellar solution, indicating a competitive effect between the fatty acid anions and lecithin. The solubility of linoleic acid increased linearly with lecithin concentration. 5. Phosphatidylethanolamine as an additive to bile salts increased the solubility of both saturated and unsaturated fatty acids in the pH range 2-3-7-4. The effectiveness of phosphatidylethanolamine as an amphiphile was similar to that of lecithin, although at pH 3.0 fatty acid solubility was greater in the presence of phosphatidylethanolamine. 6. The significance of these findings is discussed in relation to the intestinal absorption of fatty acids in sheep.

Changes in lipid metabolism in genetically different types of calves during chronic hyperthermia

1. The effects of chronically raised body temperature on lipid metabolism were studied in groups of 3-month-old British (Shorthorn × Hereford) and Zebu (Africander × British) bull calves. Calves in control groups at ambient temperature were pair-fed to calves housed in a climate room.

2. In control groups, concentrations of plasma cholesterol and phospholipid were significantly higher and the excretion of fatty acids 14:0, 16:0 and 18:0 was lower in the Zebu than in the British calves.

3. In the climate room the constant daily environmental temperature required to maintain an increase in rectal temperature of 1·3° was 33° for British and 38° for Zebu calves. Changes in respiration rate, water metabolism and nitrogen metabolism associated with hyperthermia were similar in both breeds.

4. Hyperthermia in all animals was associated with lowered plasma concentrations of cholesterol and phospholipid and increases in the ratio of free to total cholesterol. It was without effect on the circulating concentrations of non-esterified fatty acids and about twice the amount of fat was excreted without change in its fatty acid composition.

5. The results showed that, when control of normal body temperature was lost, increased rectal temperature led to similar metabolic changes in animals of both breeds.

Adsorption of palmitic, stearic and oleic acids in the sheep in the presence or absence of bile and-or pancreatic juice

1. The absorption of (3)H- and (14)C-labelled palmitic, stearic and oleic acids under control conditions and in the absence of bile and/or pancreatic juice has been studied in sheep prepared with intestinal cannulae and reentrant fistulation of the thoracic lymph duct.2. Under control conditions the percentage recoveries in thoracic duct lymph of fatty acids introduced into the duodenum were: stearic acid 59.6 +/- 3.2 (mean +/- S.E. of mean), palmitic acid 71.1 +/- 1.2 and oleic acid 80.4 +/- 1.9.3. At the peak of absorption over 90% of the radioactivity was located in the triglyceride fraction.4. It was calculated that between 1900 and 3800 ml. lymph containing up to 20 g lipid flowed daily from the thoracic duct. The major fatty acids of lymph triglycerides were C(16:0) (26%), C(18:0) (39%) and C(18:1) (19%).5. In the absence of pancreatic juice, but in the presence of bile, the absorption of stearic, palmitic and oleic acids was 0.8, 4.-8.5 and 26% respectively of the dose injected into the duodenum. In the absence of bile but presence of pancreatic juice corresponding values were 0.1-0.5, 1.8 and 7.7%.6. In the absence of pancreatic juice the total lipid content of thoracic duct lymph fell from 800 to 80 mg% within 4 hr. When bile was also absent the lipid content of lymph fell further to 50 mg%.7. It is concluded that pancreatic juice functions in the lumen of the intestine probably through the production from biliary lecithin of the lysolecithin necessary for the optimum absorption of lipids.

The influence of biliary constituents in an acid medium on the micellr solubilization of unesterified fatty acids of the duodenal digesta of sheep

1. Duodenal contents were collected from cannulated sheep that were fed on dried grass cubes. After high-speed centrifugation, the distribution of long-chain unesterified fatty acids between the aqueous and particulate fractions of the digesta was examined.

2. The unesterified fatty acids of the digesta were found to be predominantly associated with the particulate matter.

3. When gall-bladder bile was added to the digesta at 37° and pH 4.0, in vitro, it was observed that unesterified fatty acids were transferred from the particulate matter to micellar solution. This transfer was due largely to the solubilizing effect of bile salts acting together with biliary phospholipids.

4. These findings are discussed in relation to the absorption of unesterified fatty acids by the sheep.

The incorporation of linolenic and linoleic acids into the plasma lipids of sheep given intra-abomasal infusions of linseed oil, maize oil or linoleic acid

1. The fatty acid compositions of the plasma cholesteryl esters, phospholipids, triglycerides and unesterified fatty acids were determined in three sheep at various times after they had been given intra-abomasal infusions of emulsions of linseed oil, maize oil or linoleic acid.

2. The concentrations of linolenic acid or linoleic acid in the plasma triglycerides began to increase 1.5 h after infusion of the emulsions had begun. As the concentration of linolenic or linoleic acids in the plasma triglycerides increased, the concentrations of palmitic and stearic acids decreased, hut there were no appreciable changes in the concentrations of oleic acid.

3. The concentrations of linolenic or linoleic acid in the plasma phospholipids and cholesteryl esters did not begin to increase until 8–9 h and 24–25 h respectively after the infusions of the emulsions had begun.

4. It is suggested that, after absorption from the small intestine of the sheep, linolenic and linoleic acids are transported in triglyceride form to the liver where the triglycerides are partially or completely hydrolysed. These C18polyunsaturated acids are then preferentially utilized for the synthesis of phospholipids and cholesteryl esters but not for the re-synthesis of triglycerides.

The absorption of long-chain fatty acids from the small intestine of the sheep

1. Three sheep, each of which was fitted with a rumen cannula and with two pairs of reentrant cannulas in different parts of the small intestine, were used in this study. They were fed on dried grass cubes or hay plus linseed meal and oats: an aqueous solution of polyethylene glycol (PEG) was infused continuously into the rumen.

2. Total lipids were extracted from samples of the chyme entering and leaving the different lengths of the small intestine embraced by the respective cannulas. The lipids were fractionated into unesterified fatty acids, neutral lipids and phospholipids and the contribution of each fraction to the total fatty acids was determined. The samples were also analysed for their PEG content, thus affording an index of the extent to which water had been absorbed from each particular length of intestine.

3. From the above findings and a knowledge of the flow-rate of the digesta, the uptake of unesterified fatty acids and the degree of dissimilation or uptake, or of both, of esterified fatty acids was calculated.

4. The results indicated that, by the time the digesta reached the ileum (i.e. the distal half of the small intestine), the uptake of fatty acids was almost complete, as was also the hydrolytic release of esterified fatty acids.

5. Though there were no gross differences in the overall composition of the unesterified and esterified fatty acids in different parts of the small intestine, it appeared that C18mono-unsaturated acid, the principal unsaturated unesterified acid, was absorbed somewhat more efficiently than were the major saturated acids (palmitic acid and stearic acid).