Influence of dietary lipids on intestinal bile acid absorption

The enterohepatic circulation and the inability of upper small intestine to actively absorb bile acid are physiological adaptations for maintaining adequate bile acid concentrations in the intestinal lumen for use in lipid digestion and absorption. Certain lipids inhibit bile acid absorption suggesting a possible role of lipids in this scheme. Using isolated intestinal villi preparations of hamster ileum, experiments were conducted to assess the degree of inhibition of bile acid absorption by lipids of various classes and to determine the possible mechanism of inhibition. At an initial bile acid concentration of 10.0 mM, triolein significantly reduced villus uptake of taurocholic acid by 50% and cholic acid by 38%. This inhibition was similar to the degree of inhibition produced by oleic acid (58 and 48%, respectively). Likewise, representative medium-chain and short-chain triglycerides inhibited taurocholic acid uptake by 35 and 39%, respectively. Results show that triglycerides as well as oleic acid inhibit ileal bile acid uptake. Neither oleic acid nor triolein altered bile acid uptake when micelles were absent from incubation solutions. Furthermore, lipids did not alter absorption of a nonmicelle-forming bile acid, taurodehydrocholic acid. These data imply that dietary lipids in general may inhibit intestinal bile acid absorption. Oleic acid significantly reduced the intermicellar bile acid concentration from 8.9 +/- 0.2 mM to 3.9 +/- 0.2 mM while tributyrin, tricaprylin, and triolein had no effect. Results from these studies suggest that the mechanism of inhibition appears to be an enhancement of micelle formation. We speculate that this mechanism may be an additional mechanism for maintaining adequate luminal bile acid concentrations and may be the pathophysiologic mechanism contributing to bile acid malabsorption in cystic fibrosis.     

Further mechanistic study on intestinal absorption enhanced by unsaturated fatty acids: reversible effect by sulfhydryl modification.

In order to study the relationship between the sulfhydryl (SH) modification of membrane-associated proteins and the oleic acid-induced permeability enhancement of the colonic mucosa, in vitro and in situ absorption studies were performed using rat colon and carboxyfluorescein as an impermeable dye. The pretreatment of the mucosa with diamide, a bifunctional sulfhydryl modifier, in in vitro experiments with the everted colonic loops reduced the absorption enhancing effect of oleic acid in a concentration-dependent manner, less inhibitory effect, though just a little, was observed as compared to N-ethylmaleimide. The inhibition caused by the addition of diamide was absolutely restored by exposure of the mucosa to dithiothreitol. On the other hand, these SH modifiers showed no pronounced effect on the in vivo permeability of quinine which is well-known to be absorbed by a passive transport system mainly via the membrane lipid bilayer. These results obtained in the present study have identified an important role of the functional SH groups of membrane proteins on modulating the permeability alteration of the mucosal epithelium provoked by oleic acid. Furthermore, the SH proteins have been revealed as being unimportant in the intestinal absorption of lipoid-soluble compounds.     

Inhibitory effect of gymnemic acid on intestinal absorption of oleic acid in rats

Gymnemic acid, a mixture of triterpene glycosides extracted from the leaves of Gymnema sylvestre, is known to inhibit the intestinal absorption of glucose in human and rats. This work examined the effect of gymnemic acid on oleic acid absorption by the method of intestinal perfusion in rats. The results showed the following. (i) Gymnemic acid potently inhibited the absorption of oleic acid in intestine. (ii) This inhibition was dose dependent and reversible. (iii) The extent of inhibition and the recovery progress were extremely similar to that of glucose absorption. (iv) Taurocholate did not affect the inhibitory effect of gymnemic acid on oleic acid absorption, but lowering its concentration facilitated the recovery from the inhibition. (v) The absorption of oleic acid was not affected by other glycosides such as phloridzin, stevioside, and glycyrrhizin. These new findings are important for understanding the roles of gymnemic acid in therapy of diabetes mellitus and obesity.     

Use of soybean flour (dehulled, solvent-extracted soybean) as a fish meal substitute in practical diets for African catfish, Clarias gariepinus (Burchell 1822): growth, feed utilization and digestibility

Growth and digestibility trials were conducted using African catfish, Clarias gariepinus (Burchell 1822): (1) to obtain apparent digestibility coefficient (ADC) values for capelin fish meal, soybean flour and corn meal; (2) to formulate diets based on ADC values of the protein feedstuffs; and (3) to evaluate the effects of replacing 25%, 50% and 75% of fish meal in control diets with soybean flour on growth, feed utilization efficiency and carcass composition. Supplemental methionine was added to the diet formulation in which soybean flour replaced 75% of the diet. Diets were formulated (400 g digestible protein kg⁻¹ and 15 kJ digestible energy g⁻¹ dry diet) and fed to catfish fingerlings (13.1 ± 0.5 g) to apparent satiation twice daily for 70 days. The protein and energy digestibilities of fish meal and soybean flour were high (>90% and >80%, respectively; P < 0.05). At 75% fish meal replacement with soybean flour (without methionine supplementation), growth and feed utilization efficiency indicators were depressed compared with other diet treatments which had a similar (P > 0.05) growth and feed utilization efficiency to those fed the control diet. The carcass compositions of catfish in all diets were similar (P > 0.05) and the liver histology of catfish fed any of the diets showed no alterations. The results obtained indicate that 50% of fish meal protein in practical catfish diets can be replaced with soybean flour and that catfish can effectively utilize supplemental methionine, thereby allowing up to 75% of the dietary fish meal protein to be replaced by soybean flour.     

Studies on the positional integrity of glyceride fatty acids during digestion and absorption in rats

1. Rats previously starved for 24hr. were separately given by intraduodenal injections 0.5ml. of a dispersion containing 10mg. of sodium taurocholate, with 50mg. of glycerol 1,3-dioleate 2[1-(14)C]-palmitate, glycerol 1,2-dioleate 3[1-(14)C]-palmitate, a mixture of [1-(14)C]palmitic acid and triolein, or a mixture of [1-(14)C]-palmitic acid and oleic acid. 2. At the end of 30min., the net amounts, and the radioactivity, of the neutral-lipid components recovered from the intestinal lumen and mucosa, and the position of the labelled palmitic acid in the mucosal triglycerides, were determined. 3. When glycerol 1,3-dioleate 2[1-(14)C]-palmitate was administered, most of the labelled acid was retained in the di- and monoglycerides of the lumen; the triglycerides were the major components containing the radioactivity in the mucosa and 75-80% of the labelled acid was located at the beta-position of these triglycerides. 4. When glycerol 1,2-dioleate 3[1-(14)C]-palmitate was administered, the labelled acid was readily split off in the lumen and virtually no radioactivity could be traced in the monoglyceride fraction; in the intestinal mucosa, triglycerides were again the chief components containing most of the radioactivity, and 80-85% of the labelled acid was esterified at the outer positions of the glycerol. 5. When [1-(14)C]palmitic acid mixed with triolein was administered, the concentrations of free fatty acids increased markedly in the intestinal lumen and mucosa, and 80-88% of the radioactivity of the mucosal triglycerides was located at the outer positions of the glycerol. 6. When [1-(14)C]palmitic acid mixed with oleic acid was administered, the labelled acid accumulated in the lumen as well as in the cell, and it was randomly incorporated into all three positions of the mucosal triglycerides.     

Intestinal absorption of free oleic acid in the unanesthetized rat: evidence for a saturable component?

The intestinal absorption of free oleic acid at low intraluminal concentrations and the influence of luminal factors on its absorption were studied in the unanesthetized rat. The relationship between oleic acid concentration (30-2500 microM) and its rate of absorptions fitted best to a rectangular hyperbola (y = x/(2.19 + 0.0015x), r = 0.94). Oleic acid's rate of absorption increased as the hydrogen ion and sodium taurocholate concentrations were increased or as the thickness and resistance of the unstirred water layer were diminished or following the addition of lysolecithin. The additions of the artificial detergent Tween-80, or lecithin and linoleic, linolenic and arachidonic acids to the perfusate decreased oleic acid's rate of absorption. It was concluded that oleic acid absorption in this range of concentrations displays apparent saturation kinetics which are due to unstirred layer effects, limited aqueous solubility of oleic acid and possible saturation of cytosol fatty acid binding proteins. Factors which increase oleic acid's protonated concentration or diminish the unstirred layer resistance, enhance its absorption rate, while factors which enhance its micellar solubility or interfere with its transfer out of the cell membrane decrease its overall rate of absorption.     

Inhibition of lymphatic absorption of cholesterol by cholestane-3 beta, 5 alpha, 6 beta-triol

The effect of cholestane-3,5alpha,6-triol (CT) on the intestinal absorption of cholesterol and oleic acid, as well as the absorption of labeled CT, was studied in lymph ductcannulated rats. Intragastric administration of 50 mg of CT in an emulsion with cholesterol-7alpha-(3)H and oleic acid-1-(14)C resulted in 50% inhibition of sterol transfer into lymph but only 8% depression of fatty acid absorption over an 8 hr period. The absorption of labeled CT into lymph was only 2-3% compared with 50% absorption of cholesterol when each was fed alone. 10% of the fed CT was recovered in the intestinal mucosa, and of this, one-half was associated with the brush border fraction. In rats fed CT 6 days prior to cholesterol and fatty acid administration, there was no effect on fatty acid absorption, while cholesterol absorption was reduced by almost 30%. When the intestinal mucosa from these animals were investigated by electron microscopy, it appeared that CT feeding resulted in numerous enlarged mitochondria and a marked increase in length of the microvilli. If animals were allowed to recover for 6 days from the CT prefeeding regime, the intestinal mucosa appeared normal, and the absorption of cholesterol approached that in controls. A possible mechanism for CT inhibition of cholesterol absorption was shown to be competition for the enzyme cholesterol esterase which esterifies cholesterol prior to entrance into the lymphatic system. CT itself is poorly esterified and poorly absorbed, but it is effective in inhibiting esterification of cholesterol in vitro.     

Acylation of lysolecithin in the intestinal mucosa of rats

1. The presence of an active acyl-CoA-lysolecithin (1-acylglycerophosphorylcholine) acyltransferase was demonstrated in rat intestinal mucosa. 2. ATP and CoA were necessary for the incorporation of free [1-(14)C]oleic acid into lecithin (phosphatidylcholine). 3. The reaction was about 20 times as fast with [1-(14)C]oleoyl-CoA as with free oleic acid, CoA and ATP. 4. With 1-acylglycerophosphorylcholine as the acceptor, both oleic acid and palmitic acid were incorporated into the beta-position of lecithin; the incorporation of palmitic acid was 60% of that of oleic acid. 5. Of the various analogues of lysolecithin tested as acyl acceptors from [1-(14)C]oleoyl CoA, a lysolecithin with a long-chain fatty acid at the 1-position was most efficient. 6. The enzyme was mostly present in the brush-border-free particulate fraction of the intestinal mucosa. 7. Of the various tissues of rats tested for the activity, intestinal mucosa was found to be the most active, with testes, liver, kidneys and spleen following it in decreasing order.     

Fat intestinal absorption in the catfish--a histochemical study in glycol methacrylate embedded tissue

The intestinal absorption of lipids was investigated in plastic sections from glycol methacrylate embedded intestine after fat administration. In the catfish, the lipids are absorbed by the enterocytes of the proximal intestinal segment, thus forming fat cytoplasmic inclusions that were demonstrated by Sudan black B staining. The histochemical characterization of lipids by the Nile blue sulphate test revealed the neutral or triglyceride nature of the cytoplasmic droplets, both after the corn oil and oleic acid feeding. There is lipid accumulation in the lamina propria and lymphatic vessels.     

Gastrointestinal absorption and metabolism of apple polyphenols ex vivo by the pig intestinal mucosa in the Ussing chamber

Polyphenols contained in food have various positive effects on human health. The absorption and metabolism of polyphenols in the intestinal tract needs to be studied to estimate these effects. The Ussing chamber technique was used to investigate the transport behavior of apple polyphenols through pig small intestinal mucosa, which served as a model for human gastrointestinal mucosa. The identities and concentrations of polyphenols and their metabolites in the half-chambers (luminal and basolateral) within an incubation period of 4 h were determined by HPLC–MS/MS and HPLC–DAD (DAD = diode-array detection). Flux values were also measured. It was found that 5-caffeoylquinic acid and caffeic acid were absorbed and translocated to the basolateral side (1.9 and 3.7%, respectively), but other compounds, including glycosides of phloretin and quercetin, were observed without translocation. A Ussing chamber utilizing pig small intestinal mucosa is a suitable model for assessing the effect of apple polyphenols on mucosal integrity and nutrition absorption across porcine mucosa.     

Elongation of (omega-14C)oleic acid and (omega-14C)nervonic acid.

During feeding experiments with [omega-14C]oleic acid and [omega-14c]nervonic acid to adult rats, 14C-labelled C26, C28 and C30 fatty acids were recovered from the intestinal mucosa, liver, plasma, kidney and stools. The structures of these fatty acids were determined by g.l.c., radio-g.l.c. and mass spectrometry. The Schmidt and Ginger degradation methods indicated that most of the 14C found in these extra-long fatty acids remained in the omega position. These radioactive extra-long fatty acids were found mainly in the polar lipids of rats killed 3 or 15 h after being fed on labelled oleic acid or nervonic acid. Rats killed 63 h later yielded only traces of these extra-long fatty acids. When the rats were given antibiotics or received the same radioactive fatty acids by intravenous injection, the labelled extra-long fatty acids could not be detected in any of the tissues. We conclude that they were probably synthesized by elongation of oleic acid and nervonic acid by intestinal micro-organisms (probably yeasts) and then absorbed by the intestinal mucosa.     

Effects of mixed micellar lipids on carotenoid uptake by human intestinal Caco-2 cells

We reported previously that lysophosphatidylcholine remarkably enhanced β-carotene uptake from bile acid-mixed micelles by human intestinal Caco-2 cells. In the present study, we evaluated how mixed micelle components other than phospholipids, viz., fatty acids, monoolein, and cholesterol, affect carotenoid uptake by Caco-2 cells. Each component influenced the β-carotene uptake in a different way depending on micellar composition. Oleic acid at 200 μM significantly enhanced uptake in the absence of lysophosphatidylcholine. Cholesterol at 40 μM significantly reduced uptake in the presence of lysophosphatidylcholine, while no reduction was found in the presence of 200 μM oleic acid. Facilitated diffusion was suggested partly to mediate uptake in mixed micelles, except for mixed micelles containing 200 μM oleic acid. Uptake mediated by facilitated diffusion was approximately 20% of total uptake. Mixed micellar lipids have the potential to modify intestinal uptake.     

The mechanisms of intestinal absorption of the carcinogen MNNG (N-methyl-N'-nitro-N-nitrosoguanidine)

We studied the characteristics and mechanisms of MNNG (N-methyl-N'-nitro-N-nitrosoguanidine) intestinal absorption and the interaction between bile acids and fatty acids and MNNG absorption rate in vivo in male Sprague-Dawley rats. We perfused a segment of the proximal small bowel with a physiological solution containing MNNG to assess its basic kinetics and the influence of some physiological and dietary factors on carcinogen absorption. We found that MNNG was absorbed by simple passive diffusion. Transport of MNNG was the highest at pH 6.0. The addition of the bile salt, taurocholate by itself, greatly increased MNNG absorption, while the addition of the long-chain unsaturated fatty acids, oleic and linoleic, decreased the rate of absorption of MNNG. The phospholipid lecithin addition to the perfusate did not change the rate of MNNG absorption. Induction of dietary vitamin A deficiency (serum vitamin A level decreased from 40.9 to 13.7 micrograms/dl) did not change the absorption rate of MNNG. These studies demonstrate that bile acids, dietary fatty acids, and the pH of the intestinal content can modify the rate of absorption of this carcinogen by the small intestine. Since initial intestinal absorption determines serum levels and subsequent reabsorption and enterohepatic cycling determines long-term lumenal levels, serum levels, and total body content, factors which modify the rate of intestinal absorption of MNNG could also modify its carcinogenicity.     

Delivering a hydrophobic anticancer drug for photodynamic therapy by amorphous formulation

An amorphous formulation of hypocrellin A for photodynamic therapy is reported which can provide stable aqueous dispersion of such hydrophobic photosensitizers. In vitro studies have demonstrated the active uptake of amorphous formulation of hypocrellin A into the mitochondria of tumor cells. Compared with Tween-80 micelle embedded hypocrellin A, low dark-toxicity but similar light-toxicity of the amorphous one to drug impregnated tumor cells was observed. Thus, the potential of using amorphous formulation of hypocrellin A as drug delivery system for photodynamic therapy has been demonstrated.     

Resistance of adult mammalian intestinal mucosa to cytochalasin B

Cytochalasin B (CB), in concentrations previously shown to inhibit glucose and glucosamine uptake and to disrupt cytoplasmic microfilament structure in a variety of biological systems, did not alter 3-O-methyl- -glucose, glucosamine or oleic acid uptake by adult hamster small intestinal everted rings nor did CB alter microfilament structure in the microvilli, terminal web region or apical cytoplasm of intestinal absorptive cells. Moreover, glycoprotein, protein and secretory IgA synthesis and secretion were not inhibited in rabbit intestinal mucosa cultured for 24 h in CB-containing medium. Similarly, synthesis of glycerides from freshly absorbed oleic acid by everted sacs of hamster small intestine was not inhibited by addition of CB to the oxygenated buffer used for incubation. Thus the transport and synthetic functions of adult mammalian small intestine which were examined and the fine structure of adult hamster and rabbit intestinal absorptive cells seem remarkably resistant to CB.     

Green tea catechins inhibit pancreatic phospholipase A(2) and intestinal absorption of lipids in ovariectomized rats

This study was conducted to examine whether the inhibition of intestinal lipid absorption by green tea is associated with the inhibitory effect of its catechins on pancreatic phospholipase A(2) (PLA(2)). PLA(2) activity was assayed by using 1,2-dioleoylphosphatidylcholine (DOPC), porcine pancreatic PLA(2) and catechins at varying concentrations (0.075-1.80 micromol/L). The amount of 1-oleoyl-2-hydroxyphosphatidylcholine liberated was determined by HPLC. The percentage of inhibition of PLA(2) by catechins at 0.6 micromol increased in the order of (-)-epicatechin (23.3%), (+)-catechin (CAT; 24.8%), (-)-epigallocatechin (25.7%), (-)-epicatechin gallate (39.7%) and (-)-epigallocatechin gallate (EGCG; 64.9%). In an in vivo study, ovariectomized rats with lymph cannula were infused intraduodenally for 8 h with a triolein emulsion containing [dioleoyl-1-(14)C]-phosphatidylcholine, DOPC, alpha-tocopherol (alphaTOH) and retinol (ROH) without (CAT0) or with CAT or EGCG. The lymphatic total (14)C-radioactivity was significantly lowered by EGCG (45.5+/-4.9% dose) compared with CAT (56.2+/-5.2% dose) and CAT0 (64.7+/-2.0% dose). The (14)C-radioactivity remaining in the small intestinal lumen and cecum was higher in EGCG (24.1% dose) than in CAT (9.5% dose) and CAT0 rats (9.0% dose). Significantly less (14)C radioactivity was incorporated into lymph triacylglycerol and cholesteryl ester in EGCG rats. The absorption of alphaTOH, used as a marker of extremely hydrophobic lipids, was significantly lower in EGCG (7.8+/-1.7 micromol) than in CAT (14.4+/-2.8 micromol) and CAT0 rats (16.8+/-2.1 micromol). The absorption of ROH was unaffected, whereas oleic acid output was lower in EGCG rats. The results show that EGCG inhibits the intestinal absorption of lipids, which is in part associated with its inhibition of phosphatidylcholine hydrolysis. Data suggest that EGCG may inhibit the absorption of other highly lipophilic organic compounds.     

Effect of enteric micro-organisms on intestinal sugar and fatty acid absorption

The effect of micro-organisms contaminating the upper intestinal contents of malnourished children on intestinal absorption of 3-0 methyl-alpha-D-glucopyranose (3-M.G.) and oleic acid was studied in rats in vivo. Oleci acid absorption was unaffected by non-pathogenic E. coli but decreased by E. coli 0111, Salmonella paratyphi B., Shigella sonnei and Candida sp. This effect was probably explained by intestinal secretion diluting the test solution leading to a decreased diffusion gradient for solubilised fatty acid. Inhibition of sugar absorption occurred with bacterial suspensions of Staphylococcus aureus, Streptococcus faecalis, E. coli and Candida sp. and cell-free preparations of Staphylococcus aureus, Streptococcus faecalis, a non-pathogenic E. coli, Proteus sp., Klebsiella sp., Pseudomonas sp. and Candida sp. These effects were not explained by dilution of the test solution. This indicates that numerous micro-organisms and, in some instances, their cell-free preparations can interfere with intestinal active sugar transport. These findings may be relevant to the production of malabsorption in malnourished children who have a wide variety of micro-organisms contaminating their upper intestinal contents.     

Effect of phosphatidylcholine on fatty acid and cholesterol absorption from mixed micellar solutions.

Using the experimental model of the everted sac prepared from rat jejuna, kinetic studies on [14C]oleic acid uptake from bile salt micelles were conducted in the presence and absence of phosphatidylcholine. The concentration of oleic acid was varied between 0.625 and 5 mM. At every level of fatty acid concentration studied the addition of 2 mM phosphatidylcholine produced a significant inhibition of fatty acid uptake. It was further noted that the intact phospholipid molecule was required for this effect as lysophosphatidylcholine produced little, if any, inhibition of [14C]oleic acid uptake. The effect of varying the concentration of phosphatidylcholine on fatty acid uptake was also studied. The degree of inhibition was noted to be correlated grossly with media concentrations of this phospholipid although the decrease of fatty acid uptake was not strictly proportional to concentration of this material in the medium. Studies were also performed analyzing in vitro absorption of [14C]oleic acid and [3H]cholesterol simultaneously from mixed micelles composed of sodium taurocholate, oleic acid, monoolein and cholesterol. Control medium contained no phospholipid while experimental medium contained either diester or diether phosphatidylcholine, 2 mM. Both types of phosphatidylcholine caused significant inhibition of fatty acid and cholesterol uptake. In vivo absorption studies were also performed using the isolated jejunal segment technique. A mixed micellar solution containing [3H]cholesterol and [14C]oleic acid was used as the test dose. Phospholipid in the test dose for controls was supplied as lysophosphatidylcholine and for experimentals it was in the form of diether phosphatidylcholine. Significantly less radioactively labeled cholesterol and fatty acid was absorbed by experimentals as compared to controls over a 10-min period. It is concluded that the intact molecule of phosphatidylcholine inhibits intestinal uptake of cholesterol and fatty acid from mixed micellar solutions under both in vitro and in vivo conditions.     

Diminished Oleic Acid Absorption in Man by <Emphasis Type="SmallCaps">L</Emphasis>-Phenylalanine Inhibition of an Intestinal Phosphohydrolase

EXPERIMENTS in our laboratory have shown that a change in the position and concentration of intestinal alkaline phosphatase (IAP) in the jejunal mucosa is induced by absorption of oleic acid. To determine whether alkaline phosphatase and fatty acid absorption are causally related, we investigated the effect of an inhibitor of IAP, L-phenylalanine, on the rate of absorption of oleic acid (C₁₈) in man.     

Immune responses and intestinal morphology of striped catfish,Pangasianodon hypophthalmus (Sauvage, 1878), fed dietary nucleotides

The present study was conducted to evaluate the effects of dietary nucleotides (NT) on growth performance, immune responses, and intestinal morphology of striped catfish, Pangasianodon hypophthalmus. Different levels of NT (0, 0.25. 0.5, 0.75, and 1% weight per weight, WW^?1) were used in the basal diet and then randomly allocated to triplicate groups of fish with an average initial weight of 1.52 ± 0.11 g for 10 weeks. Nucleotide supplementation did not improve growth parameters significantly (P > 0.05). Feeding fish with a NT‐containing diet significantly improved ACH₅₀ activity and IgM levels in comparison to the control group (P < 0.05). The dietary NT (P < 0.05) affected the intestinal morphology, whereby the folds and enterocyte heights in the mid‐intestine were significantly increased compared to the control group (P < 0.05), while the microvilli length was not affected. The results indicate that feeding striped catfish with commercial NT have significant effects on the immune parameters and intestinal morphology of catfish and that the best NT levels are 0.25 and 0.5%.