Lipid Related Consequences of Intestinal Malabsorption

Gut: first published as 10.1136/gut.30.Spec_No.29 on 1 November 1989. Downloaded from

Glut Festschrift, 1989, 29-34

Lipid related consequences of intestinal malabsorption

GILBERT R THOMPSON

SUMMARY The small intestine plays a key role in lipid metabolism by absorbing fat and synthesising apoproteins. Fat malabsorption secondary to intestinal disease results in abnormalities of lipoprotein concentration and composition and can lead to deficiency of essential fatty acids and fat-soluble vitamins. Malabsorption offat can be induced by administration of neomycin and malabsorption ofbile acids by administration ofanion-exchange resins or by creating a partial ileal bypass. These induced forms of malabsorption are useful in the treatment of hyperlipidaemic patients liable to atherosclerosis.

This review spans 25 years of research in the field INTRAILUMINAL PHASE of lipids. During that time major advances in know- The intraluminal phase is dependent upon the pre- ledge and changes in attitude have taken place, sence of adequate concentrations of pancreatic lipase culminating in the award of the Nobel Prize to Brown to hydrolyse triglyceride into monoglyceride and and Goldstein in 1985 for their discovery of the fatty acid, and of bile salts to incorporate these low density lipoprotein (LDL) receptor. My own products of lipolysis into mixed micelles. In addition, interest in the subject was awakened while working non-polar sterols such as cholesterol and fat soluble as a registrar with Chris Booth at the Hammersmith vitamins must be incorporated into the core of mixed copyright. and seeing a patient of his with steatorrhoea and micelles as a prerequisite to their absorption. The severe osteomalacia. This led me to study vitamin D efficient uptake of monoglyceride and, to a lesser absorption and subsequently, while working with extent, of cholesterol by the jejunum contrasts with Kurt Isselbacher in Boston, I became interested in that of bile salts, which are not absorbed until the that closely related compound, cholesterol. On re- ileum. turning to the Hammersmith I studied the lipoprotein The importance of bile salts in the absorption of fat

abnormalities which accompany malabsorption syn- soluble vitamins has been known for many years. http://gut.bmj.com/ dromes and the therapeutic potential of induced Borgstrom showed that during fat absorption the malabsorption in hyperlipidaemic patients. Thus my intestinal contents could be separated by ultra- lipophilic interests have taken me from the intestinal centrifugation into a supernatant oil phase and an lumen to the arterial-wall, a logical but initially uphill infranatant aqueous, or micellar, phase. Subse- journey. I am grateful to Chris Booth for giving me quently, Hofmann and he showed that the micellar the freedom to pursue my own research interests and phase consisted of aggregates, 5 m[t in diameter, also to those colleagues who assisted my endeavours, of monoglycerides, ionised fatty acids and bile salts, whose work is reviewed in this tribute to him. termed mixed micelles whereas the oil phase con- on September 29, 2021 by guest. Protected sisted mainly of unhydrolysed triglyceride in the form Absorption of dietary fat and fat soluble vitamins of emulsion particles, 1,000 m[i in diameter. Non- polar lipids such as cholesterol and the fat-soluble Lipid absorption can be depicted as occurring in four vitamins partition themselves between the oil and separate stages, as follows: (I) An intraluminal phase micellar phases. There is some evidence that polar of lipolysis and micellar solubilisation. (2) A mucosal lipids, such as fatty acids, can be absorbed reasonably phase comprising the uptake of micellar lipids and, in well from emulsions and that incorporation into most instances, their subsequent re-esterification. (3) mixed micelles is not a prerequisite for their absorp- A lymphatic phase, which involves chylomicron tion. In the case of non-polar lipids, however, micelle formation and secretion into lymph. (4) A phase formation is essential.' of intravascular metabolism, encompassing the hydrolysis of chylomicrons in peripheral blood ves- MUCOSAI PHASE sels, the uptake of their remnants by the liver and the Electron microscopic studies show the presence of subsequent recycling and exchange of lipids and osmiophilic droplets in the region of the Golgi and in apoproteins between the various lipoproteins pre- the intracellular spaces during the mucosal phase of sent in plasma. absorption. These appearances reflect the resynthe- 29 Gut: first published as 10.1136/gut.30.Spec_No.29 on 1 November 1989. Downloaded from

30 Thonipsont sis of triglyceride from absorbed fatty acid and apoBl(1) as well as apoE. During lipolysis, some of monoglyceride, its subsequent incorporation into the free cholesterol and lecithin on the surface chylomicrons and their extrusion from the cell by of chylomicrons is released into plasma and this reverse pinocytosis. coalesces with nascent HDL3 particles which haive After uptake as the free sterol, cholesterol is been secreted by the liver and the small intestine. largely re-esterified during its passage through the As a result of the action of lecithin:cholesterol intestinal mucosa. Approximately two thirds of the acyltransferase (LCAT) the free cholesterol gets absorbed sterol undergoes esterification by acyl esterified to cholesterol ester and the particle en- cholesterol acyl transferase (ACAT), mainly to larges and becomes HDL2. Very low density lipo- cholesterol oleate. Vitamin A (retinol) is also largely protein (VLDL) containing apoBI100, is synthesised re-esterified during absorption, preferentially with predominantly in the liver but to some extent in the palmitic acid, but vitamins D, E and K pass through small intestine. This too gets hydrolysed by the action the intestinal mucosa without major modifications. of lipoprotein lipase to form VLDL remnants. The latter are then either taken up by the LDL receptor or LYMPHATIC PHASE are acted upon by hepatic lipase to give rise to LDL. During this phase of absorption numerous chylo- Low density lipoprotein circulatcs in plisma- alid microns can be seen within intestinal lymphatics.' eventually gets taken up by' LDL receptors locaited The major route of transport of triglycerides, choles- both in peripheral cells and in the liver. terol and the fat soluble vitamins from the intestinal mucosa into blood is via the lymph. The rate of exit Consequences of acquired malabsorption of free cholesterol and vitamin D into lymph is increased by the simultaneous administration of Malabsorption can be secondary to intraluminal exogenous triglyceride- and vitamin E absorption is dysfunction, as in pnacreatic insufficiency, inflam- stimulated in a similar manner.4 The enhanced matory bowel disease and intestinal resection or it absorption is probably related to the increased syn- can be associated with mucosal dysfunction, as found thesis of chylomicrons that occurs during fat absorp- in coeliac disease, tropical sprue, and after the copyright. tion, within which cholesterol and the fat soluble administration of large doses of neomycin. It can also vitamins are largely transported. Green and Glick- be caused by lymphatic dysfunction, as seen in man' have shown that the small intestine synthesises intestinal lymphangiectasia, Whipple's disease and several of the apoproteins which occur in chylo- filariasis. Lastly, it can be secondary to manoeuvres microns, including apoB48, apoA-IV and apoA-1, which stimulates bile acid excretion, such as the and Wu and Windmueller6 have shown that over administration of anion-exchange resins or the cre- 50% of the apoA-I and apoA-IV come from the small ation of a partial ileal bypass. http://gut.bmj.com/ intestine. Although intestinal lymph is undoubtedly the FAT SOLUBt E VITAMIN DEFICIENCIES major pathway of absorption for cholesterol and the There are numerous case reports of patients with fat soluble vitamins, it is probable that some absorp- malabsorptive disorders developing clinical defi- tion also occurs via the portal vein. Evidence for this ciency of vitamins D or K. Biochemical evidence of has been put forward for vitamin E by MacMahon vitamin E deficiency has also been demonstrated et al,4 who found higher concentrations of labelled quite frequently in such patients, although its clinical on September 29, 2021 by guest. Protected ca-tocopherol in portal venous than in aortic plasma significance is difficult to assess.' after oral administration. This presumably explains In view of the importance of micelle formation for why small amounts of vitamin E can be demonstrated the absorption of non-polar lipids it is not surprising in the blood of treated patients with a13-lipoprotei- that severe malabsorption of labelled vitamins A, D, naemia, in whom lymphatic transport of lipids cannot E and K, has been shown in patients with biliary occur. obstruction in whom cannulation of the thoracic duct had been undertaken. Other studies, in which the INTRAVASCUt AR METABOLISM faecal excretion of radioactivity was used as an index Chylomicrons mix with HDL in lymph, pick up apoE of absorption, showed marked malabsorption of both and C, and then get partially degraded by lipoprotein labelled vitamins D' and E in patients with steator- lipase. The resulting chylomicron remnants are rhoea of biliary or pancreatic origin. Fat soluble quickly taken up by chylomicron remnant receptors vitamin deficiencies can also occur after partial gas- in the liver. It is thought that this process is mediated trectomy or small intestinal resection. Vitamin D by specific receptors which recognise the apoE on the absorption is usually only slightly subnormal in surface of remnant particles but are metabolically patients with postgastrectomy steatorrhoea," how- distinct from LDL receptors which recognise ever, and may even be normal, suggesting that lack Gut: first published as 10.1136/gut.30.Spec_No.29 on 1 November 1989. Downloaded from

Lipid related consequences ofintestinal malabsorption 31 copyright.

Fig 2 Barium follow through showing stricture ofterminal

ileum and anomalously situated caecum and ascending colon http://gut.bmj.com/ in GH. (Reproduced with permission from ref. 11). to a stricture of the terminal ileum," as illustrated in Figures 1 and 2. It seems probable that the ileal lesion was responsible not only for malabsorption of vitamin B12 but also of bile acids, thereby impairing micelle formation and causing marked malabsorption of vitamin D, comparable with that demon- on September 29, 2021 by guest. Protected strated in rats fed cholestyramine." Fig. I Pathologicalfractures (Looser's zones) in radius and ulna of66 year old woman (GH) with osteomalacia and steatorrhoea. (Reproduced with permission from ref 11). ESSENTIAL FATTY ACID DEFICIENCY Shimoyama et al'" examined the fatty acid composition of plasma lipids in patients with malabsorption of sunlight or dietary deficiency is a more important and showed that there was a decrease in the propor- factor in causing their osteomalacia."' In contrast, tion of linoleic acid (18:2) and arachidonic acid (20:4) patients with adult coeliac disease often show marked in malabsorbers compared with healthy controls and malabsorption of vitamin D, the severity being patients without malabsorption. These changes were directly proportional to the degree of steatorrhoea.x most marked in the fatty acid composition of choles- Studies on the absorption of vitamin E showed terol esters and lecithin. marked malabsorption in two patients with intestinal When various categories of malabsorption were lymphangiectasia.7 examined it was found that the decrease in essential A particularly striking case of osteomalacia, refer- fatty acids was most marked in patients who had an red to in the Introduction, was associated with intestinal resection. Subsequently, Press et al'4 des- megaloblastic anaemia and steatorrhoea secondary cribed three patients with intestinal resection who Gut: first published as 10.1136/gut.30.Spec_No.29 on 1 November 1989. Downloaded from

32 32 Thonipsot developed clinical evidence of essential fatty acid teins by ultracentrifugation showed that VLDL con- deficiency which was confirmed by the presence centrations were either normal or raised, but LDL of omega-9 eicosatrienoic acid (20:3co9 in plasma concentrations tended to be reduced, especially in lecithin and the rapid disappearance of this patho- men, whereas in women there was a more marked logical fatty acid after the intravenous administration reduction in HDL, as shown in Figure 4. Press andi of Intralipid, as shown in Figure 3. Thompson'h measured plasma postheparin lipolytic activity in some of these patients and showed that the SECONDARY HYPOI IPOPROTEINAEMIA activity of lipoprotein lipase was markedly reduced, It is well recognised that patients with fat malabsorp- which provided an explanation for the increased tion often show evidence of hypolipoproteinaemia. levels of VLDL and triglyceride. This is particularly marked in patients with primary In addition to noticing that the concentration of malabsorption, such as abetalipoproteinaemia and LDL was reduced, Thompson and Miller'' showed familial hypobetalipoproteinaemia, but it also occurs that its composition was abnormal, in that there was in patients with malabsorption secondary to various a considerable reduction in the percentage of choles- underlying causes. terol ester in the LDL of malabsorbers and an Thompson and Miller'" measured plasma lipids in increase in the percentage of triglyceride, these patients with secondary malabsorption and found changes being inversely correlated. Miller and that plasma triglyceride concentrations were either Thompsonv then showed that LCAT activity in the normal or raised. In contrast, their plasma choles- plasma of malabsorbers was reduced and suggested terol and phospholipid concentrations were nearly that this was responsible for the decrease in the always reduced. Quantification of plasma lipopro- cholesterol ester content of their LDL. copyright. http://gut.bmj.com/ on September 29, 2021 by guest. Protected

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Fig. 3 Fatty acid pattern ofplasma lecitlhini patient witli essentialfattv alcid deficien'c \loingdt('ecr-eased lel'eLe *of 18:2 an(l 20:4 and presence of20:3 w9 before Intralipid and reeversal ofthese abnor-malities after Intralipi(d (reproduced wit/i permission from ref 14). Gut: first published as 10.1136/gut.30.Spec_No.29 on 1 November 1989. Downloaded from

Lipid related consequences ofintestinal malabsorption 33

VDL dependent upon its antibiotic properties, as was 50 confirmed in germ free pigs.' E 45 0 ° 40 OF BILE ACIDS, BY ANION-EXCHANGE RESINS E 35 AND PARTIAL ILEAL BYPASS -6 Another drug which induces malabsorption, al- w 30 0 though it affects bile acids rather than cholesterol, is o 25 the anion exchange resin cholestyramine. A marked 20 0 -,x -A- increase in bile acid excretion occurs in subjects given 0 C 15 cholestyramine but an even greater increase in occurs after the creation of a partial ileal o 10 0o 20 excretion This operation, which was invented by 0 bypass. .o- 5 6 20 10 Buchwald, involves transecting the ileum at the 0 0 distal and middle one-third and then p the p NS p < 005 p

- which was more marked It is now time to consider the other side of the coin rate of LDL catabolism, copyright. namely, the various ways in which malabsorption can after partial ileal bypass than during cholestyramine be deliberately induced in order to decrease serum therapy. There was a strong inverse correlation lipids in patients with hyperlipidaemia. One means of between the concentration of LDL cholesterol and doing this is by administering neomycin in a dose the fractional catabolic rate of LDL in these patients, of 1-2 g a day. The numerous amino groups on the lowest concentrations of LDL cholesterol and the neomycin make it highly cationic and when added in highest fractional catabolic rates occurring after titro to a solution of mixed micelles, which have a net partial ileal bypass. The latter procedure was shown http://gut.bmj.com/ negative charge, precipitation occurs due to ionic to specifically enhance receptor mediated LDL interaction. " Neomycin precipitates virtually all the catabolism, as also does cholestyramine. fatty acid, monoglyceride and cholesterol present in Cholesterol for bile acid synthesis is partly derived mixed micelles but only a small proportion of the from endogenous synthesis and partly derived from taurocholate. the influx of LDL cholesterol from plasma, mediated Neomycin was introduced as a cholesterol lower- by hepatic LDL receptors. Newly synthesised bile ing agent by Samuel who used it to treat patients acids are secreted in bile and then mainly reabsorbed with heterozygous familial hypercholesterolaemia. in the terminal ileum, only a small proportion being on September 29, 2021 by guest. Protected Studies in patients given a test meal followed by excreted in the faeces. In patients with heterozygous aspiration and ultracentrifugation of the duodenal familial hypercholesterolaemia, who have fewer contents showed that most of the cholesterol, bile hepatic LDL receptors than normal,2 a relatively acid and fatty acid was present in the micellar phase. greater proportion of bile acids is presumably derived When neomycin was given at the same time as the test from endogenous cholesterol synthesis, although meal, however, there was a very significant increase reabsorption and excretion of bile acids occurs in the amount of cholesterol and fatty acid in the normally. When such a patient is placed on precipitate. cholestyramine this results in decreased reabsorption The functional consequences of this action were of bile acids and an increase in faecal excretion. This assessed by measuring the faecal excretion of fat and provides a stimulus to bile acid synthesis and creates cholesterol off and on neomycin. The percentage of a an increased demand for cholesterol which is met dose of labelled cholesterol and the amount of fat both from endogenous synthesis and also by stimula- excreted were both increased by neomycin adminis- tion of hepatic LDL receptors; this results in an tration, indicating reduced absorption. The small increased influx of LDL from blood into the liver. dose of neomycin used was insufficient to cause After partial ileal bypass, malabsorption of bile acids mucosal damage, nor was the effect of neomycin is even more marked, causing greater stimulation of Gut: first published as 10.1136/gut.30.Spec_No.29 on 1 November 1989. Downloaded from

34 Thomipson bile acid synthesis and thus greater stimulation of In: Tlhe scientific basis of m7edicine annuillal revIiews. LDL receptors. The tendency for LDL concentra- British Postgraduate Medical Federation, University of tions to rise subsequently reflects a compensatory London. Athlone Press, 1970; 260-75. increase in endogenous cholesterol synthesis which 11 Clino-pathological Conference. A case of osteomalacia. can be suppressed by osteoporosis and hypercalcaemia. Br Med J 1967; i: administration of the HMG 219-23. CoA reductase inhibitor, mevinolin,23 now called 12 Thompson WG, Thompson GR. Effcct of choles- lovastatin. The combination of the latter drug with tyramine on the absorption of vitamin D, and calcium. cholestyramine or partial ileal bypass provides a Gut 1969; 10: 7 17-22. highly effective means of treating hypercholestero- 13 Shimoyama T, Kikuchi H, Press M, Thompson GR. laemia. Fatty acid composition of plasma lipoprotein in control subjects and in patients with malabsorption. Glut 1973; 14: 716-22. MRC Lipoprotein Team, 14 Press M, Kikuchi H, Shimoyama T, Thompson GR. Hammersmith Hospital, Diagnosis and treatment of essential fatty acid Ducane Road, deficiency in man. Br Med J 1974; ii: 247-50. London W12 15 Thompson GR, Miller JP. Plasma lipid and lipoprotcin abnormalities in patients with malabsorption. C/in Sci Molec Med 1973; 45: 583-92. References 16 Press M, Thompson GR. Plasma post-heparin lipolytic activity in patients with malabsorption: cffcct of intra- 1 MacMahon MT, Thompson GR. Comparison of the venous fat administration. Clin Sci Molec Med 1974; 46: absorption of a polar lipid, oleic acid, and a non-polar 743-51. lipid, a-tocopherol from mixed micellar solutions and 17 Miller JP, Thompson GR. Plasma cholesterol estcrifica- emulsions. EurJ Clin Invest 1970; 1: 161-6. tion in patients with secondary hypocholesterolaemia. 2 Sabesin SM, Frase S. Electron microscopic studies of Eur J Clin Invest 1973; 3: 401-6. the assembly, intracellular transport, and secretion of 18 Thompson GR, MacMahon M, Claes P. Precipitation chylomicrons by rat intestine. J Lipid Res 1977; 18: by neomycin compounds of fatty acid and cholesterol 496-511. from mixed micelles. Eur J Clin Invest 1970: 1: 40-7. copyright. 3 Thompson GR, Ockner RK, Isselbacher KH. Effect of 19 Thompson GR, Barrowman J, Gutierrez L, Dowling mixed micellar lipid on the absorption of cholesterol and RH. Action of neomycin on the intraluminal phase of vitamin D2 into lymph. J Clin Invest 1969; 48: 87-95. lipid absorption. J Clin Invest 1971; 50: 319-43. 4 MacMahon MT, Neale G, Thompson GR. Lymphatic 20 Thompson GR, Henry K, Edington N, Trexler PC. and portal venous transport of c-tocopherol and Effect of neomycin on cholesterol metabolism in the cholesterol. Eur J Clin Invest 1971; 1: 288. germ-free pig. EurJ Clin Invest 1972; 2: 365-71. 5 Green PHR, Glickman RM. Intestinal lipoprotein 21 Spengel FA, Jadhav A, Duffield RGM, Wood CB, metabolism. J Lipid Res 1981; 22: 1153-73. Thompson GR. Superiority of partial ileal bypass over http://gut.bmj.com/ 6 Wu A-L, Windmueller H. Relative contributions by cholestyramine in reducing cholesterol in familial hyper- liver and intestine to individual plasma apolipoproteins cholesterolaemia. Lancet 1981; ii: 768-70. in the rat. J Biol Chiem 1979; 254: 7316-22. 22 Harders-Spengel K, Wood CB, Thompson GR, Myant 7 MacMahon MT, Neale G. The absorption of NB, Soutar AK. Differcncees in saturable binding of ca-tocopherol in control subjects and in patients with low density lipoprotein to liver membranes from normo- intestinal malabsorption. Clin Sci 1970; 38: 197-21(0. cholesterolemic subjects and patients with heterozygous 8 Thompson GR, Lewis B, Booth CC. Absorption of familial hypercholesterolemia. Proc Natl Acad Sci USA

vitamin D2-_H in control subjects and patients with 1982; 79: 6355-9. on September 29, 2021 by guest. Protected intestinal malabsorption. J Clin Invest 1966; 45: 94-102. 23 Thompson GR, Ford J, Jenkinson M, Trayner 1. 9 Thompson GR, Lewis B, Booth CC. Vitamin D absorp- Efficacy of mevinolin as adjuvant therapy for refractory tion after partial gastrectomy. Lancet 1966; i: 457-8. familial hypercholesterolacmia. Q J Med 1986; new 10 Thompson GR. Vitamin D deficiency after gastrectomy. series: 60: 803-11.