Effect of ursodeoxycholate and its taurine conjugate on bile acid synthesis and cholesterol absorption

GASTROENTEROLOGY

LIVER

AND

BILIARY

1984:87:130-5

TRACT

Effect of Ursodeoxycholate and Its Taurine Conjugate on Bile Acid Synthesis and Cholesterol Absorption WILLIAM

G. M. HARDISON

and SCOTT M. GRUNDY

Department of Medicine, Veterans Administration Medical Center and University of California, San Diego, and Center of Human Nutrition, University of Texas Health Science Center, Dallas, Texas

Six male subjects with normal gastroenterologic function were studied to determine the effects of ursodeoxycholic (1.5 mg/kg - day) and tauroursodeoxycholic (20 mg/kg - day] acid feeding on bile acid synthesis and cholesterol absorption. Each bile acid was fed for 1 mo and withheld for the next month. Subjects remained on a metabolic ward and consumed a constant diet of 500 mg of cholesterol mixed with solid and liquid formulas. Before the study started, each subject received 50 &i of [4-‘4C]cholesterol intravenously. During the study, stools were collected for the determination of 24-h fecal acidic and neutral sterol excretion, and blood was drawn twice weekly for determination of serum cholesterol specific activity. At the end of each month an intestinal perfusion study was peflormed to measure total bile acid pool size and hourly biliary secretion rates of cholesterol, phospholipid, and bile acid. From these data, the percentage of cholesterol absorption and total endogenous bile acid synthesis could be calculated. Neither ursodeoxycholic nor tauroursodeoxycholic acid feeding decreased endogenous bile acid synthesis. During bile acid feeding periods, the percentage of cholesterol absorption was decreased. Ursodeoxycholic acid (UDCA) is a unique bile acid, in some ways superior to chenodeoxycholic acid (CDCA) for gallstone dissolution. It is capable of decreasing biliary choiesterol saturation when fed to patients in doses of 12-15 tig/kg . day while producReceived June 28, 1983. Accepted November 30, 1983. Address requests for reprints to: William G. M. Hardison, M.D., Veterans Administration Medical Center, Gastrointestinal Section (lllD), 3350 La Jolla Village Drive, San Diego, California 92161. This study was supported by the Research Service Veterans Administration and Grant AM-28446 from the National Institutes of Health. 0 1984 by the American Gastroenterological Association 0016-5085/84/$3.00

ing neither a rise in the level of serum aminotransferases nor diarrhea (l-5). The inability of UDCA to stimulate fluid production by the colonic mucosa (6,7)renders this bile acid attractive as a potential treatment for patients with ileal resection, or other conditions associated with cholerrhagic diarrhea. One could theoretically provide bile acid for fat absorption and at the same time improve diarrhea, as UDCA replaces endogenous bile acid. For UDCA to work in this fashion, it should be as effective in promoting fat absorption as endogenous bile acids, and it should also be able to suppress bile acid synthesis to replace endogenous bile acids. Evidence exists, however, that UDCA can do neither. Ursodeoxycholic acid solubilizes cholesterol poorly (8), and administration of UDCA to rats (9) and to humans (10) has been shown to suppress cholesterol absorption, although there is disagreement on the latter effect (11). Unlike CDCA, UDCA constitutes only 50%--60% of the biliary bile acids during prolonged therapy (l-5). It has been suggested that this is so because UDCA does not suppress synthesis of endogenous bile acid (12). Some investigators, however, using isotope dilution methods, maintain that endogenous bile acid synthesis is suppressed by UDCA feeding (l3,14). In the present work we used the direct technique of sterol balance and incorporation of labeled cholesterol into bile acid to answer the questions of whether cholesterol absorption is normal and whether endogenous bile acid synthesis is stippressed during UDCA therapy in humans. Because the taurine conjugate of UDCA is a more efficient solubilizer of cholesterol than its glycine conjugate (a), we studied administration of tauroursodeoxycholic acid (TUDCA) as well as of UDCA. Abbreviations used in this paper: CA, cholic acid: chenodeoxycholic acid: TUDCA, tauroursodeoxycholic UDCA, ursodeoxycholic acid.

CDCA. acid;

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IJRSODEOXYCHOLATE EFFECTS ON STEROL METABOLISM 131

Methods

Table

Six male subjects, aged 46-63 yr, with no active gastrointestinal or liver diseases were studied. All subjects were instructed in the details of the study and all gave written informed consent for the studies which were approved by the Human Subjects Committee, University of California, San Diego, Calif. Radiation dosage per subject for the entire study was calculated to be ~0.05 rad for the entire body and ~0.50 rad for the large intestine. Subjects’ characteristics are shown in Table 1. Only 1 subject was >25% over desirable body weight; three were hypertriglyceridemic. The subjects were housed on a metabolic ward during the 3-mo study with short-term (~3 days) passes permitted between the months of the study. The diet was Sustacal (Mead Johnson Nutritional, Evansville, Ind.), plus limited amounts of solids to provide weight-maintenance calories each day. Twenty percent of the calories were fat, 25% protein, and 55% carbohydrate. No subject gained or lost ~2% of his body weight during the study. Sufficient anhydrous cholesterol was added to the diet in capsule form to provide a total of 500 mg of dietary cholesterol per day. The cholesterol was taken as a single dose with the morning liquid formula. Chromium oxide, 180 mg twice daily, was given as a nonabsorbable fecal marker. At the start of the study, -2 wk before stool collections, each subject received 50 &i of [4-“Clcholesterol (60 mCii mmol) intravenously (New England Nuclear, Boston, Mass). The [“Clcholesterol was >95% pure as shown by thin-layer chromatography (TLC). Blood was drawn twice weekly for determination of serum cholesterol specific activity. Ursodeoxycholic acid and TUDCA were provided by Dr. A. F. Hofmann and were >95% pure as demonstrated by TLC. The subjects were fed an equimolar dose of -15 mgi kg . day and 20 mgikg . day respectively given in two doses daily. During each month of the study the subjects received either UDCA, TUDCA, or no bile acid. We attempted to control the order of feeding so that UDCA would be fed first to 2 subjects, TUDCA first to 2 subjects, and no bile acid first to 2 subjects. We were unable to do this because in some instances TUDCA was not available when needed. The actual order of studies is shown in Table 1.

Fecal

Collections

and

Analyses

Stool was collected each day for the last 15 days of each study period and pooled in s-day collections. They were analyzed for neutral and acidic sterol content by the methods of Grundy, Ahrens, and Miettinen (15,16). These methods involve extraction of neutral sterols from acidic sterols after mild saponification, hydrolysis of the acidic sterol fractions, further purification of each fraction by TLC, and final quantification of neutral and acidic sterols by gas-liquid chromatography (GLC). Rate of disintegration of 14C was measured in the acidic sterol fraction by combustion (Harvey Inst. Corp., Hillsdale, N.J.). A 24-h fecal excretion of neutral and acidic sterol was calculated from the chromium oxide content of the analyzed stool.

1. Subiects’ Characteristics

Serum Serum % Ideal choles- triglycAge body terol erides Patient (yr) weight (mg/dJ) (mgidl) R.K. M.K. B.N. R.F. A.C. T.F.

55 63 63 57

100

250

240

121 110 101

200 250 275 224 237

180 160 350 175 550

59

95

46

132

Experimental period 1 C UDCA UDCA

2

UDCA C TUDCA c TUDCA TUDCA C c UDCA

3 TUDCA TUDCA C UDCA UDCA TUDCA

C = control, UDCA = ursodeoxycholic acid, TUDCA = tauroursodeoxycholic acid.

Intestinal

Perfusion

Studies

These studies were performed according to the method of Grundy and Metzger (17) using a triple-lumen tube. The most proximal lumen was for perfusion. The middle lumen, 1.5-2 cm distal to the most proximal lumen, was used as the proximal aspiration port. The most distal lumen, 15 cm distal to the middle lumen, was used as the distal aspiration port. Before perfusion began, 30 ml of Sustacal was injected through the proximal port into the duodenum to stimulate gallbladder contraction. Three milliliters of fluid enriched with gallbladder bile was collected from the middle port. Next, 7 &i of [2,4-‘Hlcholic acid (15 Ci/mmol) dissolved in water was injected down the proximal tube and flushed through with 30 ml of water. The [2,4-3H]cholic acid was obtained from New England Nuclear and was >95% pure by TLC as supplied. Sustacal infusion began at a rate of 0.9 ml/min to provide 1124th of the daily caloric requirement per hour. It was infused for 2 h before infusion of /3-sitosterol. The p-sitosterol served as a nonabsorbable enteric sterol marker. After 1 h of /3sitosterol infusion, continuous aspiration of fluid from proximal and distal ports at 1 ml/h was started. Hourly collections of fluid were made for 8 h, after which the study was terminated. Intestinal fluid cholesterol, bile acid, and phospholipid were assayed and flux rates of these substances down the duodenum were calculated. In addition, intestinal fluid from the proximal port was counted for tritium in a Tracer Mark III liquid scintillation counter (Tracer Analytic, Grove Village, Ill.). Total bile acid pool size was calculated by the method of von Bergmann et al. (18). Individual bile acids in intestinal fluid samples were quantified by high pressure liquid chromatography according to Nakayama and Nakagaki (19). This method allowed quantification of the glycine and taurine conjugates of cholic, chenodeoxycholic, deoxycholic, lithocholic, and ursodeoxycholic acids. The system used was a Waters 4000 high pressure liquid chromatography unit equipped with the A400 variable wavelength detector (Waters Associates, Milford, Mass.).

Calculations Bile acid synthesis was calculated in two ways; one using a balance method and the other using the amount of

132

GASTROENTEROLOGY

HARDISON AND GRUNDY

14C radioactivity incorporated into fecal acidic sterol from precursor cholesterol of known specific activity. By the balance method (all in mg/day) Synthesis

2. Parameters

of Bile Acid Metabolism % UDCA

= fecal acidic sterol - oral bile acid intake.

The latter term was corrected for whether conjugated or unconjugated bile acid was fed. By the specific activity method dpm in 24-h fecal acidic sterol fraction Synthesis (mg/day) = dpmimg serum cholesterol The specific activity of serum cholesterol was taken for calculation 1 day before the initiation of the d-day fecal collection (20). The data from the five 3-day collections were averaged to give a figure for each experimental period. Correlation of the two methods was significant (r = 0.62, p < 0.01). Percentage of cholesterol absorption was (1 - fraction

The fraction

Table

excreted)

500 mg +(hourly

duodenal

Subject

Period

R.K.

C UDCA TUDCA C UDCA TUDCA

M.K.

cholesterol

Results Tables 2 and 3 show parameters of bile acid metabolism during the control, UDCA, and TUDCA feeding periods for the 6 subjects. The percentage of UDCA in bile was similar whether UDCA or TUDCA was fed and values approximated those reported in the literature with UDCA feeding. Feeding TUDCA did increase taurine conjugation of the UDCA in bile but deconjugation and subsequent reconjugation with glycine must have been occurring because mean percentage of taurine-conjugated UDCA during this period reached only 55%. Taurine feeding in 2 subjects did not clearly increase this percentage. The mean total bile acid pool increased 40% during UDCA feeding and 50% during TUDCA feeding; however, only in the TUDCA feeding period was this significant [Table 4). Whether UDCA and TUDCA feeding increased endogenous bile acid synthesis is uhcertain from our data. Although absolute rates of synthesis increased during both TUDCA and UDCA

as taurine conjugate

Bile acid pool size (mg)

1 57

12

3957 2902

40

30

4925

1

60 57 0 52 56 3

17 68

R.F.

UDCA TUDCA C UDCA TUDCA

38 50 3

21 64

38

44 50

A.C.

T.F.

C UDCA TUDCA C

72" a

UDCA TUDCA

51 45”

3099 5785 3866 1629 2432 3131 2790 2433 2606 1453 1490 2072 1141 4489 4467

15 49

C

flux x 24) '

where 500 mg is the dietary cholesterol, and where duodenal cholesterol flux was determined from the small bowel intubation procedure (18). The validity of this procedure has been presented in detail previously (21). After studying the results from 4 subjects, we observed that the percentage of biliary UDCA conjugated with taurine (30%-68%) during TUDCA feeding was less than we had anticipated. To increase taurine conjugation in the TUDCA feeding period, we fed 2 subjects (T.F. and AC.) 0.5 g of taurine six times daily. We achieved no obvious increase in percentage of taurine conjugation (69% and 50%) and observed no significant differences from the results of the first 4 patients in other respects. We therefore included data from all 6 subjects in our analysis.

% UDCA in bile

B.N.

x 100.

excreted was fecal 24-h neutral sterol excretion

Vol. 87. No. 1

13 69

C = control, UDCA = ursodeoxycholic acid, TUDCA = tauroursodeoxycholic acid. a Subjects fed taurine; 0.5 g six times daily, during TUDCA period.

periods, whether calculated by the balance or the specific activity method, only in 1 case (during TUDCA feeding, calculated by the balance method) was this increase significant. The data, however, give no suggestion that endogenous bile acid synthe-

Table

3. Bile Acid Balance

Subject

Period

R.K.

C UDCA TUDCA C UDCA TUDCA

M.K.

B.N.

R.F.

A.C.

T.F.

C UDCA TUDCA C UDCA TUDCA C UDCA TUDCA C UDCA TUDCA

Bile acid excreted (mg/day)

and Synthesis

Bile acid fed (mgiday)”

694

0

2132 1846 598 2121 1708 587 2226 2131 761 1932 2002 1430 1320 1338 963 2213 2295

1250 1140 0 1250 1140 0 1250 1140 0 1000 950 0 1000 950 0 1250 1140

Rates

Bile acid synthesis (mgldoy) Balance methodb 694

882 906 598 871 568 587 976 991 761 932 1052 430 320 388 963 963 1155

Isotope methodb 747 1048 1277 522 624 545 562 411 648 622 849 748 479 358 472 900 1044 -

C = control, UDCA = ursodeoxycholic acid, TUDCA = tauroursodeoxycholic acid. a Weight of TUDCA fed expressed as equivalent weight of unconjugated bile acid. b See text for method of calculation.

July 1984

URSODEOXYCHOLATE

sis decreased during feeding of UDCA or TUDCA as one might expect if these bile acids were capable of suppressing endogenous bile acid synthesis. Table 5 shows the hourly biliary lipid flux rates and the fecal neutral sterol daily excretion rates from which percentages of cholesterol absorption were calculated. It is evident that while UDCA and TUDCA feeding had no consistent effect on biliary bile acid or phospholipid secretion rate, in all cases biliary cholesterol secretion rate fell. The decrease in mean percentage cholesterol absorption was significant for each bile acid feeding period when compared with the percentage cholesterol absorption during the control period (Table 4). A few of the data appeared suspect because the change in total bile acid pool size of subject T.F. during bile acid feeding was very great. The low cholesterol flux during the control period and the increase in cholesterol absorption during the UDCA feeding period in subject B.N. stood out as did the very high biliary bile acid and lipid fluxes in subject A.C. during the control period. The raw data in these cases were reviewed, and no evident lapses in protocol, or errors in measurement or calculations were found. All the data were therefore included and averaged with all the other data.

Discussion The data from the present study answer the questions posed previously. Neither UDCA nor its taurine conjugate (TUDCA) suppresses synthesis of endogenous bile acid even when fed in high dosages. Nor is either one as effective as endogenous bile acid in promoting cholesterol absorption. The question of whether UDCA suppresses endogenous bile acid synthesis in humans has been answered both positively and negatively in the literature. Data derived from turnover studies have indicated that UDCA feeding at 12-15 mg/kg . day Table 4. Mean (*SD) Values From Tables 2, 3, and Controls 3 + 3.3

4

UDCA

TUDCA

49 k 9 20 k 12

53 2 11 55 2 15

% UDCA in bile ob UDCA as taurine conjugate BA pool size (mg)

2345 -r- 1106

3255 2 1583

3511 k 1102’

BA synthesis (m&-W Balance method Isotope method Calculated %

672 t 181 639 -c 158 57 2 9.0

824 2 250 722 L 305 47 2 9.8”

843 2 300” 738 ?I 31gb 42 2 9.0”

cholesterol absorption UDCA = ursodeoxycholic acid, TUDCA = tauroursodeoxycholic acid. a Significantly different (p < 0.05) from control by Student’s paired t-test, two-tailed. b n = 5.

EFFECTS

ON STEROL

METABOLISM

133

Table 5. Biliary Lipid Secretion and Parameters of Sterol Excretion and Absorption Biliary secretion rate (mgih)

Subject

R.K. M.K.

B.N.

R.F.

A.C.

T.F.

Period

C UDCA TUDCA C UDCA TUDCA C UDCA TUDCA C UDCA TUDCA C UDCA TUDCA C UDCA TUDCA

Bile acid

Cholesterol

Phospholipid

Fecal neutral sterol (mgiday)

730 977 1034 891 851 1027 638 920 554 468 473 395 1376 1055 460 502 640 703

53 27 29 47 33 29 31 29 23 42 27 28 100 37 32 63 35 41

234 337 341 381 271 259 260 302 248 308 303 297 1036 536 387 348 354 461

976 811 838 797 682 744 553 500 660 475 558 586 651 644 ti52 791 783 794

Calculated % cholesterol absorption” 45 29 30 51 47 38 56 58 37 69 51 50 62 54 49 59 42 46

C = control, UDCA = ursodeoxycholic acid, TUDCA = tauroursodeoxycholic acid. ” See text for method of calculation.

decreases synthesis of cholic acid (14) and CDCA (13). The question of how UDCA affects CDCA synthesis is puzzling when turnover techniques are used because CDCA can be synthesized from UDC (22). From studies of fecal bile acid composition and secretion, Thistle et al. (12) suggested that endogenous cholic acid synthesis was much less diminished by UDCA than by CDCA feeding. More recently, Nilsell et al. (23) have shown by conventional turnover techniques that synthesis rates of both cholic acid (CA) and CDCA are actually increased by UDCA feeding. Our data, derived in two different ways, both independent of isotope dilution, support these latter works in demonstrating no reduction and a possible increase in endogenous bile acid synthesis during either UDCA or TUDCA feeding. This disagreement among investigators contrasts with the unanimity of opinion concerning the effects in humans of CDCA (24,25), CA (25), or deoxycholic acid (24,26) feeding on synthesis of primary bile acids. Investigators agree that these bile acids decrease endogenous bile acid synthesis. Although during both TUDCA and UDCA feeding, UDCA constituted a large proportion of the bile acid pool, the increase in total bile acid pool size was such that the combined primary bile acid (CA and CDCA) pool size did not fall significantly, in agreement with the findings of Nilsell et al. (23). Pool sizes were 1700 t 343 mg (SEM) during the control period, 1320 + 235 mg during UDCA feeding, and

134

HARDISON

GASTROENTEROLOGY

AND GRUNDY

1466 + 330 mg during TUDCA feeding. Maintenance of these pool sizes further suggested that fed UDCA and TUDCA were not effectively suppressing endogenous bile acid synthesis. In spite of the fact that UDCA constituted only -50% of the bile acid pool, this was enough to reduce cholesterol absorption significantly. In vitro studies suggest that neither UDCA nor its glycine conjugate (GUDCA) is an efficient solubilizer of cholesterol (8).Igimi and Carey (27)have shown, moreover, that UDCA and its glycine conjugate are insoluble at the pH of small bowel fluid in the absence of other solubilizing bile acids. Tauroursodeoxycholic acid appears to be more soluble as well as more efficient than the glycine conjugate in incorporating cholesterol into micelles (8). Raicht (9)has demonstrated diminished cholesterol absorption in rats fed UDCA, and Ponz de Leon et al. (10) have shown that in humans if UDCA was fed for 1 mo the percentage of cholesterol absorption was less than normal, so that UDCA came to constitute 40% of the bile acid pool. The data of Ponz de Leon were derived by fecal isotope ratio analysis after feeding [14C]cholesterol and [3H]p-sitosterol. A recent report by LaRusso et al. contradicted the conclusions of Ponz de Leon. LaRusso et al. found no decrease in percentage of cholesterol absorption during UDCA feeding in patients fed a standard X0-mg cholesterol diet for 10 days. These investigators used the plasma isotope ratio method of Samuel et al. (28). As pointed out by Anderson (29), however, such methods do not take into account changes in endogenous cholesterol secretion, Because UDCA reduces endogenous (biliary) cholesterol secretion, a constant percentage of cholesterol absorption indicates a reduced absolute cholesterol absorption. For instance, the mean control biliary cholesterol excretion rate in the subjects we studied was 1344 mg/24 h and the mean reduction rate with UDC was 592 mg/24 h. At a dietary cholesterol intake of 250 mg/24 h, 50% cholesterol absorption would be 797 mg/24 h. With UDCA feeding, however, 50% cholesterol absorption would be only 501 mg/24 h. The results reported by LaRusso et al., therefore, imply that cholesterol absorption would be actually decreased if one considered alterations in biliary cholesterol excretion. Our data, derived by balance methods which take into account changes in endogenous cholesterol secretion, supported the earlier conclusions that cholesterol absorption decreases with UDCA feeding. Because TUDCA has theoretical advantages over UDCA as regards water solubility and incorporation of cholesterol into micelles, we studied this bile acid as well. We achieved -50% taurine conjugation. We tried to increase this percentage by feeding supplemental taurine to 2 subjects. Such feeding did not

Vol. 87, No. 1

result in very high proportions of the UDCA being conjugated with taurine as we had hoped. The degree of taurine conjugation made no obvious differences either in bile acid turnover and metabolism or in percentage cholesterol absorption. The data suggest that UDCA feeding will not be beneficial for reducing diarrhea in ileal resection or disease. Such therapy has been tried but results have been mixed. Huijbregts et al. (30) demonstrated a slight benefit in some patients but LaRusso et al. (31) showed no improvement in stool frequency with UDCA feeding. The present data explain this lack of effect. Since UDCA cannot suppress endogenous bile acid synthesis, it cannot replace endogenous bile acid in the bile acid pool, and cannot, therefore, decrease exposure of the colonic mucosa to diarrheagenie bile acids. Moreover, it is less effective than endogenous bile acid in promoting absorption of cholesterol. It is unlikely, therefore, that it can, even as the taurine conjugate, substitute effectively for endogenous bile acid in the process of fat absorption. It is possible that UDCA feeding might prove better than no bile acid supplementation in certain states of bile acid loss characterized by severe steatorrhea but the data of Huijbregts et al. (30) are not encouraging in this regard. One other condition in which UDCA therapy has been tried is in the bile reflux gastritis and esophagitis. Ursodeoxycholic acid does not break the gastric mucosal barrier as endogenous bile acid does and therefore its substitution for endogenous bile acid should benefit patients with bile reflux gastritis and esophagitis. Ursodeoxycholic acid therapy has been shown to reduce symptoms in this condition (32)but again, inability to displace endogenous bile acids limits its effectiveness, and inability to function efficiently in lipid absorption would make any major displacement undesirable even if possible. Even though the present data suggest that UDCA may have limited usefulness in a variety of diseases in which endogenous bile acids may produce symptoms or exacerbate disease, they suggest that the use of UDCA in cholesterol gallstone dissolution may offer certain advantages. The fact that UDCA does not inhibit conversion of cholesterol to bile acid and that it promotes cholesterol absorption less well than endogenous bile acid suggests that it may not adversely affect serum cholesterol and low-density lipoprotein concentrations as does CDCA (33). Such a possibility, however, could be proven or disproven only through extensive clinical trials. References 1. Makino I, Nakagawa

S. Changes in biliary lipid and biliary bile acid composition in patients after administration of ursodeoxycholic acid. J Lipid Res 1978;19:723-8.

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URSODEOXYCHOLATE

2. Tokyo Cooperative Gallstone Study Group. Efficacy and indications of ursodeoxycholic acid treatment for dissolving gallstones. A multicenter, double-blind trial. Gastroenterology 1980;78:542-8. 3. Salen G, Colalillo A, Vergo D, Bagan E, Tint GS, Shefer S. Effect of high and low doses of ursodeoxycholic acid in gallstone dissolution in humans. Gastroenterology 1980;78: 1412-8. 4. Maton PN, Murphy GM, Dowling RH. Ursodeoxycholic acid treatment of gallstones. Dose-response study and possible mechanisms of action. Lancet 1977;ii:1297-301. 5. Roda E, Bazzoli F, Labate AMM, et al. Ursodeoxycholic acid versus chenodeoxycholic acid as a cholesterol gallstone dissolving agent: a comparative randomized study. Hepatology 1982;2:804-10. 6. Chadwick VS, Gaginella TS, Carlson GL, Debongnie J-C, Phillips SF, Hofmann AF. Effect of molecular structure on bile acid-induced alterations in absorptive function, permeability and morphology in the perfused rabbit colon. J Lab Clin Med 1979;94:661-74. 7. Gordon SJ, Kinsey MD, Magen JS, Joseph RE, Kowlessar OD. Structure of bile acids associated with secretion in the rat cecum. Gastroenterology 1979;77:38-44. 8. Carey MC, Montet J-C, Phillips MC, Armstrong MJ, Mazer NA. Thermodynamic and molecular basis for dissimilar cholesterol solubilizing capacities by micellar solutions of bile salts: cases of sodium chenodeoxycholate and sodium ursodeoxycholate and their glycine and taurine conjugates. Biochemistry 1981;20:3637-48. 9. Raicht RF, Cohen Bl, Sarwal A, Takashai M. Ursodeoxycholic acid. Effects on sterol metabolism in rats. Biochim Biophys Acta 1978;531:1-8. 10. Ponz de Leon M, Carulli N, Loria N, Iori R, Zironi F. Cholesterol absorption during bile acid feeding. Effect of ursodeoxycholic acid (UDCA) administration. Gastroenterology 1980;78:214-9. 11. LaRusso NF. Effect of litholytic bile acids on cholesterol absorption in gallstone patients. Gastroenterology 1983;84: 265-71. 12. Thistle JF, LaRusso NF, Hofmann AF, Turcotte J, Carlson GL, Ott BJ. Differing effects of ursodeoxycholic or chenodeoxycholic acid on biliary cholesterol saturation and bile acid metabolism in man. Dig Dis Sci 1982;27:161-8. 13. Roda E, Roda A. Samo C. et al. Effect of ursodeoxycholic acid administration on biliary lipid composition and bile acid kinetics in cholesterol gallstone patients. Dig Dis Sci 1979;24: 123-8. 14. Lee D, Bonorris G, Cohen H, Gilmore C, Marks J, Schoenfield LJ. Effect of ursodeoxycholic acid on bile acid kinetics and hepatic lipid secretion (abstr). Hepatology 1981;1:526. 15. Grundy SM. Ahrens EH Jr, Miettinen TA. Quantitative isolation and gas-liquid chromatographic analysis of fecal bile acid. J Lipid Res 1965;6:397-410. 16. Miettinen TA, Ahrens EH Jr, Grundy SM. Quantitative isolation and gas-liquid chromatographic analysis of dietary and fecal neutral steroids. J Lipid Res 1965;6:411-24. 17. Grundy SM. Metzger AL. A physiologic method for estimation of hepatic secretion of biliary lipids in man. Gastroenterology 1972;62:1200-17. 18. von Bergmann K, Mok HY, Hardison WGM, Grundy SM.

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