Systemic spill-over of bile acids

Longitudinal characterization of plasma and fecal bile acids in dairy heifers from birth to first calving in response to transition milk feeding

This study aimed to characterize plasma bile acid changes from birth to first calving and evaluate the effects of early transition milk (TM) feeding versus milk replacer (MR) during key stages. Fecal bile acids in TM-fed calves were also analyzed, offering insights into bile acid metabolism. Thirty female Holstein calves were fed TM or MR for the first 5 d, followed by 12 L/d MR. From d 14, calves were fed MR and starter with gradual weaning between wk 8 and 14. Blood samples were collected at 7 time points: 30 min and 12 h after birth, preweaning (wk 2, 6), weaning (wk 14), 8 mo, 13 mo, 3 wk before calving, at calving, and 3 wk after calving. Fecal samples were collected from a subset of TM-fed calves (n = 10) at birth, wk 6, wk 14, 8 mo, and calving. Samples were analyzed for bile acids using the Biocrates MxP Quant 500 kit. Cholic acid (CA) in plasma showed significant time-treatment interactions, with higher levels in TM-fed calves at weaning. Taurine- and glycine-conjugated bile acids had no treatment or time-treatment interactions, but all plasma bile acids showed significant time effects. Principal component analysis revealed that bile acid profiles at birth and after colostrum intake were tightly clustered. Plasma bile acid profiles showed greater dispersion during milk feeding and weaning, with tighter clustering observed postweaning, particularly at 13 mo, and in the transition period. Significant effects were observed for CA, deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA), with CA showing a notable interaction and being higher in TM-fed calves at weaning than in MR-fed calves. Bile acid levels increased toward weaning, peaked at wk 14, and decreased after weaning. Glycine-conjugated bile acids changed over time, with glycocholic acid (GCA) and glycodeoxycholic acid (GDCA) peaking at weaning, and glycochenodeoxycholic acid (GCDCA) being elevated before weaning, decreasing thereafter, and increasing again at calving. Taurine-conjugated bile acids also showed temporal changes, peaking at wk 6. The shifts in bile acid composition from birth to postcalving, with taurolithocholic acid (TLCA), GDCA, and taurocholic acid (TCA) initially dominating, CA increasing at weaning, and GDCA and DCA dominating at calving, with CA increasing again postcalving. During the transition to calving, CA decreased whereas glycine-conjugated bile acids increased relative to taurine-conjugated bile acids in plasma, irrespective of treatment. Fecal bile acid profiles in TM-fed calves clustered distinctly at birth, evolving through pre- to postweaning and calving, with increasing profile overlap over time. Most fecal bile acids, except DCA and CA, were abundant at birth but declined over time. Both DCA and CA increased postweaning, mirroring plasma trends. From wk 6 to calving, DCA was the dominant bile acid, accounting for the highest percentage of total bile acids excreted in feces. Spearman's correlation analysis was performed to assess the relationship between plasma and fecal bile acids in TN-fed calves. A significant positive correlation was observed only for GCDCA (Spearman's rank correlation coefficient [rho] = 0.35), whereas all other bile acids were not correlated. These results illustrate the complex dynamics of bile acid profiles during calf development.

Bile acid metabolism in type 2 diabetes mellitus

Type 2 diabetes mellitus is a complex disorder associated with insulin resistance and hyperinsulinaemia that is insufficient to maintain normal glucose metabolism. Changes in insulin signalling and insulin levels are thought to directly explain many of the metabolic abnormalities that occur in diabetes mellitus, such as impaired glucose disposal. However, molecules that are directly affected by abnormal insulin signalling might subsequently go on to cause secondary metabolic effects that contribute to the pathology of type 2 diabetes mellitus. In the past several years, evidence has linked insulin resistance with the concentration, composition and distribution of bile acids. As bile acids are known to regulate glucose metabolism, lipid metabolism and energy balance, these findings suggest that bile acids are potential mediators of metabolic distress in type 2 diabetes mellitus. In this Review, we highlight advances in our understanding of the complex regulation of bile acids during insulin resistance, as well as how bile acids contribute to metabolic control.

Plasma Levels of Bile Acids Are Related to Cardiometabolic Risk Factors in Young Adults

CONTEXT

Bile acids (BA) are known for their role in intestinal lipid absorption and can also play a role as signaling molecules to control energy metabolism. Prior evidence suggests that alterations in circulating BA levels and in the pool of circulating BA are linked to an increased risk of obesity and a higher incidence of type 2 diabetes in middle-aged adults.

OBJECTIVE

We aimed to investigate the association between plasma levels of BA with cardiometabolic risk factors in a cohort of well-phenotyped, relatively healthy young adults.

METHODS

Body composition, brown adipose tissue, serum classical cardiometabolic risk factors, and a set of 8 plasma BA (including glyco-conjugated forms) in 136 young adults (age 22.1 ± 2.2 years, 67% women) were measured.

RESULTS

Plasma levels of chenodeoxycholic acid (CDCA) and glycoursodeoxycholic acid (GUDCA) were higher in men than in women, although these differences disappeared after adjusting for body fat percentage. Furthermore, cholic acid (CA), CDCA, deoxycholic acid (DCA), and glycodeoxycholic acid (GDCA) levels were positively, yet weakly associated, with lean body mass (LBM) levels, while GDCA and glycolithocholic acid (GLCA) levels were negatively associated with 18F-fluorodeoxyglucose uptake by brown adipose tissue. Interestingly, glycocholic acid (GCA), glycochenodeoxycholic acid (GCDCA), and GUDCA were positively associated with glucose and insulin serum levels, HOMA index, low-density lipoprotein cholesterol, tumor necrosis factor alpha, interleukin (IL)-2, and IL-8 levels, but negatively associated with high-density lipoprotein cholesterol, ApoA1, and adiponectin levels, yet these significant correlations partially disappeared after the inclusion of LBM as a confounder.

CONCLUSION

Our findings indicate that plasma levels of BA might be sex dependent and are associated with cardiometabolic and inflammatory risk factors in young and relatively healthy adults.

Changes in Bile Acid Metabolism, Transport, and Signaling as Central Drivers for Metabolic Improvements After Bariatric Surgery

PURPOSE OF REVIEW

We review current evidence regarding changes in bile acid (BA) metabolism, transport, and signaling after bariatric surgery and how these might bolster fat mass loss and energy expenditure to promote improvements in type 2 diabetes (T2D) and nonalcoholic fatty liver disease (NAFLD).

RECENT FINDINGS

The two most common bariatric techniques, Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG), increase the size and alter the composition of the circulating BA pool that may then impact energy metabolism through altered activities of BA targets in the many tissues perfused by systemic blood. Recent reports in human patients indicate that gene expression of the major BA target, the farnesoid X receptor (FXR), is increased in the liver but decreased in the small intestine after RYGB. In contrast, intestinal expression of the transmembrane G protein-coupled BA receptor (TGR5) is upregulated after surgery. Despite these apparent conflicting changes in receptor transcription, changes in BAs after both RYGB and VSG are associated with elevated postprandial systemic levels of fibroblast growth factor 19 (from FXR activation) and glucagon-like peptide 1 (from TGR5 activation). These signaling activities are presumed to support fat mass loss and related metabolic benefits of bariatric surgery, and this supposition is in agreement with findings from rodent models of RYGB and VSG. However, inter-species differences in BA physiology limit direct translation and mechanistic understanding of how changes in individual BA species contribute to post-operative improvements of T2D and NAFLD in humans. Thus, details of all these changes and their influences on BAs' biological actions are still under scrutiny. Changes in BA physiology and receptor activities after RYGB and VSG likely support weight loss and promote sustained metabolic improvements.

Targeting Bile Acid-Activated Receptors in Bariatric Surgery

Bariatric surgical procedures, including Roux-en-Y gastric bypass and vertical sleeve gastrectomy, are currently the most effective clinical approaches to achieve a significant and sustainable weight loss. Bariatric surgery also concomitantly improves type 2 diabetes and other metabolic diseases such as nonalcoholic steatohepatitis, cardiovascular diseases, and hyperlipidemia. However, despite the recent exciting progress in the understanding how bariatric surgery works, the underlying molecular mechanisms of bariatric surgery remain largely unknown. Interestingly, bile acids are emerging as potential signaling molecules to mediate the beneficial effects of bariatric surgery. In this review, we summarize the recent findings on bile acids and their activated receptors in mediating the beneficial metabolic effects of bariatric surgery. We also discuss the potential to target bile acid-activated receptors in order to treat obesity and other metabolic diseases.

Increased Bile Acid Synthesis and Impaired Bile Acid Transport in Human Obesity

CONTEXT

Alterations in bile acid (BA) synthesis and transport have the potential to affect multiple metabolic pathways in the pathophysiology of obesity.

OBJECTIVE

The objective of the study was to investigate the effects of obesity on serum fluctuations of BAs and markers of BA synthesis.

DESIGN

We measured BA fluctuations in 11 nonobese and 32 obese subjects and BA transporter expression in liver specimens from 42 individuals and specimens of duodenum, jejunum, ileum, colon, and pancreas from nine individuals.

MAIN OUTCOME MEASURES

We analyzed serum BAs and markers of BA synthesis after overnight fasting, during a hyperinsulinemic-euglycemic clamp, or a mixed-meal tolerance test and the association of BA transporter expression with body mass index.

RESULTS

BA synthesis markers were 2-fold higher (P < .01) and preferentially 12α-hydroxylated (P < .05) in obese subjects, and both measures were correlated with clamp-derived insulin sensitivity (r = -0.62, P < .0001, and r = -0.39, P = .01, respectively). Insulin infusion acutely reduced serum BAs in nonobese subjects, but this effect was blunted in obese subjects (δBAs -44.2% vs -4.2%, P < .05). The rise in serum BAs postprandially was also relatively blunted in obese subjects (δBAs +402% vs +133%, P < .01). Liver expression of the Na+-taurocholate cotransporting polypeptide and the bile salt export pump were negatively correlated with body mass index (r = -0.37, P = .02, and r = -0.48, P = .001, respectively).

CONCLUSIONS

Obesity is associated with increased BA synthesis, preferential 12α-hydroxylation, and impaired serum BA fluctuations. The findings reveal new pathophysiological aspects of BA action in obesity that may lend themselves to therapeutic targeting in metabolic disease.

Roux-en-Y gastric bypass normalizes the blunted postprandial bile acid excursion associated with obesity

BACKGROUND

Bile acids (BAs) are nutrient-responsive hormones that modulate energy balance through cell surface and nuclear receptors. Postprandial plasma BAs have been found to be decreased in obesity.

OBJECTIVE

We aimed to determine whether meal-stimulated circulating BA levels are altered by Roux-en-Y gastric bypass (RYGB), an operation that modifies the neurohumoral determinants of food intake and energy expenditure to cause significant and durable weight loss.

DESIGN

Longitudinal study measuring fasting and postprandial plasma BAs before and after RYGB.

SUBJECTS

Five obese surgical patients and eight lean controls underwent frequent blood sampling after a standard liquid meal. Obese subjects were also tested at 1, 4 and 40 weeks after RYGB. Primary and secondary circulating BAs, as well as their glycine and taurine conjugates, were measured via reverse-phase high-performance liquid chromatography/mass spectroscopy.

RESULTS

We found that postprandial excursion of conjugated BAs was 52.4% lower in obese than in lean individuals by area-under-the-curve (AUC) analysis (378 vs 793 μmol min l(-1), respectively, P<0.05). By 40 weeks after RYGB, the meal-induced rise in conjugated BAs increased by 55.5% to the level of healthy lean controls (378 pre-op vs 850 μmol min l(-) post-op by AUC analyses, P<0.05). In contrast, postprandial concentrations of unconjugated BAs were similar in lean and obese individuals and were not affected by surgery.

CONCLUSION

In light of the growing evidence that BAs have key roles in glucose, lipid and energy homeostasis, the observation that RYGB normalizes the blunted postprandial circulating BA response in obesity suggests that BAs may contribute to the improvement in meal-related physiology seen after RYGB. Further studies are warranted to examine this hypothesis and to determine the degree to which an augmented BA response to nutrient ingestion may mediate the increased incretin response, brown adipose tissue activation and thermic effect of feeding that has been observed after this operation.

Hypocholesterolemic action of Lactobacillus acidophilus ATCC 43121 and calcium in swine with hypercholesterolemia induced by diet

Thirty-three Yorkshire barrows (92 kg), fed a high cholesterol diet for 14 d had mean concentrations of serum cholesterol of 294.6 +/- 7.8 mg/dl. Starting on d 15 and for an additional 15 d, crystalline cholesterol was removed from the diet and pigs were assigned to one of four treatments: including two levels of calcium (0.7% and 1.4%) with and without added viable Lactobacillus acidophilus ATCC 43121 (2.5 x 10(11) cells per feeding). Serum cholesterol levels decreased, as expected, for all groups. However, the declines were initiated sooner for the groups receiving L. acidophilus. and those receiving the high level of calcium than for the respective control groups. When averaged over days, pigs fed L. acidophilus had 11.8% lower total cholesterol than pigs fed a diet without L. acidophilus. Similarly, pigs fed 1.4% calcium had a significantly lower total cholesterol than pigs fed 0.7% calcium. The effects were greater on low density lipoprotein cholesterol than on high density lipoprotein cholesterol. In addition, during the overall 15-d experimental period, serum bile acids were reduced 23.9% by dietary L. acidophilus and by 21.4% by 1.4% dietary calcium compared with those of their controls. Total bile acid concentration was positively correlated with total cholesterol concentration for pigs fed L. acidophilus or 1.4% calcium. These data suggest that both L. acidophilus and calcium can enhance the reduction of serum cholesterol in pigs that had been fed a high cholesterol diet, probably through alteration in the enterohepatic circulation of bile acids.

Serum unconjugated bile acids in patients with small bowel bacterial overgrowth

Concentrations of total and unconjugated bile acids in serum were measured fasting and 2 h postprandially in 9 patients with a positive [14C]glycocholate breath test consistent with small bowel bacterial overgrowth and in 13 controls. Gas-liquid chromatography-mass spectrometry (GLC-MS) and enzymatic-fluorometric assays were both used. In contrast to previous work, total serum bile acids were only occasionally elevated in patients with bacterial overgrowth. Total 2 h postprandial unconjugated bile acids, however, were elevated in 7/9 patients when measured by GLC-MS and in 6/9 when measured by the enzymatic-fluorometric method. The best separation between patients and controls was achieved by GLC-MS determinations of 2 h postprandial unconjugated cholic acid or primary bile acids, which were abnormal in 8/9 patients. This study indicates that measurement of serum bile acids may be a useful approach to the diagnosis of bacterial overgrowth, but would require accessible methods for separating and measuring cholic acid or unconjugated primary bile acids in post-prandial sera.

Effect of the bile-acid sequestrant colestipol on postprandial serum bile-acid concentration: evaluation by bioluminescent enzymic analysis

Chronic ingestion of bile-acid sequestrants has been shown to decrease the serum cholesterol concentration and coronary events in hypercholesterolaemic patients. To develop improved sequestrants, a rapid, convenient method for testing the bile-acid binding efficacy of sequestrants is needed. Serum bile-acid concentrations could be used to detect bile-acid binding by an administered sequestrant, since the serum bile-acid concentration is determined largely by the rate of intestinal absorption in healthy individuals. To test this, serum bile-acid concentrations were measured at frequent intervals over 24 h in five otherwise healthy hypercholesterolaemic subjects during the ingestion of three standard meals, with or without the addition of 5 g colestipol granules administered 30 min before each meal. Total serum bile-acid concentration was measured with a previously reported bioluminescent enzymic assay, that uses a 3 alpha-hydroxysteroid dehydrogenase, an oxido-reductase, and a bacterial luciferase co-immobilized on to Sepharose beads. Bile acids in 1 ml of serum were isolated by solid-phase extraction chromatography with reversed-phase C18 cartridges. Colestipol lowered the postprandial elevation of serum bile acids by one half, with a subsequent decrease in the cumulative area under the curve. The data suggest that measurement of serum bile-acid concentrations by bioluminescence is a rapid, simple way to document the efficacy of bile-acid sequestrants.

Computer simulation of portal venous shunting and other isolated hepatobiliary defects of the enterohepatic circulation of bile acids using a physiological pharmacokinetic model

The effect of three isolated defects in the enterohepatic circulation of bile acids on the size and distribution of the bile acid pool, plasma bile acid levels and bile acid secretion into the intestine was simulated using a linear multicompartmental physiological pharmacokinetic model previously used to simulate these aspects of bile acid metabolism in healthy man. Stepwise increases in portal-systemic shunting (with a reciprocal decrease in hepatic blood flow) caused an exponential increase in systemic plasma concentrations of bile acids, but no other major changes in bile acid metabolism. When the effect of varying fractional hepatic extraction was simulated, it was found that the greater the fractional hepatic extraction, the greater the elevation observed for systemic plasma bile acid levels for a given degree of portal-systemic shunting. When total hepatic blood flow was restored to normal by simulating "arterialization," systemic plasma levels of bile acids decreased strikingly, yet remained elevated. For cholate with a fractional hepatic extraction of 0.9 and 100% portal-systemic shunting, arterialization caused a decrease from a 20-fold elevation to a 5-fold elevation. This simulation thus defined the effect of the presence of the portal venous system per se on plasma bile acid levels and also quantified the circulatory route by which substances reach the liver when portal-systemic shunting is present. An isolated defect in hepatic uptake of bile acids caused little change in overall bile acid metabolism other than modestly increased plasma levels. Loss of bile acid storage by the gallbladder caused the majority of the bile acid pool to move from the gallbladder compartments to the proximal small intestine during fasting but had little effect on the dynamics of the enterohepatic circulation during eating. The results of these novel simulations of isolated defects in bile acid transport should aid in the interpretation of the more complex changes in bile acid metabolism which are likely to occur in hepatic or biliary disease.

Simulation of the metabolism and enterohepatic circulation of endogenous deoxycholic acid in humans using a physiologic pharmacokinetic model for bile acid metabolism

The metabolism and enterohepatic circulation of deoxycholic acid (DCA), a major secondary bile acid in humans, was simulated using a linear multicompartmental physiologic pharmacokinetic model. The model was similar to that previously reported and used to simulate the metabolism of cholic acid and chenodeoxycholic acid, but differed in two respects: (a) the input of newly formed DCA molecules originated from colonic absorption rather than from de novo hepatic biosynthesis and (b) a new type of transfer coefficient was proposed to describe the movement of DCA molecules from an insoluble, bound compartment to a soluble compartment. Simulations were performed to define the effect of varying fractional colonic absorption (from 0.1 to 0.6) as well as varying fractional formation of DCA from cholic acid (from 0.3 to 1). The simulations indicated that the exchangeable total DCA pool expanded up to 12-fold as fractional colonic absorption was increased from 0.1 to 0.6. The fractional turnover rate of the DCA pool showed a corresponding decrease. Increased conversion of cholic acid to DCA had an effect on DCA pool size that was similar to that resulting from increased colonic fractional absorption. So long as ileal absorption was efficient, the "soluble" colonic pool of DCA remained small relative to other organ pools, and the absorption of unconjugated DCA from the colon was less than 10% of the total DCA absorption from the ileum. It is proposed that the relatively large proportion of DCA in the biliary bile acids of white adults in the Western world as compared with that of most other mammals is attributable to (a) a high fractional absorption of DCA because of a diet relatively low in fiber, (b) the absence of hepatic 7-hydroxylation of DCA, and (c) effective competition by DCA conjugates for active transport by the terminal ileum.

Drugs as probes of organ function: evaluation of the hepatobiliary axis using oral rifampicin and novel high performance liquid chromatography

Investigation of the uptake and metabolism of drugs by organs such as the liver may allow assessment of specific aspects of organ function. Rifampicin, when orally administered, is transported into the hepatocyte from portal blood and thence passes, with its deacetylated metabolite, into the systemic circulation and into bile. This paper reports an investigation of the pharmacokinetics of a sub-therapeutic oral dose of rifampicin in healthy subjects, in patients with cirrhosis and in subjects with Gilbert's syndrome. The areas under the plasma concentration curves (AUC) in patients with cirrhosis were significantly greater than in healthy subjects. Subjects with Gilbert's syndrome had decreased AUCs compared with healthy subjects and were clearly distinguished from patients with cirrhosis. Rifampicin concentration in serum was measured by HPLC using a novel direct injection technique.

Serum unconjugated bile acids: qualitative and quantitative profiles in ileal resection and bacterial overgrowth

Qualitative and quantitative profiles of unconjugated bile acids in the serum obtained over a 24-h period from three patients with ileal resections and one with a bacterial overgrowth are described. Unconjugated serum bile acids were determined using the high sensitivity and resolution of capillary column gas liquid chromatography after their rapid extraction and isolation using reverse phase octadecylsilane bonded silica cartridges and the lipophilic gel Lipidex 1000. Unconjugated serum bile acid concentrations were elevated throughout the day in both ileum resected patients and in conditions involving bacterial overgrowth when compared to healthy subjects. Total conjugated cholic acid concentrations were expectedly low in both intestinal disorders and were without the postprandial increases generally observed in healthy subjects. Qualitative gas chromatographic profiles of serum unconjugated bile acids in bacterial overgrowth distinctly revealed a predominance of deoxycholic acid and other secondary bile acids in all samples, while, in conditions of an impaired enterohepatic circulation, deoxycholic acid was absent or present in only trace amounts. The potential significance of measuring serum unconjugated bile acids in intestinal disorders is discussed.

Serum bile acid composition in patients with cystic fibrosis

Serum bile acid composition was examined in detail using capillary column gas chromatography and mass spectrometry in 10 children with cystic fibrosis (CF) and 4 healthy children. The mean total bile acid concentration in fasting serum of CF patients was 2.33 +/- 0.84 mumol/l, slightly lower than but not statistically significantly different from healthy controls (mean 2.86 +/- 0.98 mumol/l) and appeared to show no relationship to the degree of exocrine pancreatic insufficiency. Analysis of individual serum bile acids in these children showed that cholic acid represented less than 10% of the total bile acids. Chenodeoxycholic acid was the predominant serum bile acid; the mean concentration in CF patients was 0.98 +/- 0.51 mumol/l, lower than for the healthy controls (1.69 +/- 0.84 mumol/l). Concentrations of lithocholic acid, 3 beta-hydroxy-5-cholenoic, ursodeoxycholic and 3 beta, 7 alpha, 12 alpha-trihydroxy-5 beta-cholanoic acids in fasting serum samples of the CF patients were not significantly different from the healthy control sera but were higher than those normally found in adults. Measurements of fecal bile acid excretion indicated an increased loss of primary bile acids in patients with CF consistent with an impairment of the enterohepatic circulation of bile acids.

Value of serum determinations for prediction of increased ursodeoxycholic and chenodeoxycholic levels in bile

The correlation between biliary and serum levels of ursodeoxycholic and chenodeoxycholic acids was studied in a double-blind controlled manner in 39 patients before and during treatment with ursodeoxycholic acid, 800 mg/day; ursodeoxycholic acid, 400 mg/day; chenodeoxycholic acid, 750 mg/day; chenodeoxycholic acid, 375 mg/day; and placebo, respectively. On a total of 74 occasions, fasting duodenal bile and venous blood samples were obtained simultaneously. Biliary bile acid composition was determined by gas-liquid chromatography and serum ursodeoxycholic and chenodeoxycholic acid concentrations by radioimmunoassays. There was a much closer correlation between the biliary and serum levels of ursodeoxycholic acid (r = 0.8184, P less than 0.001) than between those of chenodeoxycholic acid (r = 0.4707, P less than 0.01). In contrast to serum chenodeoxycholic, which showed many overlaps between pre- and posttreatment values, serum ursodeoxycholic acid proved to be a very sensitive, specific, and convenient means of predicting the presence of increased levels of ursodeoxycholic acid in the enterohepatic cycle.

Fasting and postprandial serum bile acid concentration with special reference to variations in the conjugate profile

Serum bile acids were group-separated by ion exchange chromatography on diethylaminohydroxypropyl Sephadex LH-20 into unconjugated bile acids and bile acids conjugated with either glycine, taurine, glucuronic acid or sulphuric acid. The conjugate moiety was hydrolysed by treatment with a combination of Helix pomatia and cholylglycine hydrolase and the released bile acids analysed by gas liquid chromatography/mass spectrometry. Analysis of fasting and postprandial serum from six healthy subjects showed that, in addition to the primary bile acids, cholic (C) and chenodeoxycholic acid (CDC), secondary bile acids were present to varying extents. Unconjugated serum bile acids were found in four of the six subjects. Glycine and taurine conjugates of C and CDC and their glucuronides and sulphates were found in all subjects. The postprandial increase of serum bile acids was mainly due to increase of the glycine conjugates of C and CDC. After the meal, the ratio C:CDC in glycine and taurine conjugates shifted to lower values.