Serum lipids, biliary lipid composition, and bile acid metabolism in vegetarians as compared to normal controls

Association between plant-based diets and plasma lipids: a systematic review and meta-analysis

Context

Although a recent meta-analysis of randomized controlled trials showed that adoption of a vegetarian diet reduces plasma lipids, the association between vegetarian diets and long-term effects on plasma lipids has not been subjected to meta-analysis.

Objective

The aim was to conduct a systematic review and meta-analysis of observational studies and clinical trials that have examined associations between plant-based diets and plasma lipids.

Data Sources

MEDLINE, Web of Science, and the Cochrane Central Register of Controlled Trials were searched for articles published in English until June 2015.

Study Selection

The literature was searched for controlled trials and observational studies that investigated the effects of at least 4 weeks of a vegetarian diet on plasma lipids.

Data Extraction

Two reviewers independently extracted the study methodology and sample size, the baseline characteristics of the study population, and the concentrations and variance measures of plasma lipids. Mean differences in concentrations of plasma lipids between vegetarian and comparison diet groups were calculated. Data were pooled using a random-effects model.

Results

Of the 8385 studies identified, 30 observational studies and 19 clinical trials met the inclusion criteria (N = 1484; mean age, 48.6 years). Consumption of vegetarian diets was associated with lower mean concentrations of total cholesterol (−29.2 and −12.5 mg/dL, P < 0.001), low-density lipoprotein cholesterol (−22.9 and −12.2 mg/dL, P < 0.001), and high-density lipoprotein cholesterol (−3.6 and −3.4 mg/dL, P < 0.001), compared with consumption of omnivorous diets in observational studies and clinical trials, respectively. Triglyceride differences were −6.5 (P = 0.092) in observational studies and 5.8 mg/dL (P = 0.090) in intervention trials.

Conclusions

Plant-based diets are associated with decreased total cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol, but not with decreased triglycerides.

Systematic Review Registration

PROSPERO number CRD42015023783. Available at: https://www.crd.york.ac.uk/PROSPERO/display_record.asp?ID=CRD42015023783.

Cholesterol Absorption and Synthesis in Vegetarians and Omnivores

SCOPE

Vegetarian diets are considered health-promoting; however, a plasma cholesterol lowering effect is not always observed. We investigate the link between vegetarian-diet-induced alterations in cholesterol metabolism.

METHODS AND RESULTS

We study male and female omnivores, lacto-ovo vegetarians, lacto vegetarians, and vegans. Cholesterol intake, absorption, and fecal sterol excretion are measured as well as plasma concentrations of cholesterol and noncholesterol sterols. These serve as markers for cholesterol absorption, synthesis, and catabolism. The biliary cholesterol secretion rate is estimated. Flux data are related to body weight. Individual vegetarian diet groups are statistically compared to the omnivore group. Lacto vegetarians absorb 44% less dietary cholesterol, synthesized 22% more cholesterol, and show no differences in plasma total and LDL cholesterol. Vegan subjects absorb 90% less dietary cholesterol, synthesized 35% more cholesterol, and have a similar plasma total cholesterol, but a 13% lower plasma LDL cholesterol. No diet-related differences in biliary cholesterol secretion and absorption are observed. Total cholesterol absorption is lower only in vegans. Total cholesterol input is similar under all vegetarian diets.

CONCLUSIONS

Unaltered biliary cholesterol secretion and higher cholesterol synthesis blunt the lowered dietary cholesterol intake in vegetarians. LDL cholesterol is significantly lower only in vegans.

The use of stable and radioactive sterol tracers as a tool to investigate cholesterol degradation to bile acids in humans in vivo

Alterations of cholesterol homeostasis represent important risk factors for atherosclerosis and cardiovascular disease. Different clinical-experimental approaches have been devised to study the metabolism of cholesterol and particularly the synthesis of bile acids, its main catabolic products. Most evidence in humans has derived from studies utilizing the administration of labeled sterols; these have several advantages over in vitro assay of enzyme activity and expression, requiring an invasive procedure such as a liver biopsy, or the determination of fecal sterols, which is cumbersome and not commonly available. Pioneering evidence with administration of radioactive sterol derivatives has allowed to characterize the alterations of cholesterol metabolism and degradation in different situations, including spontaneous disease conditions, aging, and drug treatment. Along with the classical isotope dilution methodology, other approaches were proposed, among which isotope release following radioactive substrate administration. More recently, stable isotope studies have allowed to overcome radioactivity exposure. Isotope enrichment studies during tracer infusion has allowed to characterize changes in the degradation of cholesterol via the "classical" and the "alternative" pathways of bile acid synthesis. Evidence brought by tracer studies in vivo, summarized here, provides an exceptional tool for the investigation of sterol metabolism, and integrate the studies in vitro on human tissue.

Diet and gallstones in Italy: the cross-sectional MICOL results

Fifteen thousand nine hundred ten men and 13,674 women (age, 30-69 years) were enrolled in an epidemiological survey of the general population, between December 1984 and April 1987. Each participant was submitted to ultrasonography (US) of the gallbladder and completed a food-frequency questionnaire, covering 38 food items. A common portion size was identified and subjects were asked how often each item was consumed. Nutrient intake was computed by multiplying the intake frequency and nutrient content per portion for each item, and then by summing the product over all foods. Each nutrient intake was adjusted for energy intake. Alcohol intake was calculated by summing the consumption of wine, beer, and liquor. Having excluded subjects aware of having gallstones (GS) or previously submitted to cholecystectomy (to avoid prothopatic bias), 787 males and 1,014 females with GS and 14,272 males and 10,836 females without GS were available for analysis. Relative risks (RR) of GS were computed by quintiles of nutrient intake. The overnight fasting period was calculated as the difference between the specified time of dinner and the time of the next meal (breakfast or lunch). A significant negative association was found between RR of GS and total energy intake for males (chi2 for trend = 8.37; P = .004), fiber intake for females (chi2 = 5.45; P = .02), and daily alcohol consumption for males (chi2 = 10.86; P = .001). A positive association was observed between RR of GS and carbohydrate (chi2 = 5.95; P = .01 for males; chi2 = 9.39; P = .002 for females) and protein intake only for males (chi2 = 10.92; P = .01). Prevalence of GS was higher among subjects who had an overnight fasting period of over 12 hours than subjects with that of less than 12 hours. (RR: 1.35; 95% CI: 1.01-1.80 for males; RR: 1.28; 95% CI: 1.03-1.60 for females). These data do not confirm that high energy intake is associated with an increased risk of GS. Factors protecting against GS comprise: low carbohydrate (males and females) and protein (males) intakes, high fiber (females) and moderate alcohol intake (males) consumption, and a shorter overnight fasting period for both sexes.

Effects of bile salt and phospholipid hydrophobicity on lithogenicity of human gallbladder bile

Increased biliary bile salt and phospholipid hydrophobicity may promote nucleation of cholesterol crystals and gallstone formation. We therefore compared bile salt composition (determined by gas-liquid chromatography) in patients with cholesterol (n = 35) and pigment (n = 16) gallstones (group A). Bile salt composition and cumulative bile salt hydrophobicity index were not different between both stone types. Hydrophobicity index or % of individual bile salts did not correlate with cholesterol saturation index or nucleation time. In an additional 21 cholesterol stone patients (group B) biliary bile salt and phospholipid hydrophobicity as determined by high-pressure liquid chromatography did not correlate with cholesterol saturation index or nucleation time. In both group A and group B, cholesterol stone patients with cholesterol crystals in their fresh biles had a higher % deoxycholic acid, a lower % cholic acid and a higher bile salt hydrophobicity index than crystal-negative patients. This study indicates the need for further research on the role of bile salt hydrophobicity in the pathogenesis of gallstones.

Nutrient intake of endurance runners with ovo-lacto-vegetarian diet and regular western diet

During an endurance run (1,000 km in 20 days) it was investigated whether an ovo-lacto-vegetarian diet (OLVD) could cover the nutritional requirements of endurance athletes. A regular western diet (RWD) was used as reference. Both diets were offered with an energy content of 4,500 kcal per day and an energy percentage of carbohydrate:fat:protein of 60:30:10. The runners were divided into two dietary groups according to their usual dietary habits. The results of the 55 participants who completed the race show that runners from both groups had the same intake of energy, carbohydrate, fat and protein. Runners of the OLVD group consumed more dietary fiber and polyunsaturated fatty acids as well as less cholesterol. With the exception of sodium chloride and cobalamin, the intake of the calculated minerals and vitamins was higher in the OLVD and exceeded the official recommendations. This study shows that an OLVD with a high nutrient density is adequate to cover the nutritional requirements of endurance-athletes. The intake and absorption of iron should be monitored closely in all diet groups.

Bile acid and phospholipid fatty acid composition in bile of patients with cholesterol and pigment gallstones

It has been previously reported that patients with cholesterol gallstones have increased biliary deoxycholate and arachidonate content as compared with normal subjects without gallstones. Increased biliary deoxycholate and arachidonate content might be a primary factor in the pathogenesis of cholesterol gallstones or merely an epiphenomenon due to the presence of gallstones. We therefore compared biliary bile acid composition in 46 patients with cholesterol gallstones and 22 patients with pigment stones. In addition, biliary phospholipid fatty acid composition was determined in 44 of these patients (30 cholesterol and 14 pigment stone patients). No significant differences were detected. In particular, the percentage deoxycholic acid (mean +/- SD: 20.3 +/- 8.8% and 21.5 +/- 10.9% respectively) and the percentage arachidonic acid (4.4 +/- 2.0% and 4.5 +/- 2.2%, respectively) were very similar. A significant correlation between age and biliary cholesterol saturation index was found only for the group of patients with pigment stones (R = 0.52, p less than 0.02). In conclusion, the present study does not support a primary role for increased biliary deoxycholic acid or arachidonic acid in the pathogenesis of cholesterol gallstones.

Inhibition of secondary bile acid formation in the large intestine by lactulose in healthy subjects of two different age groups

Secondary bile acids have been implicated in colonic carcinogenesis. Transformation of primary into secondary bile acids (7 alpha-dehydroxylation) in the large bowel is a pH-dependent process. Inhibition of this reaction could be achieved by lowering colonic pH. We, therefore, studied the effects of lactulose (a non-absorbable disaccharide), which is capable of acidifying colonic contents, on secondary bile acid metabolism. Because this metabolism is age dependent, lactulose was given (0.3 g kg-1 twice daily for 12 weeks) to nine middle-aged (age 31-54 years; mean 45.7) and ten elderly subjects (age 56-81 years; mean 66.4). Twice before, and after 6 and 12 weeks' lactulose administration, biliary and faecal bile acids, whole gut transit time, faecal weight and dry weight, and faecal pH were recorded. The concentration of (iso)lithocholic and deoxycholic acid in faeces was higher in elderly subjects (P less than 0.05) but the excretion was comparable. After lactulose the concentration and excretion of the major secondary bile acids decreased. The primary bile acid fraction rose from 5% before, to more than 20% after, lactulose (P less than 0.05). Faecal weight increased and faecal dry weight decreased, resulting in a higher faecal water output during lactulose. Whole gut transit time did not change. The faecal pH dropped after 6 (P less than 0.05) and further after 12 weeks' lactulose (P less than 0.05). The percentage deoxycholic acid in bile was higher, and cholic acid lower, in elderly subjects (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Biliary lipids, bile acid metabolism, gallbladder motor function and small intestinal transit during ingestion of a sub-fifty oral contraceptive

The risk of developing gallstone disease while using low dose oral contraceptives (OC) has been incompletely explored in man. In this study, biliary lipid composition, bile acid conjugation, primary bile acid kinetics, gallbladder storage and emptying by quantitative cholescintigraphy, and small intestinal transit by breath hydrogen analysis are reported in a group of non-obese healthy young women, both after 3-5 months OC, using 30 micrograms ethinyl oestradiol daily, and during an adjacent control period. OC use was associated with a significant rise of biliary cholesterol saturation in gallbladder bile. Total bile acid pool size did not change; however, mean cholic acid pool size was 36% greater than in the control period (P less than 0.001), due to its enhanced synthesis rate, at the expense of chenodeoxycholic acid and deoxycholic acid pool sizes (P less than 0.05). A rise in taurine conjugation of biliary bile acids was apparent in all subjects (P less than 0.0001). Gallbladder motor function was not influenced by ingestion of OC, whereas only a minor retardation of small intestinal transit was found. The findings show an effect of this sub-50 OC on biliary lipid composition and cholesterol saturation that is comparable with that of conventional OC. The predominance of more hydrophilic bile acid conjugates during oral contraception is in keeping with a hepatic effect of this preparation on bile acid metabolism.

Effect of long term lactulose ingestion on secondary bile salt metabolism in man: potential protective effect of lactulose in colonic carcinogenesis

This study investigated whether colonic absorption of secondary bile acids, especially deoxycholate in patients with adenomas could be decreased by oral lactulose. Bile acid metabolism was studied using bile sampling and 14C-deoxycholate kinetics in patients with colonic adenomatous polyps before and after four and 12 weeks of lactulose, 60 g/day. The results indicate that lactulose decreased the deoxycholate pool size from a mean of 22.0 (SD: +/- 13.8) to 14.3 (+/- 7.6) mumol/kg (p less than 0.025). Deoxycholate absorption fell from 3.8 (+/- 2.3) to 2.9 (+/- 1.4 mumol/kg/d (ns). The biliary bile acid composition decreased significantly in deoxycholate after four and 12 weeks with a rise in primary bile acids. There was a highly significant correlation between the %-change in DCA input and the %-change in DCA pool size (r = 0.89). Intestinal transit measured by the pellet method (4.1 +/- 1.9 to 2.4 +/- 0.6 day; p less than 0.01) and faecal pH decreased, while stool frequency and weight rose significantly. Significant correlations between the %-change in gut transit time and the %-change in DCA pool size or %-change in DCA input were absent. The results show that it is possible to lower colonic secondary bile acid absorption by long term lactulose feeding. This effect can be mediated by accelerated transit and the acidification of the colonic contents.

Fatty acid composition of phospholipids in bile in man: promoting effect of deoxycholate on arachidonate

Ninety-five percent of phospholipids (PLs) in bile is secreted as phosphatidylcholine or lecithin. The study of fatty acid patterns of phospholipids present in gallbladder bile could help clarify whether a preponderance of certain fatty acids could play a role in cholesterol gallstone formation in man. In acute bile acid-exchange experiments, it was found that more hydrophobic bile acids did promote the excretion in bile of PL rich in arachidonic acid (a prostaglandin precursor) and stearic acid. We studied, therefore, bile acid, cholesterol and phospholipid fatty acid patterns (measured by gas chromatography) in gallbladder bile, obtained by duodenal intubation and cholecystokinin-stimulation of 24 healthy volunteers with normal liver/gallbladder function (ultrasound). PL-fatty acid composition (mean % +/- SD) was 41.40 (+/- 1.41) for palmitic acid, 2.68 (+/- 0.82) for palmitoleic acid, 5.50 (+/- 1.55) for stearic acid, 12.09 (+/- 0.98) for oleic acid, 32.83 (+/- 3.04) for linoleic acid and 5.64 (+/- 1.59) for arachidonic acid. The proportion of biliary deoxycholate was positively correlated with arachidonic acid (r = 0.71; p less than 0.01), whereas chenodeoxycholate was inversely correlated with arachidonic acid (r = -0.53; p less than 0.01). There was a positive correlation between biliary chenodeoxycholate and linoleic acid (r = 0.48; p less than 0.05) and a negative correlation between biliary deoxycholate and linoleic acid composition (r = 0.68; p less than 0.01). Also a correlation was found between palmitic acid and cholesterol saturation index (r = 0.49; p less than 0.05). We conclude that the hydrophobic bile acid deoxycholate, which does not desaturate cholesterol in bile, promotes the biliary excretion of arachidonic acid. Since arachidonic acid could induce the gallbladder mucosa to produce prostaglandins and mucus, increased biliary PL-arachidonic acid composition might be a factor in cholesterol gallstone disease.

In vivo studies with fibre components

There are few studies in man examining the in vivo effects of dietary fibre on bile acid metabolism but divergent results have been recorded depending on the amount and type of fibre eaten. Studies in which the fibre components pectin, cellulose, and lignin were fed to normal volunteers for four weeks have been reported. The highly fermentable component pectin is associated with increased secondary bile acid formation in contrast to the partially fermentable component cellulose in which the opposite occurs. Lignin has no effect on bile acid metabolism. No component significantly alters biliary lipids. Since ordinary diets contain a mixture of different fibres the net effect will depend on the relative amounts of each. Significant changes in bile acid metabolism and biliary lipids in the longer term (greater than 4 weeks) however, are not precluded by the results of these short term studies.

Effect of lactulose on biliary lipid composition

It has been reported that lactulose can reduce biliary secondary bile acid (deoxycholate) composition and by this action can decrease biliary cholesterol saturation in patients with supersaturated bile. We tried to confirm this by feeding 60 g of lactulose per day during a period of 12 weeks to 8 healthy females with colonic adenomatous polyps and slightly elevated biliary cholesterol saturation. Although a 38% reduction of biliary deoxycholate was noted--38% after 4 weeks, and 32% after 12 weeks of lactulose feeding (P less than 0.01)--at the expense of a rise in both primary bile acids, cholate and chenodeoxycholate, no significant change in biliary cholesterol saturation was found after 4 nor after 12 weeks of lactulose feeding. These results do not support the concept that colonic bacterial metabolites of bile acids play a major role in the production of supersaturated bile and cholesterol gallstone formation.

Effects of the fibre components pectin, cellulose, and lignin on bile salt metabolism and biliary lipid composition in man

Randomised crossover studies in three separate groups of 10 healthy volunteers were undertaken to determine the effects of biliary lipid composition and bile salt metabolism of daily dietary supplementation for four weeks with the purified fibre components pectin (12 g/day), cellulose (15 g/day) and lignin (12 g/day). The subjects' biles were initially unsaturated with cholesterol and no significant changes in the lithogenic indices or mean percentages of cholesterol, phospholipid, or total bile acids after any of the supplements were observed. After pectin, the mean (+/- SD) percentage of cholic acid decreased significantly from 42.8 (+/- 10.8) to 39.0 (+/- 11.2), the mean (+/- SD) percentage of deoxycholic acid increased significantly from 18.2 (+/- 13.7) to 25.4 (+/- 13.5) and C14-deoxycholate metabolites were raised significantly by 65%. After cellulose, the mean (+/- SD) percentage of chenodeoxycholic acid was increased significantly from 33.6 (+/- 6.3) to 35.4 (+/- 7.0), the mean (+/- SD) percentage of deoxycholic acid decreased significantly from 18.6 (+/- 9.6) to 14.2 (+/- 8.3) and C14-deoxycholate metabolites halved. Lignin did not exert any significant effects. Though these results show that individual fibre components are associated with quite different effects on bile acid metabolism, in the short term no significant effect on biliary cholesterol saturation was observed in bile initially unsaturated with cholesterol. The bile acid changes most likely result from the different effects on colonic metabolism induced by the individual fibre components.

Intracolonic environment and the presence of colonic adenomas in man

A promoting effect of large bowel contents on colonic carcinogenesis as seen in the animal model is still incompletely explored in man. We investigated simultaneously deoxycholate absorption (as marker of colonic mucosal exposure to tumour promoting bile salt metabolites), mouth-anus transit time, and the ratio of anaerobic to aerobic bacteria in stool in 10 persons with colonic adenomas and in 10 age matched control subjects. We found that anaerobic/aerobic ratios and colonic deoxycholate absorption were higher in patients with colonic adenomas (p less than 0.002 and p less than 0.001) and that these parameters were clearly interrelated, which also applied to intestinal transit times and the anaerobic/aerobic ratios. These data are consistent with a promoting effect of the intracolonic environment on development of adenomas in man. Long term induction of a more aerobic colon flora and shortening of intestinal transit time may diminish bile-salt induced tumour promotion in adenoma patients.

Age dependent differences in human bile acid metabolism and 7 alpha-dehydroxylation

It has been suggested that transformation of secondary bile acids into (co)carcinogenic compounds may have a role in the development of cancer of the large bowel. Because of age dependent differences of this disease we undertook a study of cholic and deoxycholic acid metabolism of eleven young adults (group A, 20-30 years old) and eleven elderly persons (group B, 55-75 years old) with a double isotope dilution method. Daily food intake was standardized individually and gut transit time measured with radioopaque pellets and labelled chromium chloride. The 7 alpha-dehydroxylation fractions (the ratio of deoxycholic acid input rate from the large bowel to cholic acid synthesis rate) were higher in group B (P less than 0.01) due to higher deoxycholic acid input rates (P less than 0.005), especially when individuals from both groups with rapid gut transit were compared. As contributory factor was recognized the higher fractional turnover rate of cholic acid in group B. Pool sizes and synthesis rates of cholic acid and gut transit times were similar. In group A, but not in B, gut transit times correlated with deoxycholic acid input rates (P less than 0.01). The differences in bile acid metabolism may be related to a more effective colonic absorption of deoxycholic acid in the elderly persons with a concomitant decrease of active ileal absorption of cholic acid in the elderly persons. Differences in diet or gut transit time between both groups do not seem to be the underlying mechanism.

Effects of a standardized wheat bran preparation on biliary lipid composition and bile acid metabolism in young healthy males

The effects of adding bran to the normal diet on biliary lipid composition and bile acid metabolism in a group of young healthy males was studied. A chemically standardized coarse wheat bran product, with serum lipid lowering properties, in a dose of 0.5 kg-1 body weight per day was used. Bran feeding for 4 or 8 weeks did not change biliary lipid and biliary bile acid composition. Faecal bile acid and neutral sterol composition was similar before and after 8 weeks of bran. Bile acid kinetics, measured by double isotope dilution after simultaneous injection of [3H]cholic acid and [14C]deoxycholic acid, showed only minor differences before and during bran ingestion. The most surprising finding was an increase in 7 alpha-dehydroxylation fraction (input of deoxycholic acid divided by synthesis of cholic acid) in six out of seven subjects after 4 weeks of brain and in all four subjects after 8 weeks of bran. In conclusion, the bran product we used is not effective in lowering the biliary cholesterol saturation in healthy young males. Nor does it reduce deoxycholate input in our subjects even after 8 weeks of bran.