Evaluation of deuterated cholesterol and deuterated sitostanol for measurement of cholesterol absorption in humans

Inhibition of intestinal cholesterol absorption by ezetimibe in humans

BACKGROUND

Ezetimibe has been shown to inhibit cholesterol absorption in animal models, but studies on cholesterol absorption in humans have not been performed thus far.

METHODS AND RESULTS

The effect of ezetimibe (10 mg/d) on cholesterol absorption and synthesis, sterol excretion, and plasma concentrations of cholesterol and noncholesterol sterols was investigated in a randomized, double-blind, placebo-controlled, crossover study in 18 patients with mild to moderate hypercholesterolemia. Treatment periods lasted 2 weeks with an intervening 2-week washout period. Fractional cholesterol absorption rates averaged 49.8+/-13.8% on placebo and 22.7+/-25.8% on ezetimibe, indicating a reduction of 54% (geometric mean ratio; P< 0.001). Cholesterol synthesis increased by 89% from 931+/-1027 mg/d on placebo to 1763+/-1098 mg/d on ezetimibe (P<0.001), while the ratio of lathosterol-to-cholesterol, an indirect marker of cholesterol synthesis, was increased by 72% (P<0.001). Bile acid synthesis was insignificantly increased (placebo: 264+/-209 mg/d, ezetimibe: 308+/-184 mg/d; P=0.068). Mean percent changes from baseline for LDL and total cholesterol after ezetimibe treatment were -20.4% and -15.1%, respectively (P<0.001 for both), whereas campesterol and sitosterol were decreased by -48% and - 41%, respectively.

CONCLUSION

In humans, ezetimibe inhibits cholesterol absorption and promotes a compensatory increase of cholesterol synthesis, followed by clinically relevant reductions in LDL and total cholesterol concentrations. Ezetimibe also reduces plasma concentrations of the noncholesterol sterols sitosterol and campesterol, suggesting an effect on the absorption of these compounds as well.

Evidence that leptin contributes to intestinal cholesterol absorption in obese (ob/ob) mice and wild-type mice

In the present study, the effect of leptin on intestinal cholesterol absorption was investigated in C57 BL/6 OlaHsd Lep(ob)/Lep(ob) obese (ob/ob) mice and lean C57 BL/6 (wild-type) mice. Animals were treated either with or without recombinant leptin for 2 wk. Cholesterol absorption was measured by the constant isotope feeding method and indirectly by the ratio of campesterol to cholesterol in serum. In ob/ob mice, cholesterol absorption was significantly higher compared to wild-type mice [83.4 +/- 2.3% (SD) vs. 77.6 +/- 1.5%, P < 0.01]. Treatment with leptin significantly reduced cholesterol absorption in both ob/ob and wild-type mice by 8.5 (P < 0.001) and 5.2% (P < 0.05), respectively. Serum concentrations of campesterol and the ratio of campesterol to cholesterol in ob/ob mice were significantly higher compared to wild-type mice (2.2 +/- 0.3 mg/dL vs. 1.2 +/- 0.3 mg/dL, P< 0.001; and 36.8 +/- 2.8 microg/mg vs. 28.0 +/- 3.3 microg/mg, P < 0.001). After treatment of ob/ob mice with leptin, concentrations of campesterol and its ratio to cholesterol were significantly lower (2.2 +/- 0.3 mg/dL vs. 1.0 +/- 0.2 microg/mg, P < 0.001; and 36.8 +/- 2.8 microg/mg vs. 13.2 +/- 2.2 microg/mg, P < 0.001, respectively). In wild-type mice, the ratio of campesterol to cholesterol in serum was also significantly lower after treatment with leptin (28.0 +/- 3.3 microg/mg vs. 22.6 +/- 5.0 microg/mg, P < 0.05). A significant positive correlation (r = 0.701, P < 0.01) between cholesterol absorption and the ratio of campesterol to cholesterol in serum was found. It is concluded that leptin contributes to intestinal cholesterol absorption in ob/ob mice and lean wild-type mice.

ATP-binding cassette transporter A1 (ABCA1) affects total body sterol metabolism

BACKGROUND AND AIMS

Members of the family of ABC transporters are involved in different processes of sterol metabolism, and ABCA1 was recently identified as a key regulator of high-density lipoprotein (HDL) metabolism. Our aim was to further analyze the role of ABCA1 in cholesterol metabolism.

METHODS

ABCA1-deficient mice (ABCA1-/-) and wild-type mice were compared for different aspects of sterol metabolism. Intestinal cholesterol absorption was determined by a dual stable isotope technique, and analysis of fecal, plasma, and tissue sterols was performed by gas chromatography/mass spectrometry. Key regulators of sterol metabolism were investigated by Northern and Western blot analyses or enzyme activity assays.

RESULTS

ABCA1-disrupted sv129/C57BL/6 hybrid mice showed a significant reduction in intestinal cholesterol absorption. The decrease in cholesterol absorption was followed by an enhanced fecal loss of neutral sterols, whereas fecal bile acid excretion was not affected. Total body cholesterol synthesis was significantly increased, with enhanced 3-hydroxy-3-methyglutaryl-coenzyme A (HMG-CoA) reductase observed in adrenals and spleen. In addition, ABCA1-/- mice showed markedly increased concentrations of cholesterol precursors in the plasma, lung, intestine, and feces. Reduced HMG-CoA reductase messenger RNA and enzyme activity in the liver suggest that enhanced cholesterol synthesis in ABCA1-/- mice occurs in peripheral tissues rather than the liver.

CONCLUSIONS

The metabolism of cholesterol and cholesterol precursors is markedly affected by a lack of ABCA1 function.

Simultaneous assessment of cholesterol absorption and synthesis in humans using on-line gas chromatography/ combustion and gas chromatography/pyrolysis/isotope-ratio mass spectrometry

A number of dietary components and drugs are known to inhibit the absorption of dietary and biliary cholesterol, but at the same time can compensate by increasing cholesterol synthesis. It is, therefore, necessary to have a convenient and accurate method to assess both parameters simultaneously. Hence, we validated such a method in humans using on-line gas chromatography(GC)/combustion and GC/pyrolysis/isotope-ratio mass spectrometry (IRMS). Cholesterol absorption was measured using the ratio of [(13)C]cholesterol (injected intravenously) to [(18)O]cholesterol (administered orally). Simultaneously, cholesterol synthesis was measured using the deuterium incorporation method. Our methodology was applied to 12 mildly hypercholesterolemic men that were given a diet providing 2685 +/- 178 Kcal/day (mean +/- SD) and 255 +/- 8 mg cholesterol per day. Cholesterol fractional synthesis rates ranged from 5.0 to 10.5% pool/day and averaged 7.36% +/- 1.78% pool/day (668 +/- 133 mg/day). Cholesterol absorption ranged from 36.5-79.9% with an average value of 50.8 +/- 15.4%. These values are in agreement with already known data obtained with mildly hypercholesterolemic Caucasian males placed on a diet similar to the one used for this study. However, our combined IRMS method has the advantage over existing methods that it enables simultaneous measurement of cholesterol absorption and synthesis in humans, and is therefore an important research tool for studying the impact of dietary treatments on cholesterol parameters.

Therapeutic potential of plant sterols and stanols

Products enriched with plant sterol and stanol esters selectively lower LDL cholesterol. Consumption appears to be safe, and these functional foods thus have great potential for cardiovascular risk management. Although values remain within the normal range, one possible concern is that they lower lipid-standardized concentrations of the plasma carotenoids. Whether this affects health in the longer-term or in selected patient groups is not known. Therefore, especially in view of the increasing number of functional foods that will be on the market in the very near future, there is a clear need to establish an effective post-marketing safety net.

Lowering of plasma LDL cholesterol in hamsters by the tomato glycoalkaloid tomatine

Tomatoes contain the steroidal glycoalkaloid tomatine, which has been reported to form strong, insoluble complexes with cholesterol in vitro. To determine whether tomatine can reduce dietary cholesterol absorption and plasma levels of cholesterol and triglycerides, we fed hamsters a high-fat, high-cholesterol diet with 0.05-0.2% added tomatine in the diet. The tomatine diets induced lowering of serum low-density lipoprotein (LDL) without changing high-density lipoprotein (HDL) cholesterol. Compared to the control diets, four- to fivefold more labeled dietary cholesterol and coprostanol was excreted in the feces of the tomatine-fed hamsters. The amount of cholesterol excreted in the feces corresponded to the amount of tomatine in the diet. These observations suggest that due to the formation of an insoluble tomatine-cholesterol complex and its excretion in the feces, very little dietary tomatine is absorbed from the digestive tract into the blood stream. They are also consistent with the reported low oral toxicity of tomatine compared to other glycoalkaloids.

Beneficial effects of high dietary fiber intake in patients with type 2 diabetes mellitus

BACKGROUND

The effect of increasing the intake of dietary fiber on glycemic control in patients with type 2 diabetes mellitus is controversial.

METHODS

In a randomized, crossover study, we assigned 13 patients with type 2 diabetes mellitus to follow two diets, each for six weeks: a diet containing moderate amounts of fiber (total, 24 g; 8 g of soluble fiber and 16 g of insoluble fiber), as recommended by the American Diabetes Association (ADA), and a high-fiber diet (total, 50 g; 25 g of soluble fiber and 25 g of insoluble fiber), containing foods not fortified with fiber (unfortified foods). Both diets, prepared in a research kitchen, had the same macronutrient and energy content. We compared the effects of the two diets on glycemic control and plasma lipid concentrations.

RESULTS

Compliance with the diets was excellent. During the sixth week, the high-fiber diet, as compared with the the sixth week of the ADA diet, mean daily preprandial plasma glucose concentrations were 13 mg per deciliter [0.7 mmol per liter] lower (95 percent confidence interval, 1 to 24 mg per deciliter [0.1 to 1.3 mmol per liter]; P=0.04) and mean median difference, daily urinary glucose excretion 1.3 g (0.23; 95 percent confidence interval, 0.03 to 1.83 g; P= 0.008). The high-fiber diet also lowered the area under the curve for 24-hour plasma glucose and insulin concentrations, which were measured every two hours, by 10 percent (P=0.02) and 12 percent (P=0.05), respectively. The high-fiber diet reduced plasma total cholesterol concentrations by 6.7 percent (P=0.02), triglyceride concentrations by 10.2 percent (P=0.02), and very-low-density lipoprotein cholesterol concentrations by 12.5 percent (P=0.01).

CONCLUSIONS

A high intake of dietary fiber, particularly of the soluble type, above the level recommended by the ADA, improves glycemic control, decreases hyperinsulinemia, and lowers plasma lipid concentrations in patients with type 2 diabetes.

Plant stanol esters affect serum cholesterol concentrations of hypercholesterolemic men and women in a dose-dependent manner

The effect of plant stanol ester on serum cholesterol is dose-dependent. However, it is not clear what the dose is beyond which no additional benefit can be obtained. Therefore, we determined the dose-response relationship for serum cholesterol with different doses of plant stanol ester in hypercholesterolemic subjects. In a single-blind design each of 22 men or women consumed five different doses of plant stanol [target (actual) intake 0 (0), 0.8 (0.8), 1.6 (1.6), 2.4 (2.3), 3.2 (3.0) g/d] added as plant stanol esters to margarine for 4 wk. The order of dose periods was randomly determined. Serum total cholesterol concentration decreased (calculated in reference to control) by 2.8% (P = 0.384), 6.8% (P < 0.001), 10.3% (P < 0.001) and 11.3% (P < 0.001) by doses from 0.8 to 3.2 g. The respective decreases for LDL cholesterol were 1.7% (P = 0. 892), 5.6% (P < 0.05), 9.7% (P < 0.001) and 10.4% (P < 0.001). Although the decreases were numerically greater with 2.4 and 3.2 g doses than with the 1.6 g dose, these differences were not significant (P = 0.054-0.516). Serum plant stanols rose slightly, but significantly with the dose (P < 0.001). Apolipoprotein B concentration was decreased significantly already at the dose of 0.8 g (8.7%, P < 0.001). Apolipoprotein E genotype did not affect the lipid responses. We conclude that significant reduction of serum total and LDL cholesterol concentrations is reached with the 1.6-g stanol dose, and increasing the dose from 2.4 to 3.2 g does not provide clinically important additional effect.

Effect of lifibrol on the metabolism of low density lipoproteins and cholesterol

Lifibrol is a powerful cholesterol-lowering drug of unknown mechanism of action. This investigation was carried out to determine whether the major action of lifibrol is to enhance clearance of low density lipoproteins (LDL) through the LDL-receptor pathway, and if so, whether the drug exerts its action by altering the excretion of bile acids (acidic steroids), the absorption of cholesterol, or the synthesis of cholesterol. In a first study, in two patients with complete absence of LDL receptors, lifibrol therapy had essentially no effect on plasma LDL concentrations; in two others who had a marked reduction in LDL-receptor activity, response to the drug was attenuated. These findings suggest that lifibrol requires an intact LDL-receptor pathway to exert its action. In a second study, in patients with primary moderate hypercholesterolemia, isotope kinetic studies showed that lifibrol enhanced the fractional catabolic rate of LDL-apolipoprotein B (apo B), but had no effect on transport rates of LDL; these observations likewise support the probability that lifibrol acts mainly to increase the activity of the LDL-receptor pathway. However, in a third study in hypercholesterolemic patients, lifibrol therapy failed to increase acidic steroid excretion, inhibit cholesterol absorption, or reduce net cholesterol balance. Furthermore, lifibrol treatment did not significantly reduce urinary excretion of mevalonic acid. In contrast, in a parallel study, simvastatin therapy, which is known to inhibit cholesterol synthesis, gave the expected decrease in net cholesterol balance and reduction in urinary excretion of mevalonic acid. Thus, lifibrol, like statins, appears to increase the activity of LDL receptors; but in contrast to findings with statins, it was not possible to detect a significant decreased synthesis of cholesterol, either from balance studies or from urinary excretion of mevalonic acid. This finding raises the possibility that lifibrol activates the LDL-receptor pathway through a different mechanisms which remains to be determined.

Effects of 2 low-fat stanol ester-containing margarines on serum cholesterol concentrations as part of a low-fat diet in hypercholesterolemic subjects

BACKGROUND

Full-fat sitostanol ester-containing margarine reduces serum total and LDL cholesterol, but the effect of plant stanol ester-containing margarine as part of a low-fat, low-cholesterol diet has not been studied.

OBJECTIVE

We investigated the cholesterol-lowering effects of 2 novel, low-fat stanol ester-containing margarines as part of a low-fat diet recommended for hypercholesterolemic subjects.

DESIGN

In a parallel, double-blind study, 55 hypercholesterolemic subjects were randomly assigned after a 4-wk high-fat diet (baseline) to 3 low-fat margarine groups: wood stanol ester-containing margarine (WSEM), vegetable oil stanol ester-containing margarine (VOSEM), and control margarine (no stanol esters). The groups consumed the margarines for 8 wk as part of a diet resembling that of the National Cholesterol Education Program's Step II diet. The daily mean total stanol intake was 2.31 and 2.16 g in the WSEM and VOSEM groups, respectively.

RESULTS

During the experimental period, the reduction in serum total cholesterol was 10.6% (P < 0.001) and 8.1% (P < 0.05) greater and in LDL cholesterol was 13.7% (P < 0.01) and 8.6% (P = 0.072) greater in the WSEM and VOSEM groups, respectively, than in the control group. Serum campesterol concentrations decreased 34.5% and 41.3% (P < 0.001) in the WSEM and VOSEM groups, respectively. Serum HDL cholesterol, sitostanol, campestanol, beta-carotene, and fat-soluble vitamin concentrations did not change significantly from baseline.

CONCLUSIONS

We conclude that the low-fat, plant stanol ester-containing margarines are effective cholesterol-lowering products in hypercholesterolemic subjects when used as part of a low-fat, low-cholesterol diet. They offer an additional, clinically significant reduction in serum cholesterol concentrations to that obtained with a low-fat diet alone.

Campestanol (24-methyl-5alpha-cholestan-3beta-ol) absorption and distribution in New Zealand White rabbits: effect of dietary sitostanol

Campestanol (24-methyl-5alpha-cholestan-3beta-ol) is a naturally occurring plant stanol, structurally similar to cholesterol (5-cholesten-3beta-ol) and widely distributed in vegetable oils consumed in human diets. We measured the absorption and turnover of campestanol by the plasma dual-isotope ratio method and mathematical analysis of specific activity versus time decay curves after simultaneous oral and intravenous pulse-labeling with [3alpha-3H]- and [23-14C]-labeled campestanol, respectively, in New Zealand White (NZW) rabbits: six fed chow and six fed chow with 125 mg/d campestanol and 175 mg/d sitostanol (24-ethyl-5alpha-cholestan-3beta-ol). Plasma concentrations increased insignificantly from 0.08+/-0.01 to 0.09+/-0.01 mg/dL with dietary stanols. The percent campestanol absorption measured by the plasma dual-isotope ratio method after the rabbits were fasted for 6 hours yielded the percent absorption in the absence of competing intestinal sterols and stanols and declined insignificantly from 11.6%+/-3.5% in controls to 8.1%+/-3.7% in the treated rabbit groups. In contrast, the turnover, which measured actual absorption averaged over 24 hours, increased from 0.12+/-0.05 to 0.37+/-0.05 mg/d (P < .05) with campestanol and sitostanol added to the diet. However, the actual percent absorption declined from 3% to 0.3% of dietary intake with the campestanol and sitostanol-enriched diet. Campestanol pool sizes, although remaining small, increased slightly from 1.1+/-0.4 to 2.5+/-1.5 mg. The removal constant (KA) from pool A (MA) did not change significantly with added dietary campestanol and sitostanol (KA= -0.040+/-0.005 v -0.037+/-0.007 d(-1)). The results demonstrate small campestanol plasma concentrations and body pools even when the rabbits consumed substantial amounts because (1) intestinal absorption was limited and (2) was further reduced by competing dietary sitostanol, and (3) campestanol was removed rapidly from the body. Thus, campestanol, which shares the same basic structure and intestinal absorption pathway with cholesterol, does not accumulate when fed, and may be incorporated into the diet to block cholesterol absorption.

Postprandial studies on dietary cholesterol in human subjects using stable isotopes and gas chromatography-mass spectrometry analysis

We hypothesized that intestinal absorption and postprandial re-secretion of dietary cholesterol may be a particularly complex process in humans. To test this hypothesis, we used deuterium-enriched cholesterol to specifically label meal cholesterol and developed an improved method for quantitative measurement of traces of deuterated cholesterol as well as cholesterol with reference to two different internal standards by gas chromatography-mass spectrometry (GC-MS) measurement. In the first study, a group of healthy subjects ingested a single test meal containing deuterated cholesterol with a 7 h postprandial follow-up. In the second one, a group of healthy subjects ingested a first test meal containing deuterated cholesterol and a follow-up was performed during three consecutive test meals and later until 72 h. The most striking observations were that the occurrence of dietary cholesterol in chylomicrons is not concomitant to triglycerides and is very low after a single meal while most dietary cholesterol is re-secreted in chylomicrons after a second, and even a third, fat test meal. The data obtained show that the re-secretion of dietary cholesterol from the small intestine is a slow and complex process in humans.

Method for simultaneous measurements of traces of heptadeuterated cholesterol and cholesterol by gas chromatography-mass spectrometry: application in humans

An assay was developed to quantify deuterated cholesterol (used as a tracer) and cholesterol using gas chromatography-mass spectrometry. Ergosterol and epicoprostanol were used as internal standards. Deuterated cholesterol was quantified by comparing its peak area to that of epicoprostanol and cholesterol to ergosterol. The mean absolute recovery in spiked serum was 99.96%; the precision was in the range 0.16-10.9% and accuracy 90.4-100%; the limit of detection in plasma was 3x10(-5) mmol l(-1). Using two internal standards, the method described herein seems particularly suitable for application in humans i.e., measuring traces of deuterated cholesterol (range: 0-6.26 x 10(-4) mmol l(-1)) along with natural cholesterol (range: 0.065-4.42 mmol l(-1)) in human plasma and lipid fractions postprandially.

Inhibiting cholesterol absorption with CP-88,818 (beta-tigogenin cellobioside; tiqueside): studies in normal and hyperlipidemic subjects

CP-88,818 (beta-tigogenin cellobioside; tiqueside) is a synthetic saponin developed to treat hypercholesterolemia by inhibiting the absorption of biliary and dietary cholesterol. Two studies are reported here: one in patients to assess safety and efficacy, and one in normal volunteers to explore the mechanism of action. The former included 15 hypercholesterolemic outpatients [low-density lipoprotein cholesterol (LDL-C) > or = 160 mg/dl] treated with 1, 2, and 3 g of tiqueside daily (b.i.d.) in a crossover design for three 2-week treatment periods, each separated by a 3-week placebo period. The mechanistic study was conducted with 24 healthy male subjects who were randomized in a parallel group design to either placebo (n = 6) or tiqueside (2 or 4 g/day; n = 9 each) once daily for 3 weeks. All subjects in this study were fed a low-fat, low-cholesterol diet [National Cholesterol Education Program (NCEP) Step 1]. Fecal steroid excretion rates and plasma lipid levels were determined at baseline and after 3 weeks of treatment. Fractional cholesterol absorption was measured before and after treatment by the continuous feeding, dual-isotope method. Tiqueside produced a dose-dependent reduction in plasma LDL cholesterol levels in the hypercholesterolemic patients. In the mechanistic study, it decreased fractional cholesterol absorption rates and increased fecal neutral sterol excretion rates, changes associated with trends toward lower LDL cholesterol levels. Other lipoprotein levels were unaffected, as were fecal fat and bile acid excretion and fat-soluble vitamin absorption. Thus tiqueside dose-dependently inhibits cholesterol absorption in humans, resulting in a reduction in serum LDL cholesterol levels.

Phytosterolaemia: diagnosis, characterization and therapeutical approaches

Phytosterolaemia (sitosterolaemia) is a very rare inherited sterol storage disease characterized by tendon and tuberous xanthomas and by a strong predisposition to premature coronary atherosclerosis. In addition to increased or normal serum cholesterol, patients are found to have markedly elevated concentrations of the phytosterols sitosterol and campesterol. These sterols accumulate in all tissues, except the brain. Increased intestinal absorption of plant sterols, impaired biliary excretion, and decreased cholesterol synthesis are suggested as causes for this disease. However, the primary defect has not yet been identified. As well as dietary restrictions of cholesterol and plant sterols, therapeutic approaches based on interruption of the enterohepatic circulation of bile acids by administration of bile acid-binding resins or ileal bypass surgery have been recommended as therapeutic approaches to reduce all serum sterols. Administration of sitostanol, a nonabsorbable saturated plant sterol, showed a significant reduction of serum plant sterols and cholesterol in two patients with phytosterolaemia, presumably by competitive inhibition of sterol absorption.

Phytosterolaemia in a Norwegian family: diagnosis and characterization of the first Scandinavian case

Phytosterolaemia (sitosterolaemia) is a very rare inherited sterol storage disease characterized by tendon and tuberous xanthomas and by a predisposition to atherosclerosis. We here describe the first Scandinavian case. The 14-year-old female patient was found to have markedly elevated circulating levels of plant sterols (sitosterol, sitostanol, campesterol, stigmasterol), and the levels of these sterols were 20-50 times higher than in her healthy sister and heterozygous parents. In addition to the usual serum plant sterols we found a new major sterol in the patient tentatively identified as episterol or fecosterol (24-methyliden-cholest-7 (or 8)-en-3 beta-ol). A newly developed method based on the use of deuterium labelled cholesterol and plant sterols was used to measure sterol absorption in the patient and her relatives. Absorption of sitosterol averaged 20% in the patient and ranged from 4 to 8% in the relatives. Absorption of campesterol averaged 31% in the patient and ranged from 15 to 18% in her relatives. Absorption of cholesterol averaged 63% in the patient and ranged from 35 to 45% in the relatives. Cholesterol synthesis appeared to be reduced in the patient and was 46-52% of that of her relatives.

The role of stable isotopes in the investigation of plasma lipoprotein metabolism

The last 5 years have seen promising beginnings of the application of stable-isotope-based methods to the study of lipoprotein metabolism. Many aspects of plasma lipid transport make it an attractive system for investigating by this means. While early efforts borrowed from standard techniques of generating and interpreting kinetic parameters (FSRs), the shortcomings of these procedures as applied to lipoproteins are now appreciated. Lipoprotein heterogeneity requires that multicompartmental analysis and relatively long-term studies be employed if the information obtained is going to be a useful adjunct to that produced by radio-iodinated lipoprotein tracers. Both approaches-stable-isotope-labeled endogenous tracer and ex vivo radio-iodinated lipoprotein experiments--must be considered complementary. The first provides direct information on lipoprotein synthesis pathways while the latter is superior at following interconversions and catabolic events. Excellent agreement has been demonstrated where the methods have been used simultaneously to estimate the same kinetic parameter. Many of the questions that have arisen from decades of radioactive tracer studies relate to the nature and rate of lipoprotein synthesis, e.g. what kinds of particles are produced by the liver when different diets are taken, and how are the synthetic pathways altered in dyslipidaemic states? Endogenous tracers can address these issues, and methods are presently available which provide the means for measuring the production of all apolipoproteins and for estimating cholesterol and fatty acid biosynthesis. When techniques are developed to examine triglyceride, cholesteryl ester and phospholipid production then a much clearer picture of how lipoproteins are assembled in vivo will emerge. This kind of information will be essential to an understanding of regulation of plasma lipid transport and the subtle changes that occur in those at risk for coronary heart disease.

Comparison of intestinal absorption of cholesterol with different plant sterols in man

Intestinal absorption of cholesterol, campesterol, campestanol, stigmasterol and sitosterol were measured in 10 healthy subjects by an intestinal perfusion technique over a 50 cm segment of the upper jejunum using sitostanol as non-absorbable marker. Cholesterol absorption was highest and averaged 33%., whereas the absorption rate of sitosterol averaged 4.2% and of stigmasterol 4.8%. Higher absorption rates were found for campesterol (9.6%). Campestanol, the 5 alpha saturated derivative of campesterol, showed the highest absorption rate (12.5%) of all plant sterols. A positive correlation between the absorption rate of cholesterol and campesterol was established. In addition, there was a negative correlation between the ratio of sitosterol to cholesterol and the mass of cholesterol absorption. These results are in agreement with previous observations in animal studies, namely, that increasing the length of the side-chain of cholesterol decreases the absorbability of the sterol. Surprisingly, campestanol, the 5 alpha saturated derivate of campesterol, was shown to have higher absorbability compared with its unsaturated compound. This finding is in contrast to previous assumptions, that hydrogenisation of the nucleus double bond of a sterol causes a decrease of absorbability, as has been demonstrated for cholesterol/cholestanol and sitosterol/sitostanol.