Kinetics of chylomicron remnant clearance in normal and in hyperlipoproteinemic subjects

Measuring VLDL1 secretion in humans with an intravenous fat emulsion test

Tracer techniques to assess very-low-density lipoprotein (VLDL) secretion in humans are expensive, are time consuming, and require mathematical models to estimate VLDL kinetics. Here, we describe an alternative, time- and cost-efficient protocol to directly determine VLDL1 secretion with an intravenous (i.v.) lipid emulsion test that does not require tracers and compartmental modeling. We describe steps for intralipid infusion, blood sampling, and removal of intralipid from plasma samples, followed by density gradient ultracentrifugation to isolate VLDL1 fraction and measure the secretion rate. For complete details on the use and execution of this protocol, please refer to Bjorkegren et al. (1996),1 Al-Shayji et al. (2007),2 and Metz et al. (2022).3.

High-throughput LC-MS method to investigate postprandial lipemia: considerations for future precision nutrition research

Elevated postprandial lipemia is an independent risk factor for cardiovascular disease, yet methods to quantitate postmeal handling of dietary lipids in humans are limited. This study tested a new method to track dietary lipid appearance using a stable isotope tracer (²H₁₁-oleate) in liquid meals containing three levels of fat [low fat (LF), 15 g; moderate fat (MF), 30 g; high fat (HF), 60 g]. Meals were fed to 12 healthy men [means ± SD, age 31.3 ± 9.2 yr, body mass index (BMI) 24.5 ± 1.9 kg/m²] during four randomized study visits; the HF meal was administered twice for reproducibility. Blood was collected over 8 h postprandially, triglyceride (TG)-rich lipoproteins (TRL), and particles with a Svedberg flotation rate >400 (S_f > 400, n = 8) were isolated by ultracentrifugation, and labeling of two TG species (54:3 and 52:2) was quantified by LC-MS. Total plasma TRL-TG concentrations were threefold greater than S_f > 400-TG. Both S_f > 400- and TRL-TG 54:3 were present at higher concentrations than 52:2, and singly labeled TG concentrations were higher than doubly labeled. Furthermore, TG 54:3 and the singly labeled molecules demonstrated higher plasma absolute entry rates differing significantly across fat levels within a single TG species (P < 0.01). Calculation of fractional entry showed no significant differences in label handling supporting the utility of either TG species for appearance rate calculations. These data demonstrate the utility of labeling research meals with stable isotopes to investigate human postprandial lipemia while simultaneously highlighting the importance of examining individual responses. Meal type and timing, control of prestudy activities, and effects of sex on outcomes should match the research goals. The method, optimized here, will be beneficial to conduct basic science research in precision nutrition and clinical drug development.NEW & NOTEWORTHY A novel method to test human intestinal lipid handling using stable isotope labeling is presented and, for the first time, plasma appearance and lipid turnover were quantified in 12 healthy men following meals with varying amounts of fat. The method can be applied to studies in precision nutrition characterizing individual response to support basic science research or drug development. This report discusses key questions for consideration in precision nutrition that were highlighted by the data.

Dietary fatty acid metabolism in prediabetes

PURPOSE OF REVIEW

Experimental evidences are strong for a role of long-chain saturated fatty acids in the development of insulin resistance and type 2 diabetes. Ectopic accretion of triglycerides in lean organs is a characteristic of prediabetes and type 2 diabetes and has been linked to end-organ complications. The contribution of disordered dietary fatty acid (DFA) metabolism to lean organ overexposure and lipotoxicity is still unclear, however. DFA metabolism is very complex and very difficult to study in vivo in humans.

RECENT FINDINGS

We have recently developed a novel imaging method using PET with oral administration of 14-R,S-F-fluoro-6-thia-heptadecanoic acid (FTHA) to quantify organ-specific DFA partitioning. Our studies thus far confirmed impaired storage of DFA per volume of fat mass in abdominal adipose tissues of individuals with prediabetes. They also highlighted the increased channeling of DFA toward the heart, associated with subclinical reduction in cardiac systolic and diastolic function in individuals with prediabetes.

SUMMARY

In the present review, we summarize previous work on DFA metabolism in healthy and prediabetic states and discuss these in the light of our novel findings using PET imaging of DFA metabolism. We herein provide an integrated view of abnormal organ-specific DFA partitioning in prediabetes in humans.

Compartmental and noncompartmental modeling of ¹³C-lycopene absorption, isomerization, and distribution kinetics in healthy adults

BACKGROUND

Lycopene, which is a red carotenoid in tomatoes, has been hypothesized to mediate disease-preventive effects associated with tomato consumption. Lycopene is consumed primarily as the all-trans geometric isomer in foods, whereas human plasma and tissues show greater proportions of cis isomers.

OBJECTIVE

With the use of compartmental modeling and stable isotope technology, we determined whether endogenous all-trans-to-cis-lycopene isomerization or isomeric-bioavailability differences underlie the greater proportion of lycopene cis isomers in human tissues than in tomato foods.

DESIGN

Healthy men (n = 4) and women (n = 4) consumed (13)C-lycopene (10.2 mg; 82% all-trans and 18% cis), and plasma was collected over 28 d. Unlabeled and (13)C-labeled total lycopene and lycopene-isomer plasma concentrations, which were measured with the use of high-performance liquid chromatography-mass spectrometry, were fit to a 7-compartment model.

RESULTS

Subjects absorbed a mean ± SEM of 23% ± 6% of the lycopene. The proportion of plasma cis-(13)C-lycopene isomers increased over time, and all-trans had a shorter half-life than that of cis isomers (5.3 ± 0.3 and 8.8 ± 0.6 d, respectively; P < 0.001) and an earlier time to reach maximal plasma concentration than that of cis isomers (28 ± 7 and 48 ± 9 h, respectively). A compartmental model that allowed for interindividual differences in cis- and all-trans-lycopene bioavailability and endogenous trans-to-cis-lycopene isomerization was predictive of plasma (13)C and unlabeled cis- and all-trans-lycopene concentrations. Although the bioavailability of cis (24.5% ± 6%) and all-trans (23.2% ± 8%) isomers did not differ, endogenous isomerization (0.97 ± 0.25 μmol/d in the fast-turnover tissue lycopene pool) drove tissue and plasma isomeric profiles.

CONCLUSION

(13)C-Lycopene combined with physiologic compartmental modeling provides a strategy for following complex in vivo metabolic processes in humans and reveals that postabsorptive trans-to-cis-lycopene isomerization, and not the differential bioavailability of isomers, drives tissue and plasma enrichment of cis-lycopene. This trial was registered at clinicaltrials.gov as NCT01692340.

In vivo evidence for chylomicrons as mediators of postprandial inflammation

BACKGROUND AND AIMS

The postprandial situation is a pro-inflammatory condition most likely linked to the development of atherosclerosis. We evaluated the relationship between apolipoprotein (apo) B48 and fasting and postprandial leukocyte activation markers.

METHODS

Leukocyte activation markers and apo B48 were determined in 80 subjects with and without coronary artery disease (CAD). Twelve healthy subjects underwent an oral fat loading test (up to 8 h).

RESULTS

Fasting apo B48 was significantly higher in patients with CAD (n = 47, 8.1 ± 5.2 mg/L) than in subjects without CAD (n = 33, 5.9 ± 3.9 mg/L, p = 0.022). Fasting apo B48 and triglycerides correlated positively with fasting monocyte CD11b and neutrophil CD66b expression. Plasma apo B48 and leukocyte activation markers increased after an oral fat load. No correlations were found between fasting or postprandial triglycerides and postprandial leukocyte activation markers. We observed no correlations between postprandial apo B48 and postprandial neutrophil CD11b or CD66b expression.

CONCLUSION

This study suggests that chylomicron remnants may be responsible for postprandial leukocyte activation in the circulation. The postprandial chylomicron response may be a stronger mediator of postprandial inflammation than postprandial triglyceridemia.

An LC/MS/MS method for stable isotope dilution studies of β-carotene bioavailability, bioconversion, and vitamin A status in humans

Isotope dilution is currently the most accurate technique in humans to determine vitamin A status and bioavailability/bioconversion of provitamin A carotenoids such as β-carotene. However, limits of MS detection, coupled with extensive isolation procedures, have hindered investigations of physiologically-relevant doses of stable isotopes in large intervention trials. Here, a sensitive liquid chromatography-tandem mass spectrometry (LC/MS/MS) analytical method was developed to study the plasma response from coadministered oral doses of 2 mg [(13)C10]β-carotene and 1 mg [(13)C10]retinyl acetate in human subjects over a 2 week period. A reverse phase C18 column and binary mobile phase solvent system separated β-carotene, retinol, retinyl acetate, retinyl linoleate, retinyl palmitate/retinyl oleate, and retinyl stearate within a 7 min run time. Selected reaction monitoring of analytes was performed under atmospheric pressure chemical ionization in positive mode at m/z 537→321 and m/z 269→93 for respective [(12)C]β-carotene and [(12)C] retinoids; m/z 547→330 and m/z 274→98 for [(13)C10]β-carotene and [(13)C5] cleavage products; and m/z 279→100 for metabolites of [(13)C10]retinyl acetate. A single one-phase solvent extraction, with no saponification or purification steps, left retinyl esters intact for determination of intestinally-derived retinol in chylomicrons versus retinol from the liver bound to retinol binding protein. Coadministration of [(13)C10]retinyl acetate with [(13)C10]β-carotene not only acts as a reference dose for inter-individual variations in absorption and chylomicron clearance rates, but also allows for simultaneous determination of an individual's vitamin A status.

Immunological detection of large oxidized lipoproteins in hypertriglyceridemic serum

BACKGROUND

Triglyceride-rich, low-density lipoproteins (TG-rich LDL) have been reported as an oxidized lipoprotein species in patients with severe liver disease. Using TG-rich LDL as an immunogen, we obtained a monoclonal antibody (G11-6) that reacted with TG-rich LDL from patients with liver disease and with metal-oxidized LDL only in the early process of the oxidation reaction. This study determined the G11-6-reactive lipoproteins in hypertriglyceridemic serum.

METHODS

Serum samples from healthy volunteers (n = 12) and hypertriglyceridemic patients (n = 9) were fractionated by gel filtration and subjected to a sandwich enzyme-linked immunosorbent assay (ELISA) using G11-6 and polyclonal anti-apolipoprotein B antibodies.

RESULTS

Small LDL and larger lipoproteins reacted with G11-6. G11-6-reactive small LDL was identified in both the healthy subjects and hypertriglyceridemic patients, whereas G11-6-reactive larger lipoproteins were found only in the hypertriglyceridemic patients.

CONCLUSIONS

G11-6 is a useful tool for detecting increased large oxidized lipoproteins in hypertriglyceridemic patients.

Biofortified cassava increases β-carotene and vitamin A concentrations in the TAG-rich plasma layer of American women

Biofortification of cassava with the provitamin A carotenoid β-carotene is a potential mechanism for alleviating vitamin A deficiency. Cassava is a staple food in the African diet, but data regarding the human bioavailability of β-carotene from this food are scarce. The objective of the present study was to evaluate provitamin A-enhanced cassava as a source of β-carotene and vitamin A for healthy adult women. The study was a randomised, cross-over trial of ten American women. The subjects consumed three different porridges separated by 2 week washout periods. Treatment meals (containing 100 g cassava) included: biofortified cassava (2 mg β-carotene) porridge with added oil (15 ml peanut or rapeseed oil, 20 g total fat); biofortified cassava porridge without added oil (6 g total fat); unfortified white cassava porridge with a 0·3 mg retinyl palmitate reference dose and added oil (20 g total fat). Blood was collected six times from - 0·5 to 9·5 h post-feeding. TAG-rich lipoprotein (TRL) plasma was separated by ultracentrifugation and analysed using HPLC with coulometric array electrochemical detection. The AUC for retinyl palmitate increased after the biofortified cassava meals were fed (P< 0·05). Vitamin A conversion was 4·2 (sd 3·1) and 4·5 (sd 3·1) μg β-carotene:1 μg retinol, with and without added oil, respectively. These results show that biofortified cassava increases β-carotene and retinyl palmitate TRL plasma concentrations in healthy well-nourished adult women, suggesting that it is a viable intervention food for preventing vitamin A deficiency.

Vitamin K nutrition, metabolism, and requirements: current concepts and future research

In 2001, the US Food and Nutrition Board concluded that there were insufficient data with which to establish a RDA for vitamin K, in large part because of a lack of robust endpoints that reflected adequacy of intake. Knowledge of the relative bioavailability of multiple vitamin K forms was also poor. Since then, stable isotope methodologies have been applied to the assessment of the bioavailability of the major dietary form of vitamin K in its free state and when incorporated into a plant matrix. There is a need for stable isotope studies with enhanced sensitivity to expand knowledge of the bioavailability, absorption, disposition, and metabolism of different molecular forms of vitamin K. Another area for future research stems from evidence that common polymorphisms or haplotypes in certain key genes implicated in vitamin K metabolism might affect nutritional requirements. Thus far, much of this evidence is indirect via effects on warfarin dose requirements. In terms of clinical endpoints, vitamin K deficiency in early infancy continues to be a leading cause of intracranial bleeding even in developed countries and the reasons for its higher prevalence in certain Asian countries has not been solved. There is universal consensus for the need for vitamin K prophylaxis in newborns, but the effectiveness of any vitamin K prophylactic regimen needs to be based on sound nutritional principles. In contrast, there is still a lack of suitable biomarkers or clinical endpoints that can be used to determine vitamin K requirements among adults.

Lack of influence of test meal fatty acid composition on the contribution of intestinally-derived lipoproteins to postprandial lipaemia

The extent and duration of postprandial lipaemia have been linked to risk of CHD but the influence of dietary variables on, and the relative contributions of, exogenous (chylomicron) and endogenous (VLDL) triacylglycerols to the total lipaemic response have not been comprehensively evaluated. In the present study the triacylglycerol, apolipoprotein (apo) B-48 and retinyl ester (RE) responses to three test meals of varying monounsaturated (MUFA) and saturated fatty acid (SFA) content were measured in the triacylglycerol-rich lipoprotein (TRL) fraction of plasma (ρ = 1·006 g/ml) for 9 h after meal consumption. Fifteen healthy normolipidaemic young men consumed, on separate occasions, three test meals which were identical apart from their MUFA and SFA contents. Expressed as a percentage of total energy the MUFA/SFA contents of the meals were: (1) 12 %/17 %; (2) 17 %/12 % and (3) 24 %/5 %. The contribution of the intestinally-derived lipoproteins (chylomicrons) to the lipaemic response was investigated by determining the time to reach peak concentration and the total and incremental areas under the time response curves (AUC and incremental AUC) for RE, apoB-48 and triacylglycerol in the TRL fraction. No significant differences in these measurements were observed for the three meals. However, visual comparison of the postprandial responses to the three meals suggested that as meal MUFA content increased there was a tendency for the triacylglycerol, apoB-48 and RE responses to become biphasic as opposed to the typical monophasic response seen with the 12 % MUFA/17 % SFA meal. Comparison of the apoB-48 and RE responses for the three test meals confirmed other workers’ findings of delayed entry of RE relative to apoB-48 in TRL. The value of the two markers in investigating dietary fat absorption and metabolism is discussed.

Postprandial Lipemia and Remnant Lipoproteins

Increased postprandial lipemia or elevated levels of triglyceride-rich remnant lipoproteins in fasting plasma are associated with increased risk of coronary artery disease. Despite many studies showing that postprandial triglyceride-rich lipoproteins play a central role in the pathogenesis of atherosclerosis, suitably standardized methods to measure postprandial lipemia or remnant lipoproteins in the clinical setting are lacking. This approach for cardiovascular risk assessment is confined to research laboratories and for the time being is not a standard procedure in clinical practice.

Low-density lipoproteins generated during an oral fat load in mild hypertriglyceridemic and healthy subjects are smaller, denser, and have an increased low-density lipoprotein receptor binding affinity

Triglyceride-rich lipoproteins generated during the postprandial phase are atherogenic. Large very low-density lipoproteins (LDLs) or chylomicrons (CMs) are not as atherogenic as their remnants (Rem). Small and dense LDLs are associated with cardiovascular disease. Low-density lipoprotein size is partly under genetic control and is considered as a relatively stable LDL feature. In this article, we present data on retinyl palmitate kinetics correlated with the modification of LDL features in terms of size, density, and in vitro receptor binding affinity after an oral fat load. Six nondiabetic, hypertriglyceridemic (HTG) patients and 6 healthy controls were examined. Low-density lipoprotein size was assessed by gradient gel electrophoresis, and LDL density by density gradient ultracentrifugation. Low-density lipoprotein binding affinity was tested by in vitro competition binding assay on normal human skin fibroblasts (HSFs) and hepatoma cells (HepG2). Kinetic parameters were estimated in CM and Rem fractions by compartmental modeling. Hypertriglyceridemic patients showed significantly higher triglyceride area and a slower CM fractional catabolic rate. Postprandial LDL density increased both in HTG patients and in the control group with a significant difference between groups at 6 hours. Fasting LDL size was lower in HTG patients vs controls but decreased similarly in the postprandial phase. Low-density lipoprotein size and density postprandial modifications were not correlated with any investigated parameter. Postprandial LDLs were internalized more efficiently by HSF than baseline LDL only in the HTG group. In conclusion, postprandial LDLs are smaller and denser compared with fasting LDLs after an oral fat load. Postprandial LDLs also slightly increased their affinity to the HSF cell receptors.

Metabolism of apoB lipoproteins of intestinal and hepatic origin during constant feeding of small amounts of fat

We aimed to identify mechanisms by which apolipoprotein B-48 (apoB-48) could have an atherogenic role by simultaneously studying the metabolism of postprandial apoB-48 and apoB-100 lipoproteins. The kinetics of apoB-48 and apoB-100, each in four density subfractions of VLDL and intermediate density lipoprotein (IDL), were studied by stable isotope labeling in a constantly fed state with half-hourly administration of almond oil in five postmenopausal women. A non-steady-state, multicompartmental model was used. Despite a much lower production rate, VLDL and IDL apoB-48 shared a similar secretion pattern with apoB-100: both were directly secreted into all fractions with similar percentage mass distributions. Fractional catabolic rates (FCRs) of apoB-48 and apoB-100 were similar in VLDL and IDL. We identified a fast turnover compartment of light VLDL that had a residence time of <30 min for apoB-48 and apoB-100. Finally, a high secretion rate of apoB-48 was associated with a slow FCR of VLDL and IDL apoB-100. In conclusion, the intestine secretes a spectrum of apoB lipoproteins, similar to what the liver secretes, albeit with a much lower secretion rate. Once in plasma, intestinal and hepatic triglyceride-rich lipoproteins have similar rates of clearance and participate interactively in similar metabolic pathways, with high apoB-48 production inhibiting the clearance of apoB-100.

Challenges to understanding and measuring carotenoid bioavailability

Carotenoids are an excellent example of where poor understanding of food structure, complexity of behaviour during digestion, and inter-individual differences in response, lead to misinterpretation of study results. Four challenges associated with understanding and measuring carotenoid bioavailability are discussed: release of carotenoids from food structure and processing into an absorbable form (bioaccessibility), passage of carotenoids from gut lumen into the body (absorption), interpreting plasma response and inter-individual variation. Bioaccessibility of carotenoids is governed by characteristics of the food matrix, which affect the efficiency of physical, enzymic and chemical digestion. Carotenoids used as colorants are likely to be better absorbed because of the form in which they are dispersed in food. Extent of absorption of carotenoid supplements will depend on the proximity of dosing to the consumption of a fat-containing meal. Release of carotenoids from food plants occurs only when the plant cell is fractured and this occurs only during food preparation, processing and/or mastication, not during digestion. Following release from the food matrix, the major limiting factor is solubility of carotenoids in digesta. Absorption studies are best carried out by measuring chylomicron carotenoid excursion, with modelling of chylomicron turnover rate. In this way, inter-individual differences in lipoprotein metabolism can, in part, be taken into account before formulating conclusions on the rate and extent of absorption.

Effect of moderate changes in dietary fatty acid profile on postprandial lipaemia, haemostatic and related CVD risk factors in healthy men

OBJECTIVE

To investigate the effect of moderate changes in dietary fatty acid profile on postprandial risk factors for cardiovascular disease (CVD).

DESIGN

Double-blind, randomised, crossover, intervention trial.

SETTING

: University of Auckland Human Nutrition Unit, New Zealand.

SUBJECTS

A total of 18 lean healthy men.

INTERVENTION

A dairy butter fat modified to reduce the saturated:unsaturated fatty acid ratio and a conventional high saturated butter fat were given on two separate occasions as a high-fat test meal (59+/-4 g fat; 71 en% fat) at breakfast. A fat exclusion lunch, dinner and snacks were also given. Blood samples were collected at 0 (baseline), 1, 3, 6, 10 and 24 h.

RESULTS

Maximum peak in total triacylglycerol (TAG) occurred 3 h postprandially and was highest on modified treatment (diet, P<0.05) due predominantly to increased TAG within the chylomicron-rich fraction. Transient peaks in total-, LDL- and HDL-cholesterol occurred postprandially, but did not differ between dietary treatments (P>0.05). There were no differential effects of diet on postprandial free fatty acids, apo A, apo B, glucose, insulin, amylin or haemostatic clotting factors (P>0.05).

CONCLUSIONS

In a group of healthy young men, replacement of 16% of total saturated fatty acids by mono- and polyunsaturated fats within a dairy lipid did not induce postprandial changes in CVD risk that may be considered beneficial for health.

SPONSORSHIP

Fonterra, Wellington; New Zealand.

A physiological pharmacokinetic model describing the disposition of lycopene in healthy men

A physiological pharmacokinetic model was developed to describe the disposition of lycopene, delivered as a tomato beverage formulation in five graded doses (10, 30, 60, 90, or 120 mg), for a phase I study in healthy male subjects (five per dose). Blood was collected before dose administration (0 h) and at scheduled intervals until 672 h. Serum concentrations of carotenoids and vitamins were measured by high performance liquid chromatography analysis. The model was comprised of seven compartments: gastrointestinal tract, enterocytes, chylomicrons, plasma lipoproteins, fast-turnover liver, slow-turnover tissues, and a delay compartment before the enterocytes. As predicted, the percent absorption at the 10 mg dose (33.9 +/- 8.1%) was significantly greater than at the higher doses; however, the amount of lycopene absorbed (mg) was not statistically different (mean: 4.69 +/- 0.55 mg) between doses, suggesting a possible saturation of absorptive mechanisms. The slow-turnover tissue compartment served as a slow-depleting reservoir for lycopene, and the liver represented the fast-turnover pool. Independent of dose, 80% of the subjects absorbed less than 6 mg of lycopene. This may have important implications for planning clinical trials with pharmacological doses of lycopene in cancer control and prevention if absorption saturation occurs at levels that are already being consumed in the population.

Chylomicron remnant concentrations in patients with coronary artery disease

Persisting chylomicron remnants have been linked to premature atherosclerosis. The analysis of chylomicron remnant concentrations by an oral triglyceride tolerance test, however, is time-consuming for the study subjects and requires large resources in the laboratory. Therefore, only small numbers of subjects have been studied in the past. The aim of this study was to elucidate the prevalence of elevated chylomicron remnants, to identify effectors of chylomicron remnant clearance and to compare chylomicron remnants in the prediction of coronary artery disease with other risk factors. We applied a novel oral triglyceride tolerance test to 423 patients (368 males, 55 females) with a confirmed diagnosis of coronary artery disease (CAD) and to 390 control subjects (295 males, 95 females) in a case-control setting. This study revealed that elevated chylomicron remnant concentrations (retinyl esters > 1.5 micromol/l) are present in 20% of all subjects. Male gender, the apolipoprotein E2 isoform, and higher body mass index were associated with increased chylomicron remnant concentrations. However, chylomicron remnants were lower and plasma triglycerides higher in patients with CAD. We conclude that screening for a delayed clearance of chylomicron remnants is of little clinical value in CAD.

Atorvastatin enhances the plasma clearance of chylomicron-like emulsions in subjects with atherogenic dyslipidemia: relevance to the in vivo metabolism of triglyceride-rich lipoproteins

Delayed chylomicron clearance is a characteristic of patients with coronary artery disease. In vivo study of the clearance of labeled chylomicron-like emulsions constitutes a valid model system for evaluation of chylomicron catabolism. The effects of atorvastatin at low (10 mg) and high (40 mg) dose upon the intravascular metabolism and plasma kinetics of chylomicron-like emulsions were evaluated in fasting hyperlipidemic subjects (n=45). Subjects were randomized to a 6-week treatment period with placebo (n=15), low dose or high dose atorvastatin (10 mg/day, n=17 and 40 mg/day, n=13). The chylomicron-like emulsion, double-labeled with 14C-Cholesteryl oleate (14C-CE) and 3H-triolein (3H-TG), was injected in a bolus after a 12-h fast, and blood samples were collected up to 60 min. Plasma decay curves were determined for labeled emulsion CE and TG and residence times (RT) calculated by the occupancy principle. The 14C-CE RT was decreased by 50% after low dose atorvastatin and by 73% after atorvastatin at high dose in comparison to placebo (P<0.05). The 3H-TG RT was significantly reduced (-55%) after high dose atorvastatin, but in contrast was not significantly reduced after placebo or low dose statin. By compartmental analysis, both doses of atorvastatin led to marked elevation in the slow removal component of emulsion remnant particles (10 mg/day=107%; 40 mg/day=195%, P=0.01). Equally, the rapid removal component was increased (+99%) at high dose (P=0.015). Recirculation of 3H-fatty acids was significantly reduced at both statin doses (43 and 83%, respectively) in comparison to placebo (P=0.01). In conclusion, atorvastatin treatment accelerates the plasma clearance of chylomicron-like emulsions and reduces recirculation of fatty acids in subjects with atherogenic hyperlipidemia. Such effect might implicate in reduction of cardiovascular risk.

Intra-individual variations of fasting plasma lipids, apolipoproteins and postprandial lipemia in familial combined hyperlipidemia compared to controls

BACKGROUND

The intra-individual variability of plasma lipids and apolipoproteins has not been studied systematically in familial combined hyperlipidemia (FCHL).

METHODS

Intra-individual changes in fasting plasma lipids and apolipoproteins B and AI were determined in 18 untreated FCHL subjects and 16 unrelated, normolipidemic subjects. Participants were matched for gender, age and body mass index. The mean follow-up period of fasting plasma lipids was 48.91 +/- 35.46 (mean +/- S.D.) days. Postprandial lipemia was determined on 3 different days in 1 week in 90 healthy controls and 17 untreated FCHL subjects by the area under the diurnal capillary triglyceride curve (TGc-AUC).

RESULTS

The coefficients of variation (CVs) for fasting plasma TG were similar between FCHL (23.2 +/- 10.2%) and controls (20.4 +/- 8.2%). The CVs for HDL-C, apo B and apo AI were the lowest of all fasting plasma measurements in both groups and there was no significant difference between FCHL (12.8 +/- 8.2%, 13.2 +/- 15.8% and 6.4 +/- 5.2%, respectively) and controls (11.4 +/- 4.3%, 11.3 +/- 10.6% and 7.8 +/- 4.6%, respectively). The CVs for postprandial lipemia were not different between FCHL (15.9 +/- 11.3%) and controls (15.1 +/- 11.0%), and were significantly lower than the CV of fasting capillary TG (TGc) in the same period (36.3 +/- 24.7% and 24.9 +/- 17.2%, respectively).

CONCLUSIONS

Our study does not provide evidence for short-term major changes in fasting or postprandial lipemia or apolipoproteins in FCHL when systematically compared to healthy controls.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.

The uptake of lipoprotein-borne phylloquinone (vitamin K1) by osteoblasts and osteoblast-like cells: role of heparan sulfate proteoglycans and apolipoprotein E

Vitamin K is essential for the gamma-carboxylation of Gla-containing bone proteins such as osteocalcin and a suboptimal vitamin K status has been linked to osteoporosis but nothing is known of how the lipoprotein-borne vitamin accesses the bone matrix. We have studied the mechanism of transport of lipoproteins labeled with [3H]-phylloquinone (vitamin K1 [K1]) into osteoblasts using both tumor-derived cell lines and normal osteoblast-rich cell populations. We also investigated the effect of heparin in this model since long-term heparin treatment causes osteopenia and the anticoagulant is known to impair normal lipoprotein metabolism. Heparinase treatment, which removes heparan sulfate proteoglycans (HSPG), reduced uptake of [3H]-K1 from triglyceride-rich lipoproteins (TRL) and low-density lipoproteins (LDL). The effect of heparin in this model was complex depending on cell type, concentration, and time but, overall, the results were consistent with an inhibition of vitamin K uptake by osteoblasts. Anti-apolipoprotein E (apoE) antiserum reduced uptake of TRL-[3H]-K1 by 55 +/- 4% and LDL-[3H]-K1 uptake by 35 +/- 2%. Exogenous apoE4 increased uptake of TRL-[3H]-K1 by 90 +/- 1% compared with 53 +/- 11% for apoE3 and 52 +/- 5% for apoE2. Our findings show that HSPG on the cell surface and apoE in the lipoprotein particles contribute to lipoprotein-K1 uptake by osteoblasts as is known for lipoprotein uptake by hepatocytes. This mechanism is significant in view of the epidemiological association of both undercarboxylation of osteocalcin and the presence of an apo epsilon4 allele with increased fracture risk and reduced bone mineral density (BMD). The inhibition by heparin of lipoprotein-mediated carriage of vitamin K and possibly other lipids to bone may provide a basis for the future understanding of heparin-induced osteoporosis.

Post-transcriptional stimulation of the assembly and secretion of triglyceride-rich apolipoprotein B lipoproteins in a mouse with selective deficiency of brown adipose tissue, obesity, and insulin resistance

A mouse model of insulin resistance and its associated dyslipidemia was generated by crossing mice expressing human apolipoprotein B (apoB) with mice lacking only brown adipose tissue (BATless). On a high fat diet, male apoB/BATless mice became obese, hypercholesterolemic, hypertriglyceridemic, and hyperinsulinemic compared with control apoB mice. Fast performance liquid chromatography revealed increased triglyceride concentrations in intermediate density lipoprotein/low density lipoprotein (LDL) and reduced high density lipoprotein cholesterol concentrations. Inhibition of lipolysis by the drug, tetrahydrolipostatin, demonstrated that very low density lipoprotein-sized particles were initially secreted. Metabolic studies employing Triton WR-1339 and either [(3)H]glycerol or [(3)H]palmitate showed that the hypertriglyceridemia in apoB/BATless mice was due to the increased synthesis and secretion of triglyceride. Furthermore, lipoprotein lipase and hepatic lipase activities were not defective. ApoB was also secreted at increased rates in the apoB/BATless mice. Similar levels of apoB mRNA in apoB and apoB/BATless mice indicated that apoB secretion was regulated post-transcriptionally. LDL receptor mRNA was increased in the apoB/BATless mice, indicating that the observed increase in apoB-lipoprotein secretion was not due to their decreased reuptake. Finally, mRNA levels of the large subunit of microsomal triglyceride transfer protein, a required component for very low density protein assembly, were not different between apoB and apoB/BATless mice. This rodent model should prove useful in exploring mechanisms underlying the regulation of apoB secretion in the context of insulin resistance.

Use of water-miscible retinyl palmitate as markers of chylomicrons gives earlier peak response of plasma retinyl esters compared with oil-soluble retinyl palmitate

Delayed peak response of plasma retinyl esters (RE) relative to plasma triacylglycerols (TAG) and apolipoprotein (Apo) B-48 responses following a fat load supplemented with vitamin A raised doubts about the use of vitamin A to label dietary-derived lipids and lipoproteins. The present study compared the use of water-miscible and oil-soluble retinyl palmitate (RP) as markers of dietary-derived lipoproteins in healthy subjects along with the measurements of postprandial plasma TAG and ApoB-48 responses to investigate whether the delayed peak response observed was due to delayed intestinal output of RE from oil-based solutions. Nine healthy female subjects were given a standard test meal containing a dose (112 mg) of RP in either water-miscible or oil-soluble form in random order, on two separate occasions after a 12 h overnight fast. The results showed that the mean plasma RE concentrations reached a peak significantly later than mean plasma TAG and ApoB-48 concentrations when oil-soluble RP was consumed, whereas plasma RE peaked earlier relative to plasma TAG and ApoB-48 responses when water-miscible RP was used. The results suggested a more rapid absorption with a significantly higher and earlier peak response of plasma RE when water-miscible RP was consumed. This was in contrast to the delayed initial appearance and later sustained higher concentrations of plasma RE during the late postprandial period when oil-soluble RP was consumed. The RE response to the water-miscible RP showed better concordance with plasma TAG response than that of oil-soluble RP.

An improved method for the rapid assessment of persisting chylomicron remnant concentrations

Persisting chylomicron remnant concentrations have been linked to premature atherosclerosis. The analysis of persisting chylomicron remnant concentrations by an oral triglyceride tolerance test, however, is time-consuming for the study subjects and requires large resources in the laboratory. Therefore, only small numbers of subjects have been studied in the past. We describe major improvements of the testing procedure in regard of composition of the fatty meal, of patient testing, and measurement of postprandial remnants. Shifting the time of the (ready-to-use) fatty drink from the morning hours to bedtime was well accepted by the study subjects and allowed the analysis of blood samples drawn at the morning with minimal impact on the participants' time and with minimal interferences by confounding factors (e.g. smoking, additional food intake, physical activity). Chylomicron remnants were measured by fluorometry of the supernatant after ultracentrifugation. This procedure was sensitive, was specific for chylomicron remnants, and was easy to perform. The biological validity of the improved procedure was evaluated by studying type III hyperlipoproteinemia patients and normolipemic apolipoprotein (Apo) E2 homozygotes. In conclusion, this improved test permits the rapid testing for persisting chylomicron remnants in the clinical routine and in large epidemiological studies.

Effects of ApoE genotype on ApoB-48 and ApoB-100 kinetics with stable isotopes in humans

Subjects with the apolipoprotein (apo) E4 allele have been shown to have higher low density lipoprotein (LDL) cholesterol and apoB levels than do subjects with the other alleles. To elucidate the metabolic mechanisms responsible for this finding, we examined the kinetics of apoB-48 within triglyceride-rich lipoproteins (TRLs) and of apoB-100 within very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), and LDL by using a primed constant infusion of [5,5,5-(2)H(3)]leucine in the fed state (hourly feeding) during consumption of an average American diet in 18 normolipidemic subjects, 12 of whom had the apoE3/E3 genotype and 6, the apoE3/E4 genotype. Lipoproteins were isolated by ultracentrifugation and apolipoproteins, by sodium dodecyl sulfate gels; isotope enrichment was assessed by gas chromatography-mass spectrometry. Kinetic parameters were calculated by multicompartmental modeling of the data with SAAM II software. Compared with the apoE3/E3 subjects, the apoE3/E4 subjects had significantly higher levels of total apoB, 100. 1+/-17.8 versus 135.4+/-34.0 mg/dL (P=0.009), and significantly higher levels of LDL apoB-100, 88.1+/-19.2 versus 127.5+/-32.7 mg/dL (P=0.005), respectively. The pool size of TRL apoB-48 was 17.4% lower for apoE3/E4 subjects compared with apoE3/E3 subjects due to a 33.3% lower production rate (P=0.28). There was no significant difference in the TRL apoB-48 fractional catabolic rate (5.1+/-2.2 versus 5.0+/-2.1 pools per day). The pool size for VLDL apoB-100 was 36% lower for apoE3/E4 subjects compared with apoE3/E3 subjects due entirely to a 30% lower production rate (P=0.04). The LDL apoB-100 pool size was 57.8% higher (P=0.003) for apoE3/E4 subjects compared with apoE3/E3 subjects due to a 35.5% lower fractional catabolic rate of LDL apoB-100 (P=0.003), with no significant difference in production rate. In addition, 77% of VLDL apoB-100 was converted to LDL apoB-100 in apoE3/E4 subjects compared with 58% in apoE3/E3 subjects (P=0.05). In conclusion, the presence of 1 E4 allele was associated with higher LDL apoB-100 levels owing to lower fractional catabolism of LDL apoB-100 and a 33% increase in the conversion of VLDL apoB-100 to LDL apoB-100.

Postprandial chylomicrons and VLDLs in severe hypertriacylglycerolemia are lowered more effectively than are chylomicron remnants after treatment with n-3 fatty acids

BACKGROUND

n-3 Fatty acids lower plasma triacylglycerols not only in the fasting state but also in the postprandial state. However, it is not known whether chylomicrons, chylomicron remnants, and VLDLs are all affected equally or whether some lipoprotein species are lowered preferentially.

OBJECTIVE

Lipoproteins, including large and small chylomicron remnants, were determined specifically with the aid of a newly developed method involving a combination of size-exclusion chromatography and fluorometric determination of retinyl palmitate, which served as a marker for exogenous fat.

DESIGN

Twelve hypertriacylglycerolemic men were treated for 6 wk with 4 capsules containing 85% fish-oil concentrate/d; each capsule contained 850 mg n-3 fatty acid ethyl esters (49.1% eicosapentaenoic acid by wt and 32.2% docosahexaenoic acid by wt). Oral-fat-tolerance tests were performed before and after the treatment. Blood samples were drawn in the fasting state and until 8 h postprandially.

RESULTS

Treatment with n-3 fatty acids reduced the fasting VLDL-triacylglycerol concentration by 44% (P < 0.05) and postprandial chylomicrons and VLDLs at 4, 6, and 8 h (P < 0.05) by 49-64% and 36-43%, respectively. Chylomicron remnants were reduced only in the late postprandial phase: large chylomicron remnants by 19% at 6 h and by 43% at 8 h (P < 0.05) and small chylomicron remnants by 31% at 8 h (P < 0.05).

CONCLUSION

n-3 Fatty acids effectively lower chylomicrons and VLDLs, but their effect on chylomicron remnants was observed only in the late postprandial phase.

Hormone replacement therapy enhances postprandial lipid metabolism in postmenopausal women

Postmenopausal estrogen therapy reduces cardiovascular morbidity and mortality, except in women with advanced coronary disease. This beneficial effect is partly attributed to a reduction of fasting plasma total and low-density lipoprotein cholesterol (LDL-C) and an elevation of plasma high-density lipoprotein cholesterol (HDL-C) concentrations. Since postprandial lipemia seems to play a role in the pathogenesis of coronary artery disease, we evaluated the effect of hormone replacement therapy (HRT) on postprandial lipoprotein metabolism in 14 normolipemic postmenopausal women. A vitamin A fat-loading test before and after three cycles of treatment with a sequential combination of conjugated equine estrogen (CEE) and medroxyprogesterone acetate (MPA) was used to label chylomicrons and chylomicron remnants with retinyl palmitate (RP), and RP clearance was assessed over an 8-hour period postprandially. Following 3 months of HRT, fasting total cholesterol and LDL-C levels were reduced 9.8% (P = .049) and 16.5% (P = .023), respectively. Fasting HDL-C levels increased 18.9% (P = .001). Fasting triglycerides (TGs) increased, but not significantly. Postprandial integrated plasma TGs did not change significantly. The integrated RP levels in whole plasma and chylomicron (Svedberg flotation units [Sf] > 1,000) and nonchylomicron (Sf < 1,000) fractions were reduced 58% (P = .043), 78% (P = .041), and 75% (P = .001), respectively, after hormonal treatment. Enhanced clearance of chylomicrons and chylomicron remnants by HRT may contribute to the protective effect of estrogens against cardiovascular disease in normolipemic postmenopausal women.

Delayed changes in postprandial lipid in young normolipidemic men after a nocturnal vitamin A oral fat load test

The oral fat load tests (OFLT) used to study postprandial lipemia are generally conducted during the day. A nocturnal fat load test could be convenient and physiologically more appropriate. We have therefore compared the lipemic responses of 9 normolipidemic young men to OFLT given at 2200 h (nocturnal) and at 0700 h (diurnal). Triglyceride and retinyl palmitate concentrations were measured for 10 h. Peak plasma concentrations or areas under curves (AUC) for triglyceride after the diurnal and nocturnal tests were not significantly different [2.17 +/- 0.78 (diurnal) vs. 2.04 +/- 0.87 mmol/L (nocturnal) and 13.12 +/- 4.45 (diurnal) vs. 13.74 +/- 5.79 mmol/(L. h) (nocturnal)]. Peak plasma concentrations and AUC retinyl palmitate for the two tests were not different [1.71 +/- 0.69 (diurnal) vs. 1.42 +/- 0.66 mg/L (nocturnal) and 7.17 +/- 3.98 (diurnal) vs. 6.63 +/- 4.23 mg/(L. h) (nocturnal)]. The diurnal triglyceride peak occurred significantly earlier (4.3 +/- 1.2 h) than the nocturnal peak (5.8 +/- 1.7 h, P < 0.05). We have developed a model using only three sample time points to predict AUC [triglyceride at 0 h, triglyceride at average peak-time (4 h for diurnal and 6 h for nocturnal tests), and triglyceride at 10 h], thus reducing the number of blood samples. The predicted AUC was well correlated with the total AUC after nocturnal OFLT (r = 0.98, P < 0.0001). The nocturnal test appeared to be well tolerated by the subjects. The three-point simplified protocol may well be suitable for studies on large groups of subjects.

Thyroxine replacement therapy enhances clearance of chylomicron remnants in patients with hypothyroidism

To further confirm the benefit of replacement therapy in terms of risk for coronary artery disease, we evaluated the effect of T4 on postprandial lipoproteins in patients with hypothyroidism. Nine normolipidemic patients (aged 62.75+/-7.6 yr) with TSH of 32.2+/-13.2 mU/L and free T4 of 0.66+/-0.17 ng/mL were treated with T4 (50-100 microg/day) for at least 4 months. The behavior of postprandial lipoproteins was assessed before and during treatment by determining retinyl palmitate levels in the total plasma, chylomicrons (Sf >1000) and chylomicron remnants (Sf <1000) fractions for 8 h after a mixed meal plus vitamin A. During T4 treatment, serum levels of TSH and FT4 were 4.4+/-4.9 mU/L and 1.2+/-0.34 ng/mL (P = 0.001 and P = 0.002), respectively. Fasting low density lipoprotein cholesterol decreased from 166+/-35 to 135+/-23 mg/dL (P = 0.035). Retinyl palmitate (RP) levels in the chylomicron remnant fraction was reduced significantly during therapy from 6948+/-2790 to 5174+/-2401 microg/L x h (area under the curve +/-SD; P = 0.014). Total plasma RP and chylomicron RP remained unchanged. We conclude that T4 enhances the clearance of chylomicron remnants in normolipidemic patients with hypothyroidism.

The HMG-CoA reductase inhibitor atorvastatin increases the fractional clearance rate of postprandial triglyceride-rich lipoproteins in miniature pigs

We have previously shown in vivo that the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor atorvastatin decreases hepatic apolipoprotein B (apoB) secretion into plasma. To test the hypothesis that atorvastatin modulates exogenous triglyceride-rich lipoprotein (TRL) metabolism in vivo, an oral fat load (2 g fat/kg body wt) containing retinol (50 000 IU) was given to 6 control miniature pigs and to 6 animals after 28 days of treatment with atorvastatin 3 mg. kg-1. d-1. A multicompartmental model was developed by use of SAAM II and kinetic analysis performed on the plasma retinyl palmitate (RP) data. Peak TRL (d<1.006 g/mL; Sf>20) triglyceride concentrations were decreased 29% by atorvastatin, and the time to achieve this peak was delayed (5.2 versus 2.3 hours; P<0.01). The TRL triglyceride 0- to 12-hour area under the curve was decreased by 24%. In contrast, atorvastatin treatment had no effect on peak TRL RP concentrations, time to peak, or its rate of appearance into plasma; however, the TRL RP 0- to 12-hour area under the curve was decreased by 20%. Analysis of the RP kinetic parameters revealed that the TRL fractional clearance rate was increased significantly, 1.4-fold (3.093 versus 2.276 pools/h; P=0.012), with atorvastatin treatment. The percent conversion of TRL RP from the rapid-turnover to the slow-turnover compartment was decreased by 47% with atorvastatin treatment. The TRL RP fractional clearance rate was negatively correlated with very low density lipoprotein apoB production rate measured in the fasting state (r=-0.49). Thus, although atorvastatin had no effect on intestinal TRL assembly and secretion, plasma TRL clearance was significantly increased, an effect that may relate to a decreased competition for removal processes by hepatic very low density lipoprotein.

Kinetics of triglyceride rich lipoproteins: chylomicrons and very low density lipoproteins

Lipoprotein dynamics are complex during the postprandial state. A significant rise in chylomicron concentration is associated with increased competition for LPL with VLDL particles. This results in an increased concentration of large VLDL. The concentration of small VLDL is reduced as a result of diminished conversion of large to small VLDL. Such changes, induced in the postprandial state, complicate the application and development of models that describe lipoprotein particle kinetics. The development of models that integrate chylomicron and VLDL particle information, rather than surrogate markers, together with data including other variables will provide insight into the complexity of lipoprotein metabolism in the postprandial state.

The influence of chylomicron remnants on endothelial cell function in the isolated perfused rat aorta

A system for the perfusion of the isolated rat aorta which allowed the study of both the uptake of chylomicron remnants by the artery wall and their effects on endothelial function was developed. Perfusion for 2 h with 125I-labelled native or oxidised (by treatment with copper sulphate) chylomicron remnants showed that small amounts became associated with the artery wall (0.111 +/- 0.034 and 0.216 +/- 0.082 ng protein/mg tissue, respectively). Tests on endothelial function were carried out in vessel rings prepared after perfusion of the aortas in the presence or absence of chylomicron remnants for 2 h. After perfusion of the vessels with oxidised chylomicron remnants, the maximum response to phenylephrine (PE) was significantly increased (from 0.34 +/- 0.06 to 0.51 +/- 0.04 g/mg tissue; P < 0.05), while the maximum % relaxation to carbachol (CCh) was significantly decreased (from 91.6 +/- 2.4 to 71.5 +/- 7.2; P < 0.05) and the response to S-nitroso-N-acetylpenicillimine (SNAP) was unaffected. Perfusion with native chylomicron remnants showed a tendency to induce similar effects, although the changes observed did not reach statistical significance. As the lipoproteins were not present in the solution bathing the vessel rings during these tests, these effects can be attributed to perfusion of the aortas with chylomicron remnants, despite only small quantities being associated with the artery wall. The results suggest that oxidised chylomicron remnants influence vascular endothelial function by interfering with the L-arginine-nitric oxide (NO) pathway. The observed potentiation of contraction to PE may be due to inhibition of the basal release of NO or to the release of contractile factors. These findings support a role for dietary lipoproteins in the modulation of endothelial cell function which occurs in the pathogenesis of atherosclerosis.

Measurement of postprandial remnant-like particles (RLPs) following a fat-loading test

To assess the usefulness of remnant-like particles (RLPs), which represent potentially atherogenic lipoprotein remnants, as an index of postprandial hyperlipidemia, RLPs and other lipids were measured in 19 healthy adults (23-44 years old) after ingestion of a test meal (fat content, 32.9%). The lipoprotein derivatives, with a marked increase after fat-loading, were triglycerides, retinyl palmitate and RLPs. The retinyl palmitate peak lagged behind the RLP and triglyceride peaks, indicating that it may have different kinetics than RLP and triglyceride. While triglycerides are considered as an index of atherosclerotic risk factor, RLPs also may be a suitable index for evaluating postprandial hyperlipidemia. RLPs remained high even 8 h after fat-loading, suggesting that the postprandial state persists for almost the whole day. Accordingly, it may be important to assess postprandial remnant concentrations.

Intestinal absorption of beta-carotene, lycopene and lutein in men and women following a standard meal: response curves in the triacylglycerol-rich lipoprotein fraction

A high intake of fruit and vegetables is believed to be protective against heart disease and cancer. beta-Carotene has been closely examined for evidence of these protective properties but evidence is still conflicting and there are many other carotenoids in plant foods which deserve attention. This paper reports studies on the concentrations of lutein and lycopene in the triacylglycerol-rich lipoprotein (TRL) fraction of plasma in comparison with beta-carotene following a large dose of the respective carotenoids fed with a standard meal after an overnight fast. beta-Carotene (40 mg) was given to twelve volunteers (six men and six women) and six of the same volunteers (three men and three women) also received 31.2 mg lutein or 38 mg lycopene. Plasma was collected at hourly intervals for 8 h and the TRL fraction was separated and subsequently analysed for the respective carotenoids and retinyl palmitate in the case of beta-carotene. Intestinal uptake of the three carotenoids was estimated using the 'area under the curve' method and apparent absorption was calculated from these results. The response curves in the TRL fraction for beta-carotene and retinyl palmitate occurred maximally over the fourth to fifth hour postprandially. There was a correlation between the TRL concentrations of beta-carotene and retinyl palmitate (males r 0.62, P < 0.001; females r 0.52, P < 0.001) and there was no significant difference between men and women either in the total amount of beta-carotene appearing in the TRL fraction or in the amount converted to retinol. On estimation, approximately 1.4 mg of the 40 mg beta-carotene dose was absorbed and this was not significantly different from the amount of lycopene (1.0 mg) but significantly different (P < 0.05) from the amount of lutein (0.8 mg) absorbed, after correction for the smaller doses administered. There was approximately a twofold difference between subjects in the uptake of beta-carotene into the TRL fraction, a two- to threefold variation in lycopene and a two- to threefold variation in lutein. Despite these inter-subject differences, in three volunteers between whom there was a threefold difference in beta-carotene in the TRL fraction and a twofold difference in retinol formation, repeat experiments with beta-carotene 4 months later found differences of only 3-6% in the TRL beta-carotene content and 4-9% for the TRL retinol formed. In conclusion, large inter-subject variation in TRL carotene uptake precluded any differences between sexes but surprising intra-subject consistency was observed in TRL beta-carotene uptake of three subjects.

Olestra ingestion and retinyl palmitate absorption in humans

This study examined the effect of olestra, a zero-calorie fat replacement, on the absorption of retinyl palmitate in humans. After a 30-d adaptation period during which they consumed 10 g olestra/d in potato chips under free-living conditions, 68 healthy male subjects were housed in a metabolic ward and given a single dose of retinyl palmitate (0.33 RDA) containing a trace amount of 3H-retinyl palmitate with a breakfast that contained 0, 8, 20 or 32 g of olestra and about 38 g of triglyceride. Blood was collected at defined intervals for 48 h and plasma analyzed for 3H-retinyl esters by HPLC and liquid scintillation spectrometry. There was no significant effect on retinyl palmitate absorption as determined from the area under the plasma 3H-retinyl esters concentration-time curve. However, an area under the plasma concentration-time curve in the 32-g olestra group that was 81% (mean value) or 70% (median value) of the area under the curve for the placebo group suggested that olestra may have affected retinyl palmitate absorption. Inclusion or exclusion of 13 high responders did not change the results.

Kinetics of retinyl esters during postprandial lipemia in man: a compartmental model

Orally ingested vitamin A (retinol) is incorporated into intestinal chylomicrons (CHYLO) in the form of retinyl esters (RE) along with newly absorbed dietary triglycerides (TG). As the intestinal lipoproteins undergo hydrolysis in the circulation, the majority of the RE remain with the secreted intestinal particles and have been used as a marker for intestinally derived lipoproteins during the early phase of the postprandial state. A multicompartmental model was developed for the kinetics of RE during postprandial lipemia in individuals with normal lipid levels (n = 16) and in patients with hyperlipidemia (n = 44). The assumptions used in the development of the model are presented in this report. Some of the key findings include (1) as much as 50% of the newly synthesized RE may be secreted by the intestine as very-low-density lipoprotein (VLDL)-sized particles of S(f) 20 to 400 following consumption of a test meal containing a moderate amount of fat (20 to 30 g); (2) in most individuals, approximately 50% of the RE secreted in S(f) greater than 400 are converted to smaller, less buoyant fractions, and 50% are irreversibly removed directly from the plasma; (3) as much as 5% to 20% of the ingested retinol may be secreted as small intestinal lipoproteins with the buoyance of low-density lipoprotein (LDL) in some individuals; and (4) less than 5% of RE flux through S(f) 20 to 400 is converted to S(f) less than 20, and the primary catabolic pathway for RE in this fraction is direct uptake. Comparable estimates can be obtained for the kinetic parameters when repeat studies are made in the same subjects under comparable conditions.

Clearance of postprandial lipoproteins in normolipemics: role of the apolipoprotein E phenotype

The hepatic clearance of triglyceride-rich lipoproteins is mediated via apolipoprotein (apo) E which occurs in three common isoforms, apoE2, apoE3 and apoE4. To study the importance of the apoE isoforms on the response curves of different triglyceride-rich lipoproteins and the effect of chylomicron remnants on the composition of HDL, 37 normolipemics were investigated after a standardized fatty meal (8 apoE2/E2, 8 apoE2/E3, 8 apoE3/E3, 7 apoE3/E4 and 6 apoE4/E4). These individuals were matched for age, body mass index, fasting triglycerides, HDL-cholesterol, and apoA-I. A delayed chylomicron remnant clearance was observed only in apoE2 homozygotes, and this delay was neither correlated with fasting lip ds nor with peak lipoprotein concentrations. In apoE2/E3 heterozygotes, in contrast, the defective isoform E2 appears to be compensated for by the normal apoE isoform E3. In non-apo-E2/E2 individuals, the chylomicron remnant response was highly correlated with the magnitude of chylomicron and VLDL responses, with fasting triglycerides, and with the triglycerides enrichment and cholesterol depletion of HDL. These correlations were not observed in apoE2/E2. From these results we conclude that the chylomicron remnant response curve is an indicator of the extent of postprandial lipemia in non-apoE2/E2 individuals only.

Bezafibrate therapy in patients with isolated low high-density lipoprotein cholesterol levels may have a beneficial effect in prevention of atherosclerosis

Although a low plasma high-density lipoprotein cholesterol (HDL-C) level is a well-accepted risk factor for coronary artery disease (CAD), it is unclear whether pharmacologic agents can effectively increase HDL-C levels and/or reduce the incidence of CAD in patients with isolated low HDL-C levels. An important determinant of HDL levels is the efficiency of postprandial lipoprotein catabolism. The purpose of the present study was to evaluate the efficacy of bezafibrate therapy in increasing HDL-C levels in these patients and to examine its effect on postprandial lipoprotein levels. Fasting and postprandial lipid and lipoprotein levels were studied in 23 patients with isolated low HDL-C levels before and during 3 and 6 months of bezafibrate treatment. Postprandial lipoprotein levels were evaluated using the vitamin A-fat loading test, in which these intestinally derived lipoproteins are specifically labeled with retinyl palmitate (RP). Patients with isolated low HDL had significantly higher levels of chylomicron RP than a control group of 19 normolipidemic subjects. The area below the chylomicron RP curve was 17,773 +/- 6,821 versus 13,936 +/- 6,217 micrograms/L.h, respectively (P < .005). No differences were found in chylomicron remnant levels between the groups. Bezafibrate therapy reduced the chylomicron RP area by 27%, from 17,773 +/- 6,821 to 12,895 +/- 2,576, and the nonchylomicron RP area by 25%, from 6,059 +/- 3,310 to 4,430 +/- 1,963 (P < .0001). It increased fasting HDL-C levels from 35 +/- 3 to 38 +/- 1.4 mg/dL after 3 months (P < .001) and to 40 +/- 2.2 mg/dL after 6 months (P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Postprandial vitamin A and squalene clearances and cholesterol synthesis off and on lovastatin treatment in type III hyperlipoproteinemia

Postprandial fat clearance and absorption, fecal elimination and synthesis of cholesterol, bile acid synthesis, and cholesterol precursors and plant sterols in serum were studied in five patients with type III dyslipoproteinemia off and on lovastatin. The basal values were related to those in nontreated normolipidemic control subjects with apolipoprotein E3/3 phenotype (apo E3 controls, n = 16). On regular home diets, cholesterol precursor concentrations and cholesterol precursor/cholesterol ratios were high in the type III group. However, cholesterol absorption efficiency, bile acid and cholesterol synthesis measured with sterol balance technique and the precursor sterol/plant sterol ratios in serum were similar to the control values, suggesting that cholesterol absorption and metabolism was normal in these subjects. Lovastatin normalized the increased lipoprotein concentrations and reduced biliary cholesterol secretion, absolute absorption of cholesterol, precursor sterol/cholesterol and precursor sterol/plant sterol ratios in serum, fecal neutral and total sterol outputs and cholesterol synthesis. Lovastatin had no effect on cholesterol absorption efficiency or bile acid synthesis. Despite normalization of the triglyceride-rich lipoprotein levels by lovastatin, the postprandial vitamin A and squalene peak concentrations and the areas under the curves remained above the control ranges. The findings show that in type III hyperlipidemia, the precursor sterol/cholesterol ratios do not predict cholesterol synthesis. The latter, bile acid synthesis, precursor sterol/plant sterol ratios in serum, and cholesterol absorption are normal under basal conditions. The normalization of increased lipids by lovastatin is mainly due to reduced synthesis and absolute absorption of cholesterol, while the retarded postprandial fat clearance was not normalized by the drug.