Delayed clearance of postprandial chylomicrons and their remnants in the hypoalphalipoproteinemia and mild hypertriglyceridemia syndrome

Assessment of the Validity and Reproducibility of a Novel Standardized Test Meal for the Study of Postprandial Triacylglycerol Concentrations

Lipotest^® is a standardized fat-rich meal designed for use as a test meal during a fat tolerance test (FTT) for the study of postprandial triacylglycerol (TAG) concentrations. Herein we examined the precision and reproducibility of examination using Lipotest^® on postprandial TAG levels. A total of 26 healthy consenting subjects were examined twice after 8–10 h fasting with an interval of approximately 1 week apart. Blood samples were collected at baseline and 1, 2, 3, and 4 h after consumption of the test meal for measurement of plasma total TAG levels. We examined agreement, precision, and accuracy between the two visits using the Altman plots and correlation coefficient. Reproducibility was tested using the coefficient of variation (CV) and intraclass correlation coefficient (ICC). Moreover, the area under the curve (AUC) as a summary measure of the overall postprandial TAG levels was calculated. The agreement, precision (r ≥ 0.74, p < 0.001), and accuracy (≥0.99) between the measurements in plasma TAG during Lipotest^® testing in the two visits were high. In terms of reproducibility, the values of CV were 15.59–23.83% while those of ICC were ≥0.75. The values of the AUCs in the visits were not different (p = 0.87). A single measurement of plasma TAG levels at 4 h after Lipotest^® consumption depicted peak postprandial TAG concentration. A FTT using Lipotest^® as a standardized meal has good precision and reproducibility for the study of postprandial TAG levels in healthy individuals. A single determination of plasma TAG concentration at 4 h after Lipotest^® consumption captures peak postprandial TAG response.

Validity of abbreviated oral fat tolerance tests for assessing postprandial lipemia

BACKGROUND & AIMS

Postprandial lipemia is assessed using the oral fat tolerance test (OFTT), a six-hour procedure requiring hourly blood sampling. In order to simplify the test, we investigated whether a) a single postprandial triacylglycerol concentration, b) an OFTT lasting less than six hours or c) an OFTT requiring fewer blood samples, may accurately assess postprandial lipemia under various conditions.

METHODS

Seventy-two subjects underwent a conventional OFTT. Predictability of single-point concentrations and time-shortened tests was assessed using linear regression and re-sampling analysis. Validity of reduced-sampling tests was assessed using Pearson's correlation coefficients and Bland-Altman analysis.

RESULTS

a) A single-point triacylglycerol concentration (3 or 4h postprandially) did not present significant correlation with postprandial lipemia in the hypetriacylglycerolemic, exercise and energy restriction groups (P>0.05), b) Time-shortened OFTT (4h) was able to predict postprandial lipemia in every group studied (R(2)=0.707-0.970, P<0.01), except the hypertriacylglycerolemics (P=0.338), c) Reduced-sampling OFTT (3 or 4 samples) was able to assess postprandial lipemia in every group as well as in the overall sample (r=0.874-0.997, P<0.05).

CONCLUSIONS

A more convenient, reduced-sampling OFTT may be used alternatively to the conventional OFTT, whereas a time-shortened OFTT may be appropriate only for healthy people.

Gemfibrozil Improves Postprandial Hypertriglyceridemia in Patients with Isolated Low HDL

Aim

To assess the response of postprandial (PP) hypertriglyceridemia to genfibrozil in healthy male subjects with isolated low HDL-Cholesterol but without the metabolic syndrome (MS).

Patients and methods

14 male subjects with isolated low HDL (HDL-C ≤ 33), normal fasting triglycerides and LDL-C levels and without any feature of the MS, were studied. 13 male subjects with HDL-C > 38 mg/dl served as controls. They also had normal fasting triglycerides and LDL-C levels and without any feature of the MS. The 2 groups were statistically similar with respect to age, blood pressure, BMI, body fat composition, waist circumference, waist to hip ratio, fasting insulin, fasting and PP blood sugar, baseline fasting TG level and baseline LDL-C. Postprandial TG levels were measured at 2, 4, and 6 hours following a morning meal. Ten of the patients with PP hypertriglyceridemia were treated with gemfibrozil 600 mg PO twice/day for one month.

Results

Patients had markedly higher levels of the peak PP TG at 4 hours compared to controls (296.0 ± 37.7 vs. 206.7 ± 29.9 mg/dl; P < 0.05) the other two postprandial levels were also higher in patients but the difference was not significant. Treatment with gemfibrozil significantly decreased the levels of fasting and postprandial TG and increased HDL-C by around 3.6 mg/dl (11.7%) without affecting LDL-C.

Conclusion

Postprandial increase in serum TG may be present in patients with isolated HDL-C without the MS. Treatment of patients with PP hypertriglyceridemia with gemfibrozil improves the low HDL-C and postprandial rise in TG.

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.

Impaired HDL response to fat in men with coronary artery disease

A low HDL cholesterol is found frequently in subjects with premature coronary artery disease. We speculated that individuals with a normal total cholesterol and coronary artery disease have an impaired HDL response to dietary fat. Twenty-one men with recently diagnosed coronary artery disease and total plasma cholesterol of <6 mmol/l were matched by age, weight and cholesterol with 26 men with no personal or family history of coronary artery disease. They were placed sequentially on a 25% fat diet for 2 weeks, a high carbohydrate supplement which reduced fat to 16% of energy for 3 weeks and a high monounsaturated fat supplement which increased fat to 35% for a final 3-week period. Half of the subjects underwent an intravenous glucose tolerance test at the end of the intervention periods. The high fat supplement increased HDL cholesterol from 0.79 to 0.89 mmol/l in the men with coronary artery disease while HDL increased from 0.88 to 1.05 mmol/l in the control group (P<0.05 for group difference). Plasma triglyceride fell by 0.79 and 0.45 mmol/l in cases and controls respectively (P<0.05 for group difference). LDL cholesterol fell by 0.2 mmol/l in both groups. Men with coronary artery disease had an enhanced insulin response during the intravenous glucose tolerance test (P<0.03) particularly in the low fat phase. Thus men with premature coronary artery disease and a low HDL cholesterol appear to have an impaired elevation of HDL cholesterol in response to dietary fat, and insulin resistance may underlie this response.

The atherogenic significance of an elevated plasma triglyceride level

The importance of hypertriglyceridemia as an independent predictor of coronary artery disease (CAD) remains unsettled. Hypertriglyceridemia, with or without associated hypercholesterolemia, occurs more frequently in premature CAD subjects than does hypercholesterolemia alone. With univariate analysis, most studies show a positive correlation between plasma triglyceride (TG) level and risk for CAD, but with multivariate analysis plasma TG level is no longer an independent risk factor except in women and diabetics. Prospective studies have shown that subjects with a high LDL/HDL cholesterol ratio and a high plasma TG level have the highest risk for CAD. Hypertriglyceridemia signifies the presence of excess triglyceride-rich lipoproteins (TRL), including chylomicrons, VLDL, and their remnants. The question then becomes one of whether TRL are directly or indirectly involved in atherogenesis. TRL were thought to be too big to infiltrate the arterial wall, and histopathological studies have shown cholesterol but not triglyceride accumulation in the atherosclerotic plaque. However, there was a recent demonstration of undegraded VLDL and IDL in atherosclerotic plaques. Larger TRL may undergo hydrolysis on the arterial surface to become smaller particles before entry into the intima. Possible cellular pathways for the uptake of TRL by macrophages have been described. The smaller TRL (Sf 20-60), including postprandial chylomicron remnants, are believed to be the most atherogenic of all TRL particles. Because large amounts of TRL are produced in the postprandial period, atherogenesis involving TRL may be primarily a postprandial phenomenon. Once in the intima, TG may undergo hydrolysis, releasing free fatty acids and mono- and diacyl glycerol, accounting for the dearth of TG in atherosclerotic lesions. Particle for particle, VLDL delivers five times as much cholesterol as LDL does to the macrophage.

Postprandial responses of plasma lipids and lipoproteins in subjects with apoA-I(Lys107-->0)

Subjects with apoA-I(Lys107-->0) deletion mutation have reduced levels of plasma HDL cholesterol, apoA-I, apoA-II and Lp(AI:AII). In the present study were describe the postprandial responses of apoA-I(Lys107-->0) subjects (n=6) to the ingestion of a fat rich meal compared to the responses of their unaffected family members (n=6). The postprandial plasma triglyceride responses were comparable in the two groups of subjects. Plasma postprandial HDL cholesterol levels fell in both groups; patients (0.89+/-0.05-0.76+/-0.06 mmol/l, P=0.0032) and control subjects (1.32+/-0.25-1.18+/-0.21 mmol/l; P=0.0022). HDL2 cholesterol levels tended to rise, but the changes were not significant. By contrast, in both patients and control subjects, the HDL3 cholesterol levels fell; patients (0.52+/-0.15-0.37+/-0.11 mmol/l, P=0.0068) and control subjects (0.63+/-0.10-0.49+/-0.08 mmol/l, P=0.0078), respectively. In both patients and control subjects the plasma HDL2 mass increased in response to the fat meal; patients (94.7+/-37.3-114.3+/-28.8 mg/dl, P=0.0397) and control subjects (142.1+/-32.8-168.1+/-29.8 mg/dl, P=0.0298), whereas HDL3 mass decreased; patients (162.3+/-36.8-131.4+/-28.9 mg/dl, P=0.0251) and control subjects (185.5+/-34.1-161.1+/-29.0 mg/dl, P=0.0021), respectively. In control subjects the triglyceride levels increased in both HDL2 (0.10+/-0.06-0.17+/-0.06 mmol/l; P=0.0005) and to a lesser extent in HDL3 (0.10+/-0.03-0.12+/-0.02 mmol/l, P=0.0017). In patients, triglyceride levels in both HDL subclasses remained unchanged. Changes in the concentration of Lp(AI) in HDL2 and HDL3 were comparable in the two groups of subjects. The Lp(AI:AII) concentration in HDL2 remained unchanged in the patients, but increased in control subjects (change+27%. P=0.0026). Consequently, the 20% difference at baseline in the concentration of Lp(AI:AII) in HDL2 between the patients and control subjects increased postprandially to 45% (P=0.0240). The Lp(AI:AII) concentration in HDL3 decreased in both groups, but the changes were non-significant. Our findings show that in postprandial state, no accumulation of triglycerides in HDL subclasses occurs in patients with apoA-I(Lys[107]-->0) and that these patients appear to lack the ability to respond to fat feeding by increasing the Lp(AI:AII) concentration in HDL2.

An NCEP II diet reduces postprandial triacylglycerol in normocholesterolemic adults

We compared the fasting and postprandial response to a National Cholesterol Education Program (NCEP) II diet with that of a diet high in total (40% of energy) and saturated fat. In free-living conditions, 17 healthy normolipidemic, normoglycemic men and women consumed a high-fat diet, a maintenance diet and the NCEP II diet, for 1 mo each. At the completion of each dietary period, an oral fat load (70 g/m2 body surface area) was administered and plasma triacylglycerol (TAG) determined every 2 h for 8 h. Compared with the high-fat phase, the NCEP II diet was associated with significantly lower energy intake (12.1 +/- 1.1 vs. 7 +/- 0.7 MJ/d) and final body weight (78 +/- 3.8 vs. 76.3 +/- 3.5 kg) (P < 0.01). Plasma high density lipoprotein cholesterol, apolipoprotein (apo) A-I and ApoB concentrations were also significantly lower when subjects consumed the NCEP II diet rather than the high-fat diet (P </= 0.004). There were no significant differences in subjects fasting TAG, glucose or insulin concentration between the high fat and NCEP II diet periods. However, the postprandial plasma TAG response to the fat load was lower after completing the NCEP II than after the high-fat diet period (P = 0.045). Under free-living conditions, a NCEP II diet was associated with weight loss and a decrease in postprandial but not fasting TAG. Because dietary alteration may not affect fasting TAG levels, thorough assessment of a dietary intervention should include measurements of postprandial TAG.

Clinical pharmacokinetics of fibric acid derivatives (fibrates)

Beginning with the description of clofibrate in 1962, derivatives of fibric acid (fibrates) have been used clinically to treat dyslipidaemias. Subsequently, gemfibrozil, fenofibrate, bezafibrate, ciprofibrate and long-acting forms of gemfibrozil, fenofibrate and bezafibrate have been developed. Clinically, this class of drugs appears to be most useful in lipoprotein disorders characterised by elevations of very low density lipoprotein and plasma triglycerides, which are often accompanied by reductions in high density lipoprotein (HDL) levels. The principal effects are a reduction in triglyceride and increase in HDL levels, with increases in the activity of hepatic lipase and lipoprotein lipase. There is some reduction of low density lipoprotein (LDL), lipoprotein (a), fibrinogen and uric acid. As a class, these drugs are generally well absorbed from the gastrointestinal tract (immediate-acting fenofibrate being the exception) and display a high degree of binding to albumin. Fibrates are metabolised by the hepatic cytochrome P450 (CYP) 3A4. All members of this class are primarily excreted via the kidneys and display some increase in plasma half-life in individuals with severe renal impairment. The long-acting forms of gemfibrozil and bezafibrate have pharmacokinetic properties similar to those of their immediate-acting parent compounds. The long-acting form of fenofibrate, produced by the process of micronisation, has increased oral bioavailability with less variability in absorption compared with the immediate-acting form of fenofibrate. Drug interactions are seen with other drugs that share a high degree of binding to albumin or are metabolised by CYP3A4. Clinically the most important and most commonly reported drug interactions are with HMG-CoA reductase inhibitors (lovastatin, simvastatin, pravastatin and fluvastatin), warfarin, cyclosporin and oral hypoglycaemic agents [including metformin, tolbutamide and glibenclamide (glyburide)]. The main potential for drug interactions is with drugs or compounds that are metabolised by or affect CYP3A4, including imidazoles, grapefruit juice, erythromycin, mibefradil and others.

Dietary preparation before rest and exercise testing

To compare the effectiveness of four dietary preparations for stabilizing resting and exercise measurements, seven male recreational exercisers (27 +/- 4 y) participated in four dietary preparations, each repeated in successive weeks: (1) 24-h random diet including an overnight fast (RAN); (2) 24-h random diet, including fast, followed by a standard meal 3 h before testing (RANM); (3) 24-h prescribed diet including an overnight fast (PRES); and (4) 24-h prescribed diet, including fast, followed by a standard meal (PRESM). After each preparation, metabolic rate (VO2) and respiratory exchange ratio (RER) were measured at rest and in association with moderate treadmill exercise. Plasma was analyzed for glucose, cholesterol, and high-density lipoprotein (HDL)-cholesterol. Repeated measures analyses of variance (ANOVAs) followed by Tukey posthoc tests indicated that resting VO2, RER, and blood parameters were not different between the two trials on the same diet. Exercise RER, however, was slightly different in trial 1 than in trial 2 for all preparations except PRESM. Combining both trials, resting VO2 and exercise RER were higher when a pretesting meal was administered. Plasma values were not different for the four dietary preparations. These results suggest that a standard overnight fast appears to be adequate for establishing representative and reproducible rest and exercise values for the parameters measured, except possibly for exercise RER reliability.

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.

The genetic determinants of plasma cholesterol and response to diet

In general, risk factors for multifactorial disorders such as atherosclerosis and hyperlipidaemia show a continuous distribution in the population, and this is the result of both interaction between genetic variation at genetic loci, and genetic and environmental interaction. Therefore, the investigation of the genetics of intermediate phenotypes such as levels of plasma lipid traits is likely to be particularly informative. Once the genes involved in determining the levels of these phenotypes have been identified, it should be possible to use the information to obtain a better understanding of the way these genetic variations determine the clinical end points. In the population it will be possible to identify a number of polygenes that are having a small effect on determining the trait, but for a particular individual, or the relatives of that individual, only a subset of all these polygenes will determine the level of the trait and therefore the risk of developing the disorder. In general, mutations with a large effect on the trait are rare in the population, By contrast, polymorphisms with a small effect on the trait may be common, such as is found with the effect of the apoE alleles and variation at the apoB gene locus on lipid levels. In the field of hyperlipidaemia and atherosclerosis research, molecular techniques have already given a great deal of information on how specific sequence variations in some of the candidate genes are involved in determining levels of plasma apoproteins, lipoproteins and lipids. As more mutations and sequence variations are identified, this will not only aid our understanding of the underlying pathology, but should be useful for identifying individuals who are at risk of developing atherosclerosis because of their particular genotype or combination of genotypes.

Low-density lipoprotein heterogeneity

As a key feature of the ALP, a raised level of small, dense LDL forms part of what is potentially the most common collection of lipoprotein abnormalities to influence the risk of CHD in the general population. The consistency of the association between a prevalence of small, dense LDL and increased risk of CHD is impressive, though the practical constraints of our current methods would limit the clinical application of this information for screening purposes. This highlights the need to elucidate the underlying metabolic and genetic determinants of LDL heterogeneity and to develop alternative forms of analysis based on the structural and functional properties of LDL subclasses and their genotypes. The relationship between enhanced post-prandial lipaemia and raised VLDL is of particular relevance in providing insight into the way in which diet and drugs may target LDL heterogeneity via the insulin-dependent regulation of the post-prandial response. In alluding to the impact of genetic influences on LDL subclasses, twin studies identify a major role for environmental factors as determinants of LDL heterogeneity and, more importantly, as modulators of environmentally susceptible genes. The potential for interactions between dietary factors alone and putative LDL heterogeneity genes is considerable and yet poorly understood, as, for example, the reduced penetrance of the 'ATHS' gene caused through variation in dietary fat and carbohydrate (Nishina et al, 1992). Finally, small, dense LDL has been implicated in at least two major steps of the atherogenic process. Elucidation of the molecular basis of these interactions will be crucial for the identification of genetically susceptible individuals and in the design of appropriate diets and treatment to reduce the risk of CHD mediated through LDL.

Effects of pravastatin with niacin or magnesium on lipid levels and postprandial lipemia

This study was designed to evaluate the therapeutic effectiveness of 3 different pharmacologic lipid-lowering regimens in the treatment of patients with clustered lipid risk factors. Sixty-five patients with low high-density lipoprotein (HDL) levels and hypertriglyceridemia were randomized to 1 of 3 treatment arms: pravastatin/niacin, pravastatin/magnesium, or pravastatin/placebo. After 18 weeks, patients in the pravastatin/niacin group had a -41% change in the total cholesterol/HDL ratio compared with -13% in the pravastatin/magnesium arm and -16% in the pravastatin/placebo group. The HDL2 and HDL3 subfractions, as well as the apolipoprotein A-I levels, were increased significantly only in the pravastatin/niacin arm. The levels of small dense low-density lipoprotein (LDL) cholesterol (LDL3) were decreased to a greater extent in the pravastatin/niacin arm (-43%) than in either the pravastatin/magnesium (-13%) or the pravastatin/placebo (-20%) arm. Only the pravastatin/niacin regimen significantly diminished postprandial lipemia (-32% change in the remnant particle triglyceride concentration and decreased very-low-density lipoprotein remnant levels). Thus, in this group of patients with clustered risk factors, the combination of pravastatin and niacin resulted in significant improvements in HDL and triglyceride levels, total cholesterol to HDL ratio, small dense LDL levels, and postprandial lipemia. Pravastatin alone or in combination with magnesium resulted in less significant changes that were largely limited to LDL cholesterol reduction.

Postprandial lipoprotein responses in hypertriglyceridemic subjects with and without cardiovascular disease

Three groups of age- and weight-matched men (aged 40 to 70 years) without diabetes were studied: controls (n = 10), plasma triglycerides (TG) less than 180 mg/dL and no cardiovascular disease (CVD); HTG-CVD (n = 11), hypertriglyceridemic (HTG) (TG > 240 mg/dL) without CVD; and HTG+CVD (n = 10), HTG (TG > 240 mg/dL) with documented CVD. HTG+CVD subjects had higher fasting and post-oral glucose tolerance test insulin levels than the other two groups, respectively. Very-low-density lipoprotein (VLDL)+chylomicrons (CMs), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and three high-density lipoprotein (HDL) subfractions (HDL-L, HDL-M, and HDL-D, from least to most dense) were isolated by gradient ultracentrifugation. Fasting lipoproteins were similar in HTG groups, except for higher VLDL lipid to apolipoprotein (apo) B ratios (P < .04) in the HTG+CVD group. Subjects were fed a high-fat mixed meal, and lipoprotein composition was determined at 3, 6, 9, and 12 hours postprandially. Postprandial responses of the core lipids (TG and cholesterol esters [CE]) in all of the lipoprotein subfractions were similar in the two HTG groups at each time point. However, both controls and HTG-CVD subjects had increases in HDL-M phospholipid (PL) at 9 and 12 hours with no change in HDL-D PL. The HTG+CVD group, on the other hand, had no increase in HDL-M PL and had a substantial reduction in HDL-D PL. These changes resulted in significant increases in HDL-M and HDL-D PL to apo A-I ratios in both controls and HTG-CVD subjects between 6 and 12 hours, whereas there was no increase seen in the HTG+CVD group. The HTG-CVD group also had a significantly greater increase in the VLDL+CM PL to apo B ratio (P = .038) at 3 hours than the HTG+CVD group. This diminished amount of surface lipid per VLDL particle may account for the late decrease in the HDL-D PL to apo A-I ratio seen in HTG+CVD patients. There were no other postprandial lipid or apolipoprotein differences between the two HTG groups. We conclude therefore that the major postprandial lipoprotein abnormality in these HTG+CVD patients was a failure to increase the PL content per particle in VLDL+CM, HDL-M, and HDL-D. This abnormality could prevent the usual increase in reverse cholesterol transport seen in postprandial plasma and therefore contribute to their increased incidence of CVD. The greater insulin resistance seen in these patients also appears to contribute significantly to their CVD.

Common variation in the gene for apolipoprotein B modulates postprandial lipoprotein metabolism: a hypothesis generating study

We have carried out a pilot study to examine the influence on postprandial lipid and lipoprotein metabolism of common genetic variation in the gene coding for apolipoprotein (apo) B, in a previously described group of 30 individuals who had survived a myocardial infarction (MI) before the age of 45 (normo (NTG)- and hypertriglyceridaemic (HTG) patients) and 11 age-matched healthy individuals. Postprandial lipid or lipoprotein levels were examined by genotypes in the three groups separately and after adjustment for fasting triglycerides (TG) in the whole group combined. For the signal peptide polymorphism in the apo B gene, individuals with one or more SP-24 alleles had a 38% smaller mean area under curve (AUC) (P = 0.06) for postprandial large chylomicron remnants and a 29% smaller mean AUC (P = 0.01) for large very low density lipoproteins (VLDLs) compared to individuals homozygous for the wild type SP-27 allele. Previously in this patient group, small chylomicron remnants (apo B-48 levels in the Sf 20-60 range) were found to relate significantly and positively to progression of coronary atherosclerosis suggesting that these lipoproteins are implicated in progression of atherosclerosis. For the apo B Val591-Ala polymorphism (Ag a1/d), individuals homozygous for the V591 allele had a 33% greater AUC for Sf 20-60 postprandial triglycerides (P = 0.006), with higher postprandial levels of both apo B-48- and apo B-100-containing lipoproteins in this fraction. This pilot study gives insight into the mechanisms of the previously reported associations between polymorphisms in the apo B gene and fasting plasma lipids and lipoproteins.

Quantification of postprandial triglyceride-rich lipoproteins in healthy men by retinyl ester labeling and simultaneous measurement of apolipoproteins B-48 and B-100

The metabolism of chylomicrons, very-low-density lipoprotein (VLDL), and their remnants in the postprandial state was studied in normolipidemic healthy men by measuring apoB-48 and apoB-100 and retinyl palmitate (RP) in fractions of triglyceride-rich lipoproteins after a mixed meal type of oral fat load supplemented with vitamin A. ApoB-48 was present at low concentrations in the fasting plasma samples in most subjects and increased in response to the test meal in Svedberg's flotation rate (Sf) > 20 lipoprotein fractions. Concomitantly, the level of Sf 60 to 400 apoB-100 (large VLDL) had doubled at 3 hours and returned to baseline at 9 hours. The number of apoB-48-containing lipoprotein particles did not exceed 20% of the total number of apoB-containing lipoproteins contained in Sf 12 to 400 fractions at any time point after fat intake. The peak plasma level of RP was delayed compared with the peak plasma concentration of apoB-48, suggesting that retinyl ester labeling of chylomicrons is questionable as a means of quantifying postprandial triglyceride-rich lipoproteins of intestinal origin. Approximately 2000 and 4000 RP molecules were carried in each chylomicron particle in the 3- and 6-hour samples, respectively, in contrast to the remnant fractions in which 100 to 600 RP molecules were found for each lipoprotein particle. The limited RP exchange between lipoprotein particles indicates that the smaller intestinal lipoproteins do not originate primarily from larger Sf > 400 chylomicron particles but instead are secreted directly into the Sf 20 to 400 fraction and subsequently converted to smaller chylomicron remnants.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of gemfibrozil and lovastatin on postprandial lipoprotein clearance in the hypoalphalipoproteinemia and hypertriglyceridemia syndrome

Eleven men with hypoalphalipoproteinemia (HPAL; fasting plasma high density lipoprotein (HDL) cholesterol level of < 0.9 mmol/l), mild hypertriglyceridemia (HTG; triglycerides (TG) level of 1.75-7.5 mmol/l) and a normal calculated LDL cholesterol level (< 3.7 mmol/l) participated in a randomized, double-blind, double-placebo, crossover trial to compare the effect of two drugs, lovastatin (40 mg once daily) and gemfibrozil (600 mg twice daily), on clearance of postprandial lipoproteins. A 2-week washout period separated drug treatment periods of 6 weeks each. Ten subjects completed each treatment period. After ingestion of a vitamin A fat load, plasma, chylomicron and non-chylomicron retinyl palmitate (RP) and TG responses (areas under curves) were reduced in all subjects on gemfibrozil therapy and in 7 on lovastatin therapy. There was close correlation between change in fasting TG (but not fasting HDL-cholesterol) and change in postprandial RP areas on gemfibrozil but not lovastatin therapy. Postheparin lipoprotein lipase (LPL) and hepatic lipase (HL) activities were increased by gemfibrozil therapy while only a mild elevation in LPL activity alone was seen on lovastatin therapy. These data indicate that improvement in HTG is the main feature associated with improvement in postprandial lipemia and this is likely due to LPL-mediated enhancement of lipolytic hydrolysis. Gemfibrozil is more effective than lovastatin in attenuating postprandial lipemia in the HPAL/HTG syndrome.