Triacylglycerol-rich lipoprotein margination: a potential surrogate for whole-body lipoprotein lipase activity and effects of eicosapentaenoic and docosahexaenoic acids

Omega-3 fatty acid regulation of lipoprotein lipase and FAT/CD36 and its impact on white adipose tissue lipid uptake

Lipid uptake by white adipose tissue (WAT) is critically important for storage of excess energy and to protect peripheral tissues from ectopic lipid deposition. When WAT becomes dysfunctional (i.e., with obesity), it is characterized by impaired lipid uptake and increased lipolysis which, together, promote whole-body dyslipidemia. Omega-3 polyunsaturated fatty acids (N-3 PUFA) are widely studied for their triacylglycerol (TAG)-lowering properties and cardiometabolic health benefits. One potential mechanism underlying these benefits is the modification of WAT lipid uptake; however, there are gaps in our understanding regarding the specific mechanisms by which N-3 PUFA function. Evidence to date suggests that N-3 PUFA promote TAG clearance by increasing lipoprotein lipase (LPL) activity and the abundance of fatty acid transporters. Specifically, N-3 PUFA have been shown to increase LPL activity through increased gene transcription and modifications of endogenously produced LPL regulators such as apolipoprotein C-II/III and angiopoietin-like proteins. This review presents and discusses the available in vitro and in vivo research to provide a comprehensive overview of N-3 PUFA regulation of WAT lipid uptake in healthy and obese contexts. Additionally, we highlight areas where more research is necessary to better understand the contribution of increased WAT lipid uptake in relation to the TAG-lowering properties associated with N-3 PUFA.

Lipoprotein Lipase Activity Does Not Differ in the Serum Environment of Vegans and Omnivores

Although vegan diets have been reported to be associated with a reduced risk of cardiovascular disease, it was not known whether this might be partly due to vegan diets' effects on plasma triglyceride metabolism. This study aimed to investigate if there are differences in the activity of lipoprotein lipase (LPL), an enzyme that functions at the vascular endothelium and is responsible for triglyceride breakdown, in sera obtained from vegans and omnivores. LPL activity was assessed using isothermal titration calorimetry, which allows measurements in undiluted serum samples, mimicking physiological conditions. Fasted sera from 31 healthy participants (12F 2M vegans, 11F 6M omnivores) were analyzed. The results indicated no significant differences in average LPL activity between the vegan and omnivore groups. Interestingly, despite similar triglyceride levels, there were considerable variations in LPL activity and total very-low-density lipoprotein triglyceride breakdowns between individuals within both groups. Biomarker analysis showed that vegans had lower total cholesterol and LDL-C levels compared to omnivores. These findings suggest that the lipid-related benefits of a vegan diet, in terms of atherogenic risk, may primarily stem from cholesterol reduction rather than affecting serum as a medium for LPL-mediated triglyceride breakdown. In healthy individuals, lipid-related changes in serum composition in response to a vegan diet are likely overshadowed by genetic or other lifestyle factors.

Association of Fish and Omega-3 Fatty Acid Intake with Carotid Intima-Media Thickness in Middle-Aged to Elderly Japanese Men and Women: The Toon Health Study

Fish and omega-3 fatty acid consumption is known to be beneficial for cardiometabolic health. However, the related evidence for individuals with a relatively higher intake of fish or omega-3 unsaturated fatty acids, e.g., Japanese individuals, is scarce. Therefore, this study aimed to examine the association of fish and omega-3 fatty acid intakes with the carotid intima-media thickness (C-IMT) in the Japanese population. In total, 1803 Japanese men and women aged 30–84 years without a history of myocardial infarction or angina pectoris were included in the study. The fish and omega-3 fatty acid intakes were estimated using food frequency questionnaires. The C-IMT was measured using ultrasound imaging, and the participants were classified into three groups: normal, moderate (1.1 to 1.4 mm of maximum C-IMT), and severely increased C-IMT (≥1.5 mm). Multinomial logistic regression models were used to calculate the odds ratio (OR) and 95% confidence intervals (95% CI) of the presence of moderately and severely increased C-IMT. The omega-3 fatty acid intake was shown to be associated with lower odds of severely increased C-IMT. The multivariable-adjusted OR (95%CI) was 0.55 (0.31–0.97; p for trend = 0.04). We also found a borderline significant negative association between fish intake and the presence of severely increased C-IMT. In conclusion, omega-3 fatty acid intake might protect against the development of atherosclerosis in the Japanese population.

Lipid-Lowering Therapy in Woman of Childbearing Age: a Review and Stepwise Clinical Approach

PURPOSE OF REVIEW

Women are less often recognized to have cardiovascular disease (CVD) risk and are underrepresented in randomized trials of lipid-lowering therapy. Here, we summarize non-pharmacologic and pharmacologic strategies for lipid-lowering in women of childbearing age, lipid changes during pregnancy and lactation, discuss sex-specific outcomes in currently available literature, and discuss future areas of research.

RECENT FINDINGS

While lifestyle interventions form the backbone of CVD prevention, some women of reproductive age have an indication for pharmacologic lipid-lowering. Sex-based evidence is limited but suggests that both statin and non-statin lipid-lowering agents are beneficial regardless of sex, especially at high cardiovascular risk. Pharmacologic lipid-lowering therapies, both during the pregnancy period and during lactation, have historically been and continue to be limited by safety concerns. This oftentimes limits lipid-lowering options in women of childbearing age. In this review, we summarize lipid-lowering strategies in women of childbearing age and the impact of therapies during pregnancy and lactation. The limited sex-specific data regarding efficacy, adverse events, and cardiovascular outcomes underscore the need for a greater representation of women in randomized controlled trials. More data on lipid-lowering teratogenicity are needed, and through increased clinician awareness and reporting to incidental exposure registries, more data can be harvested.

Comparative Analysis of the Effects of Fish Oil and Fenofibrate on Plasma Metabolomic Profiles in Overweight and Obese Individuals

SCOPE

The drug fenofibrate and dietary fish oils can effectively lower circulating triglyceride (TG) concentrations. However, a detailed comparative analysis of the effects on the plasma metabolome is missing.

METHODS AND RESULTS

Twenty overweight and obese subjects participate in a double-blind, cross-over intervention trial and receive in a random order 3.7 g day-1 n-3 fatty acids, 200 mg fenofibrate, or placebo treatment for 6 weeks. Four hundred twenty plasma metabolites are measured via gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS). Among the treatments, 237 metabolites are significantly different, of which 22 metabolites change in the same direction by fish oil and fenofibrate, including a decrease in several saturated TG-species. Fenofibrate additionally changes 33 metabolites, including a decrease in total cholesterol, and total lysophosphatidylcholine (LPC), whereas 54 metabolites are changed by fish oil, including an increase in unsaturated TG-, LPC-, phosphatidylcholine-, and cholesterol ester-species. All q < 0.05.

CONCLUSION

Fenofibrate and fish oil reduce several saturated TG-species markedly. These reductions have been associated with a decreased risk for developing cardiovascular disease (CVD). Interestingly, fish oil consumption increases several unsaturated lipid species, which have also been associated with a reduced CVD risk. Altogether, this points towards the power of fish oil to change the plasma lipid metabolome in a potentially beneficial way.

Spotlight on Icosapent Ethyl for Cardiovascular Risk Reduction: Evidence to Date

Icosapent ethyl is a highly purified formulation of eicosapentaenoic acid, a type of omega-3 fatty acid contained in fish oil. While omega-3 fatty acids have long been thought to have cardioprotective benefits, the Reduction of Cardiovascular Events with EPA-Intervention Trial (REDUCE-IT) has helped to establish icosapent ethyl as an evidence-based therapy for risk reduction of atherosclerotic cardiovascular disease (ASCVD). REDUCE-IT, however, was by no means an overnight success story. Close examination of the evidence shows that the trial was a culmination of many lessons learned from previous studies. The purpose of this manuscript is to review contemporary evidence of icosapent ethyl in ASCVD risk reduction and the clinical implication of this promising therapy.

The Differential Effects of Eicosapentaenoic Acid and Docosahexaenoic Acid on Cardiometabolic Risk Factors: A Systematic Review

A large body of evidence supports the cardioprotective effects of the long-chain omega-3 polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). There is increasing interest in the independent effects of EPA and DHA in the modulation of cardiometabolic risk factors. This systematic review aims to appraise the latest available evidence of the differential effects of EPA and DHA on such risk factors. A systematic literature review was conducted up to May 2017. Randomised controlled trials were included if they met strict eligibility criteria, including EPA or DHA > 2 g/day and purity ≥ 90%. Eighteen identified articles were included, corresponding to six unique studies involving 527 participants. Both EPA and DHA lowered triglyceride concentration, with DHA having a greater triglyceride-lowering effect. Whilst total cholesterol levels were largely unchanged by EPA and DHA, DHA increased high-density lipoprotein (HDL) cholesterol concentration, particularly HDL₂, and increased low-density lipoprotein (LDL) cholesterol concentration and LDL particle size. Both EPA and DHA inhibited platelet activity, whilst DHA improved vascular function and lowered heart rate and blood pressure to a greater extent than EPA. The effects of EPA and DHA on inflammatory markers and glycaemic control were inconclusive; however both lowered oxidative stress. Thus, EPA and DHA appear to have differential effects on cardiometabolic risk factors, but these need to be confirmed by larger clinical studies.

Omega-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid and their mechanisms of action on apolipoprotein B-containing lipoproteins in humans: a review

Background

Epidemiological and genetic studies suggest that elevated triglyceride (TG)-rich lipoprotein levels in the circulation increase the risk of cardiovascular disease. Prescription formulations of omega-3 fatty acids (OM3FAs), mainly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), reduce plasma TG levels and are approved for the treatment of patients with severe hypertriglyceridemia. Many preclinical studies have investigated the TG-lowering mechanisms of action of OM3FAs, but less is known from clinical studies.

Methods

We conducted a review, using systematic methodology, of studies in humans assessing the mechanisms of action of EPA and DHA on apolipoprotein B-containing lipoproteins, including TG-rich lipoproteins and low-density lipoproteins (LDLs). A systematic search of PubMed retrieved 55 articles, of which 30 were used in the review; 35 additional arrticles were also included.

Results

In humans, dietary DHA is retroconverted to EPA, while production of DHA from EPA is not observed. Dietary DHA is preferentially esterified into TGs, while EPA is more evenly esterified into TGs, cholesterol esters and phospholipids. The preferential esterification of DHA into TGs likely explains the higher turnover of DHA than EPA in plasma. The main effects of both EPA and DHA are decreased fasting and postprandial serum TG levels, through reduction of hepatic very-low-density lipoprotein (VLDL)-TG production. The exact mechanism for reduced VLDL production is not clear but does not include retention of lipids in the liver; rather, increased hepatic fatty acid oxidation is likely. The postprandial reduction in TG levels is caused by increased lipoprotein lipase activity and reduced serum VLDL-TG concentrations, resulting in enhanced chylomicron clearance. Overall, no clear differences between the effects of EPA and DHA on TG levels, or on turnover of TG-rich lipoproteins, have been observed. Effects on LDL are complex and may be influenced by genetics, such as APOE genotype.

Conclusions

EPA and DHA diminish fasting circulating TG levels via reduced production of VLDL. The mechanism of reduced VLDL production does not involve hepatic retention of lipids. Lowered postprandial TG levels are also explained by increased chylomicron clearance. Little is known about the specific cellular and biochemical mechanisms underlying the TG-lowering effects of EPA and DHA in humans.

Eicosapentaenoic acid and docosahexaenoic acid containing supplements modulate risk factors for cardiovascular disease: a meta-analysis of randomised placebo-control human clinical trials

BACKGROUND

Over 200 clinical trials have examined the effect of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) supplements on risk factors associated with cardiovascular disease. However, an updated analysis of the evidence is lacking. The aim of the present meta-analysis was to quantify the effect of supplements containing EPA and DHA on risk factors for cardiovascular disease.

METHODS

An analysis was carried on 171 clinical trials with acceptable quality (Jadad score ≥3) that were identified from a comprehensive electronic search strategy of two databases (Pubmed and Cochrane Library). A random effect model was used to obtain an overall estimate on outcomes of interest. Heterogeneity between trial results was tested for using a standard chi-squared test.

RESULTS

Compared with control, EPA and DHA supplements produced significant reductions of triglycerides of 0.368 mmol L^-1 [95% confidence interval (CI) = -0.427 to -0.309], systolic blood pressure of 2.195 mmHg (95% CI = -3.172 to -1.217), diastolic blood pressure of 1.08 mmHg (95% CI = -1.716 to -0.444), heart rate of 1.37 bpm (95% CI = -2.41 to -0.325) and C-reactive protein of 0.343 mg L^-1 (95% CI = -0.454 to -0.232). This analysis indicates an increase in both low-density lipoprotein cholesterol (mean difference = 0.150 mmol L^-1 ; 95% CI = 0.058-0.243) and high-density lipoprotein cholesterol (mean difference = 0.039 mmol L^-1 ; 95% CI = 0.024-0.054). The triglyceride-lowering effect was dose-dependent.

CONCLUSIONS

The lipid-lowering, hypotensive, anti-arrhythmic and anti-inflammatory actions of EPA and DHA supplements were confirmed in this analysis of randomised placebo-control blinded clinical trials.

Emerging strategies of targeting lipoprotein lipase for metabolic and cardiovascular diseases

Although statins and other pharmacological approaches have improved the management of lipid abnormalities, there exists a need for newer treatment modalities especially for the management of hypertriglyceridemia. Lipoprotein lipase (LPL), by promoting hydrolytic cleavage of the triglyceride core of lipoproteins, is a crucial node in the management of plasma lipid levels. Although LPL expression and activity modulation is observed as a pleiotropic action of some the commonly used lipid lowering drugs, the deliberate development of drugs targeting LPL has not occurred yet. In this review, we present the biology of LPL, highlight the LPL modulation property of currently used drugs and review the novel emerging approaches to target LPL.

Challenges in enriching milk fat with polyunsaturated fatty acids

Milk fatty acid composition is determined by several factors including diet. The milk fatty acid profile of dairy cows is low in polyunsaturated fatty acids, especially those of the n-3 series. Efforts to change and influence fatty acid profile with longer chain polyunsaturated fatty acids have proven challenging. Several barriers prevent easy transfer of dietary polyunsaturated fatty acids to milk fat including rumen biohydrogenation and fatty acid esterification. The potential for cellular uptake and differences in fatty acid incorporation into milk fat might also have an effect, though this has received less research effort. Given physiological impediments to enriching milk fat with polyunsaturated fatty acids, manipulating the genome of the cow might provide a greater increase than diet alone, but this too may be challenged by the physiology of the cow.

Omega-3 fatty acids improve postprandial lipemia and associated endothelial dysfunction in healthy individuals - a randomized cross-over trial

BACKGROUND

Postprandial elevation of triglycerides impairs endothelial function and contributes to the development of atherosclerosis. We investigated the effects of omega-3 fatty acids on postprandial endothelial function and lipid profiles.

METHODS

Healthy volunteers [10] were given supplementation at 4g/day omega-3 fatty acids (or were not treated) for 4 weeks in a randomised crossover study. Postprandial levels of various lipids were monitored and endothelial function assessed by brachial artery flow-mediated dilation during fasting and after a standard cookie test.

RESULTS

Omega-3 fatty acids reduced postprandial endothelial dysfunction compared with the control diet (flow-mediated dilation at 4h=-0.5±1.2 vs. -2.0±1.6%, P=0.03). Postprandial levels of triglycerides, apolipoprotein B-48, and remnant lipoprotein-cholesterol increased in untreated subjects, peaked at 2-4h, and returned to baseline at 8h, whereas low-density lipoprotein-cholesterol levels did not change. Supplementation with omega-3 fatty acids significantly suppressed postprandial elevation of triglycerides (incremental area under the curve=220±209 vs. 374±216mg/h/dL, P=0.04) and remnant lipoprotein-cholesterol (incremental area under the curve=21.7±13.8 vs. 13.3±12.9mg/h/dL, P=0.04). Supplementation with omega-3 fatty acids significantly suppressed the increase in triglyceride content in chylomicrons as well as in very-low-density lipoproteins from baseline to 4h after the cookie test.

CONCLUSION

Omega-3 fatty acids significantly decreased postprandial triglyceride elevation and postprandial endothelial dysfunction, suggesting that omega-3 fatty acids may have vascular protective effects in postprandial state.

Mammary uptake of fatty acids supplied by intravenous triacylglycerol infusion to lactating dairy cows

Supplementing dairy cows with n-3 fatty acid-rich feeds does not easily increase quantities in milk fat. Previous results demonstrated very long-chain n-3 fatty acids are primarily transported in the PL fraction of blood, making them largely unavailable to the mammary gland for enrichment of milk fat. Our objective was to compare mammary uptake of fatty acids of increasing chain length and unsaturation delivered intravenously as TAG emulsions. Late lactation dairy cows were assigned to a completely randomized block design. Treatments were intravenous TAG emulsions enriched with oleic acid (OLA), linoleic acid (LNA), alpha-linolenic acid (ALA), or docosahexaenoic acid (DHA) and were delivered continuously at 16 mL/h for 72 h. Each treatment supplied 30 g/day of the target fatty acid. Treatment did not affect feed intake, milk yield, or milk composition, but all treatments reduced intake and yield. The proportion of DHA increased in plasma FFA, TAG, and PL with infusion. Increases of n-3 fatty acids, ALA, EPA, and DHA, were evident in the plasma PL fraction, suggesting re-esterification in the liver. Transfer efficiencies were 37.8 ± 4.1, 27.6 ± 5.4, and 10.9 ± 4.1 %, and day 3 total milk fatty acyl yields were 37.0 ± 3.4, 10.8 ± 0.4, and 3.3 ± 0.3 g for LNA, ALA, and DHA. Variation in oleic acyl yield prevented calculation of OLA transfer efficiency. Mammary uptake of fatty acids was reduced with increased chain length and unsaturation. Both liver and mammary mechanisms may regulate transfer of long-chain polyunsaturates.

A diet enriched in docosahexanoic Acid exacerbates brain parenchymal extravasation of apo B lipoproteins induced by chronic ingestion of saturated fats

Chronic ingestion of saturated fatty acids (SFAs) was previously shown to compromise blood-brain barrier integrity, leading to brain parenchymal extravasation of apolipoprotein B (apo B) lipoproteins enriched in amyloid beta. In contrast, diets enriched in mono- or polyunsaturated (PUFA) oils had no detrimental effect. Rather, n3 and n6 oils generally confer protection via suppression of inflammation. This study investigated in wild-type mice if a PUFA diet enriched in docosahexanoic acid (DHA) restored blood-brain barrier integrity and attenuated parenchymal apo B abundance induced by chronic ingestion of SFA. Cerebrovascular leakage of apo B was quantitated utilising immunofluorescent staining. The plasma concentration of brain-derived S100β was measured as a marker of cerebrovascular inflammation. In mice fed SFA for 3 months, provision thereafter of a DHA-enriched diet exacerbated parenchymal apo B retention, concomitant with a significant increase in plasma cholesterol. In contrast, provision of a low-fat diet following chronic SFA feeding had no effect on SFA-induced parenchymal apo B. The findings suggest that in a heightened state of cerebrovascular inflammation, the provision of unsaturated fatty acids may be detrimental, possibly as a consequence of a greater susceptibility for oxidation.

Fish oil -- how does it reduce plasma triglycerides?

Long chain omega-3 fatty acids (FAs) are effective for reducing plasma triglyceride (TG) levels. At the pharmaceutical dose, 3.4g/day, they reduce plasma TG by about 25-50% after one month of treatment, resulting primarily from the decline in hepatic very low density lipoprotein (VLDL-TG) production, and secondarily from the increase in VLDL clearance. Numerous mechanisms have been shown to contribute to the TG overproduction, but a key component is an increase in the availability of FAs in the liver. The liver derives FAs from three sources: diet (delivered via chylomicron remnants), de novo lipogenesis, and circulating non-esterified FAs (NEFAs). Of these, NEFAs contribute the largest fraction to VLDL-TG production in both normotriglyceridemic subjects and hypertriglyceridemic, insulin resistant patients. Thus reducing NEFA delivery to the liver would be a likely locus of action for fish oils (FO). The key regulator of plasma NEFA is intracellular adipocyte lipolysis via hormone sensitive lipase (HSL), which increases as insulin sensitivity worsens. FO counteracts intracellular lipolysis in adipocytes by suppressing adipose tissue inflammation. In addition, FO increases extracellular lipolysis by lipoprotein lipase (LpL) in adipose, heart and skeletal muscle and enhances hepatic and skeletal muscle β-oxidation which contributes to reduced FA delivery to the liver. FO could activate transcription factors which control metabolic pathways in a tissue specific manner regulating nutrient traffic and reducing plasma TG. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

n-3, but not n-6 lipid particle uptake requires cell surface anchoring

Omega-3 (n-3) fatty acids are emerging as bioactive agents protective against cardiovascular disease. However, their cellular delivery pathways are poorly defined. Here we questioned whether the uptake of n-3 triglyceride-rich particles (TGRP) is mediated by cell surface proteoglycans (PG) using LDL receptor (LDLR)+/+ and LDLR-/- cell models. LDLR+/+ but not LDLR-/- cells showed higher n-6 over n-3 TGRP uptake. Removal of cell surface proteins and receptors by pronase markedly enhanced the uptake of n-3 but not n-6 TGRP. Lactoferrin blockage of apoE-mediated pathways decreased the uptake of n-6 TGRP by up to 85% (p<0.05) but had insignificant effect on n-3 TGRP uptake. PG removal by sodium chlorate in LDLR+/+ cells substantially reduced n-3 TGRP uptake but had little effect on n-6 TGRP uptake. Thus, while n-6 TGRP uptake is preferentially mediated by LDLR-dependent pathways, the uptake of n-3 TGRP depends more on PG and non-LDLR cell surface anchoring.

Dose-response of n-3 polyunsaturated fatty acids on lipid profile and tolerability in mildly hypertriglyceridemic subjects

The n-3 polyunsaturated fatty acids (PUFAs) are known to reduce risk for coronary heart disease partly by altering blood lipids. The purpose of this study was to determine the effect of low doses of n-3 PUFAs on the lipid profile and their tolerability in mildly hypertriglyceridemic subjects. Subjects with a serum triacylglycerol (TG) level between 100 and 300 mg/dL were placed on a corn oil placebo (seven capsules per day; single-blind) for 4 weeks and then randomized to continue seven capsules per day of placebo (n = 49) or one of the following doses of n-3 PUFAs for 20 weeks: 0.5 g (n = 51), 1 g (n = 23), or 2 g (n = 12). During the treatment period, 48-50% of the 1-g and 2-g dose groups reported noticing burping (P < .05), and only about half as many in the 0.5-g group noticed burping. Interestingly, 2% of those assigned to the placebo did report a fishy burping during the study, but this dropped back to nearly 0% by week 20. There was no significant effect for any dosage group on fasting and postprandial serum TG, chylomicron TG, very-low-density lipoprotein-cholesterol, or high-density lipoprotein-cholesterol concentration. However, 1-g and 2-g n-3 PUFA treatments significantly increased total cholesterol and low-density lipoprotein-cholesterol concentration. In addition, all doses of n-3 PUFA treatments significantly increased plasma phospholipid n-3 PUFAs. We conclude that doses of n-3 PUFAs of 0.5-2 g/day in healthy volunteers with above average TG levels did not have beneficial effects on their lipid profiles.

Reduction in systemic and VLDL triacylglycerol concentration after a 3-month Mediterranean-style diet in high-cardiovascular-risk subjects

The first results of the PREDIMED (PREvencion con Dieta MEDiterranea) randomized trial, after 3-month intervention, showed that the Mediterranean Diet (MD), supplemented with either virgin olive oil (VOO) or nuts, reduced systolic blood pressure, serum cholesterol and triacylglycerol (TG) concentrations and increased high-density lipoprotein (HDL)-cholesterol when compared to a control (low-fat diet) group. Serum TG levels are an independent risk factor for coronary heart disease and are strongly determined by very low-density lipoprotein (VLDL) composition, which can be specifically modified by dietary lipid source. Within the context of the PREDIMED study, we assessed the VLDL composition in 50 participants after 3 months of intake of two MD, supplemented with VOO or nuts, compared with a low-fat diet. Total and low-density lipoprotein cholesterol concentrations were reduced in subjects on the MD+nuts, whereas HDL-cholesterol increased after consumption of the MD+VOO. Serum TG concentrations were significantly lowered in both intervention groups (either the MD+nuts or MD+VOO). However, only the MD+VOO reduced the VLDL-cholesterol and VLDL-TG content and the TG/apolipoprotein B ratio in VLDL, which was used to estimate particle size. Although VLDL-TG fatty acids were very slightly modified, VLDL-TG molecular species in VLDL after consumption of the MD+nuts were characterized by a higher presence of linoleic acid (18:2, n-6), whereas after the intake of MD+VOO, they were rich in oleic acid (18:1, n-9). Therefore, we conclude that the reduction in systemic TG concentrations observed after consumption of the MD may be explained by reduction of the lipid core of VLDL and a selective modification of the molecular species composition in the particle.

VLDL-TG kinetics: a dual isotope study for quantifying VLDL-TG pool size, production rates, and fractional oxidation in humans

Very-low-density lipoproteins (VLDLs) are large, complex particles containing both surface proteins (e.g., ApoB100) and core lipids, e.g., cholesterol and triglycerides (TG). Whereas ApoB100 kinetics have been thoroughly studied, accurate measurement of VLDL-TG kinetics have proven difficult due to either complex mathematics or laborious procedures. The present study was therefore designed to measure VLDL-TG kinetics by dual isotope ex vivo labeled VLDL-TG tracers and well-established kinetics equations (bolus injection or the primed continuous infusion). Ten healthy Caucasian men [age, 23 +/- 3 yr old (mean +/- SD); body mass index, 24.7 +/- 1.3 kg/m(2)] were included in the study. VLDL-TG rate of appearance (Ra) was measured using a dual-tracer technique ([9,10-(3)H]-labeled VLDL-TG and [1-(14)C]-labeled VLDL-TG) to allow comparison of various bolus decay curve fits with the Ra obtained by the primed continuous infusion (PCI; considered the gold standard). In addition, VLDL-TG fatty acid oxidation was measured as (14)CO(2) in exhaled breath, using the hyamine trapping technique. Following a bolus injection, tracer decay was better described by a biexponential than a monoexponential fit (r(2) = 0.99 +/- 0.01 vs. 0.97 +/- 0.04, respectively, P = 0.01). VLDL-TG Ra calculated using the PCI correlated significantly with the biexponential fit (rho = 0.62, P < 0.05), whereas this was not the case for the monoexponential fit (rho = -0.18, P = not significant). VLDL-TG Ra using the best fit of the bolus injection method (biexponential) was less than values obtained by the constant infusion technique [biexponential, 34.3 (range, 27.1-69.6) vs. PCI, 44.4 (range, 33.0-72.7), P < 0.05]. Fractional oxidation of VLDL-TG was 37.2 +/- 8.8% at 240 min corresponding to 198.8 +/- 55.9 kcal/day or 10.6 +/- 3.3% of resting energy expenditure (REE). Our data demonstrate that VLDL-TG Ra measured by a biexponential fit to a bolus decay curve correlates well with VLDL-TG Ra measured by a primed continuous infusion, and therefore that a "second" peripheral VLDL-TG compartment with rapid exchange of TG exists. VLDL-TG volume of distribution is therefore greater than previously anticipated. Finally our data supports that VLDL-TG contributes quantitatively to REE.

Long-chain n-3 polyunsaturated fatty acids: new insights into mechanisms relating to inflammation and coronary heart disease

Evidence from observational studies, prospective cohort studies and randomized clinical intervention studies indicate that moderate doses of long-chain n-3 polyunsaturated fatty acids (LC n-3 PUFA) significantly decrease risk of fatal coronary heart disease (CHD). Higher doses and longer duration of intervention may also protect from non-fatal CHD events. The exact mechanisms through which LC n-3 PUFA has an effect on CHD are not well established but may include a decrease in fasting and postprandial triacylglycerol levels, a decrease in arrhythmias, modulation of platelet aggregation and decreased synthesis of pro-inflammatory agents. The mechanistic relation between LC n-3 PUFA and inflammation has attracted great interest, and in vitro studies have revealed that these fatty acids decrease endothelial activation, affect eicosanoid metabolism (including epoxygenation pathways) and induce inflammatory resolution. However, the effects of LC n-3 PUFA on established biomarkers of inflammation and endothelial activation in vivo are not strong. Consequently we need new and more sensitive and systemic biomarkers to reveal the effects of LC n-3 PUFA on localized inflammatory processes.

Role of n-3 fatty acids in the treatment of hypertriglyceridemia and cardiovascular disease

n-3 Fatty acids (FAs) when used in doses of 3-4 g/d eicosapentaenoic acid and docosahexaenoic acid have profound effects on triacylglycerol (TG) concentrations. The mechanism for their TG reduction relates to their favorable effects on reducing hepatic production and secretion of VLDL and VLDL apolipoprotein B particles, along with favorable effects on plasma lipolytic activity through lipoprotein lipase-mediated clearance, as well as stimulation of beta-oxidation of other FAs in the liver. Their hypotriglyceridemic properties are related to both the dose of n-3 FAs used and the baseline TG concentrations of the population. In patients with TG concentrations >500 mg/dL, 4 g n-3 FAs have been shown to reduce TGs by 45%, VLDL by 42%, and non-HDL by 10.2%. A recent pooled meta-analysis with multiple doses of n-3 FAs ranging from 0.8 to 5.4 g revealed changes in TGs of -27 mg/dL (95% CI: -33, -20), in HDL of +1.6 mg/dL (95% CI: + 0.8, +2.3), and in LDL cholesterol of +6 mg/dL (95% CI: + 3, +8). The clinical uses of n-3 FAs include treatment of severe and moderate hypertriglyceridemia, use in statin-treated patients with elevated TG concentrations or non-HDL cholesterol (mixed hyperlipidemia), and use in the secondary and primary prevention of cardiovascular disease. Existing large-scale clinical trials such as the GISSI-Prevenzione Study and JELIS with low doses of n-3 FAs (1-2 g) show clinical benefit in reducing coronary heart disease without substantial changes in concentrations of TGs or other lipids. Future clinical trials need to determine whether the TG-lowering doses of n-3 FAs (3-4 g/d) result in additional risk reduction.

Fish oil in the treatment of dyslipidemia

PURPOSE OF REVIEW

The recent availability of a concentrated prescription omega-3 fatty acid preparation provides physicians with an additional anti-dyslipidemic agent at a time when limitations of statin therapy in dyslipidemic high-risk patients are recognized. This review examines the evidence supporting the use of omega-3 fatty acid treatment in dyslipidemic states.

RECENT FINDINGS

There is now considerable evidence that omega-3 fatty acid treatment at the prescription strength of 4 g/day effectively and safely lowers triglyceride levels and increases low-density lipoprotein size, as well as affecting high-density lipoprotein metabolism. Similar effects have been noted in patients treated with statins, and add-on prescription omega-3 fatty acid therapy significantly increases the proportion of statin-treated dyslipidemic patients reaching their non-high-density lipoprotein cholesterol goals. In addition to past studies showing a cardioprotective effect of low-dose omega-3 fatty acid treatment against sudden death, a recent controlled clinical trial showed that 1.8 g of omega-3 fatty acid in statin-treated patients reduced major coronary events by 19% compared with statin plus placebo treatment.

SUMMARY

Omega-3 fatty acid treatment should be considered in patients with severe hypertriglyceridemia as well as in high-risk patients with an atherogenic lipoprotein phenotype.

Omega-3 fatty acid containing diets decrease plasma triglyceride concentrations in mice by reducing endogenous triglyceride synthesis and enhancing the blood clearance of triglyceride-rich particles

BACKGROUND & AIMS

Intake of n-3 fatty acids can reduce both fasting and postprandial triglyceride (TG) concentrations in humans as well as in experimental animals, but the mechanisms by which this occurs are not completely known. We investigated in mice the effects of dietary fish oil (a source of n-3 fatty acids) on endogenous TG synthesis and exogenous TG-rich particle removal.

METHODS

C57 BL/6J mice were fed for 4 months with three types of high-fat diets (18% fat wt/wt) - soy oil, fish oil and a mixture of soy oil and fish oil (soy/fish) (5:1 wt/wt), and a chow diet with 6% fat from soy oil (wt/wt) served as a control. Plasma TG and apolipoprotein B (apoB) concentrations and lipoprotein lipase (LPL) activity were measured. Triton WR 1339 was used to assess hepatic synthesis of very low density lipoprotein, and intravenous injection of chylomicron-like lipid emulsions was conducted to determine the effects of dietary fish oil n-3 fatty acids on exogenous TG clearance.

RESULTS

Both fish and soy/fish oil diets reduced plasma TG levels in fed and fasted states compared to soy oil alone. Plasma pre- and post-heparin LPL activities were significantly higher with fish and soy/fish oil diets than soy oil diet in fed mice. No differences in plasma TG levels and LPL activity were shown among groups of fish oil, soy/fish oil and normal chow diets. Levels of hepatic TG and apoB synthesis were 30-50% and 42% lower in mice fed with the fish oil diet compared to the other three diets. In addition, compared to soy oil diet, fish oil feeding significantly increased blood clearance of chylomicron-like lipid emulsions by 21-26%.

CONCLUSIONS

Our data suggest that reduced endogenous TG synthesis, increased LPL activities and more rapid blood clearance of TG-rich particles all distinctly contribute to the TG-lowering effects of fish oil n-3 fatty acids.

Dietary oleic and palmitic acids modulate the ratio of triacylglycerols to cholesterol in postprandial triacylglycerol-rich lipoproteins in men and cell viability and cycling in human monocytes

The postprandial metabolism of dietary fats produces triacylglycerol (TG)-rich lipoproteins (TRL) that could interact with circulating cells. We investigated whether the ratios of oleic:palmitic acid and monounsaturated fatty acids (MUFA):SFA in the diet affect the ratio of TG:cholesterol (CHOL) in postprandial TRL of healthy men. The ability of postprandial TRL at 3 h (early postprandial period) and 5 h (late postprandial period) to affect cell viability and cycle in the THP-1 human monocytic cell line was also determined. In a randomized, crossover experiment, 14 healthy volunteers (Caucasian men) ate meals enriched (50 g/m(2) body surface area) in refined olive oil, high-palmitic sunflower oil, butter, and a mixture of vegetable and fish oils, which had ratios of oleic:palmitic acid (MUFA:SFA) of 6.83 (5.43), 2.36 (2.42), 0.82 (0.48), and 13.81 (7.08), respectively. The ratio of TG:CHOL in postprandial TRL was inversely correlated (r = -0.89 to -0.99) with the ratio of oleic:palmitic acid and with the MUFA:SFA ratio in the dietary fats (P < 0.05). Postprandial TRL at 3 h preferentially increased the proportion of necrotic cells, whereas postprandial TRL at 5 h increased the proportion of apoptotic cells (P < 0.05). Cell cycle analysis showed that postprandial TRL blocked the human monocytes in S-phase. Our findings suggest that the level of TG and CHOL into postprandial TRL is associated with the ratios of oleic:palmitic acid and MUFA:SFA in dietary fats, which determines the ability of postprandial TRL to induce cytotoxicity and disturb the cell cycle in THP-1 cells.

Triglycerides in fish oil affect the blood clearance of lipid emulsions containing long- and medium-chain triglycerides in mice

Lipid emulsions containing long-chain triglycerides (LCT) and medium chain triglycerides (MCT) are widely used in parenteral nutrition. Recently, fish oil (FO) triglyceride (TG)-derived emulsions are considered therapeutic because of their many beneficial biological modulatory actions. We investigated in mice whether adding 10% FO to an intravenous lipid emulsion with MCT and LCT (MCT:LCT:FO -50:40:10% by wt) would affect particle blood clearance and tissue targeting in comparison to LCT (100% by wt) and MCT:LCT (50:50% by wt) emulsions. The 3 emulsions were labeled with [3H] cholesteryl oleoyl ether and administered by bolus injection (400 microg TG/mouse) to C57BL/6J mice. Contributions of LDL receptor (LDL-R) and LDL-R-related protein to emulsion catabolism were assessed using LDL-R-deficient mice and preinjection of lactoferrin, and the effects of lipoprotein lipase (LPL) were determined by preinjection of heparin and Triton WR 1339. Although fractional catabolic rates did not differ among the 3 emulsions, blood removal at each time point after injection was greater for MCT:LCT:FO particles due to their higher initial margination volume. Compared with MCT:LCT and LCT emulsions, patterns of tissue uptake of the MCT:LCT:FO emulsions were different, e.g. MCT:LCT:FO emulsion particle uptake was lower in heart, adipose tissue, and muscle, and higher in lung, and the removal of MCT:LCT:FO emulsion particles was less dependent on LPL, LDL-R, and lactoferrin-sensitive pathways. These data suggest that the addition of a low percentage of FO to MCT:LCT emulsions substantially changes their particle clearance and tissue uptake mechanisms.

Why do omega-3 fatty acids lower serum triglycerides?

PURPOSE OF REVIEW

Fish oils rich in n-3 fatty acids reduce serum triglyceride levels. This well known effect has been shown to be caused by decreased very low-density lipoprotein triglyceride secretion rates in kinetic studies in humans. Animal studies have explored the biochemical mechanisms underlying this effect. Triglyceride synthesis could be reduced by n-3 fatty acids in three general ways: reduced substrate (i.e. fatty acids) availability, which could be secondary to increase in beta-oxidation, decreased free fatty acids delivery to the liver, decreased hepatic fatty acids synthesis; increased phospholipid synthesis; or decreased activity of triglyceride-synthesizing enzymes (diacylgylcerol acyltranferase or phosphatidic acid phosphohydrolase).

RECENT FINDINGS

Rarely were experimental conditions used in rat studies physiologically relevant to the human situation in which 1.2% energy as n-3 fatty acids lowers serum triglyceride levels. Nevertheless, the most consistent effect of n-3 fatty acids feeding in rats is to decrease lipogenesis. Increased beta-oxidation was frequently, but not consistently, reported with similar numbers of studies reporting increased mitochondrial compared with peroxisomal oxidation. Inhibition of triglyceride-synthesizing enzymes was only occasionally noted.

SUMMARY

As the vast majority of studies fed unphysiologically high doses of n-3 fatty acids, these findings in rats must be considered tentative, and the mechanism by which n-3 fatty acids reduce triglyceride levels in humans remains speculative.

Dietary intake of n-6 fatty acids modulates effect of apolipoprotein A5 gene on plasma fasting triglycerides, remnant lipoprotein concentrations, and lipoprotein particle size: the Framingham Heart Study

BACKGROUND

Apolipoprotein A5 gene (APOA5) variation is associated with plasma triglycerides (TGs). However, little is known about whether dietary fat modulates this association.

METHODS AND RESULTS

We investigated the interaction between APOA5 gene variation and dietary fat in determining plasma fasting TGs, remnant-like particle (RLP) concentrations, and lipoprotein particle size in 1001 men and 1147 women who were Framingham Heart Study participants. Polymorphisms -1131T>C and 56C>G, representing 2 independent haplotypes, were analyzed. Significant gene-diet interactions between the -1131T>C polymorphism and polyunsaturated fatty acid (PUFA) intake were found (P<0.001) in determining fasting TGs, RLP concentrations, and particle size, but these interactions were not found for the 56C>G polymorphism. The -1131C allele was associated with higher fasting TGs and RLP concentrations (P<0.01) in only the subjects consuming a high-PUFA diet (>6% of total energy). No heterogeneity by sex was found. These interactions showed a dose-response effect when PUFA intake was considered as a continuous variable (P<0.01). Similar interactions were found for the sizes of VLDL and LDL particles. Only in carriers of the -1131C allele did the size of these particles increase (VLDL) or decrease (LDL) as PUFA intake increased (P<0.01). We further analyzed the effects of n-6 and n-3 fatty acids and found that the PUFA-APOA5 interactions were specific for dietary n-6 fatty acids.

CONCLUSIONS

Higher n-6 (but not n-3) PUFA intake increased fasting TGs, RLP concentrations, and VLDL size and decreased LDL size in APOA5 -1131C carriers, suggesting that n-6 PUFA-rich diets are related to a more atherogenic lipid profile in these subjects.

The effect of high-dose simvastatin on triglyceride-rich lipoprotein metabolism in patients with type 2 diabetes mellitus

Statins decrease triglycerides (TGs) in addition to decreasing low density lipoprotein-cholesterol. Although the mechanism for the latter effect is well understood, it is still unclear how TG decrease is achieved with statin therapy. Because hypertriglyceridemia is common in obese patients with type 2 diabetes mellitus, we studied triglyceride-rich lipoprotein triglyceride (TRL-TG) turnover in 12 such subjects using stable isotopically labeled glycerol. The diabetic subjects were studied after 12 weeks of placebo and after a similar course of therapy with simvastatin (80 mg daily) in a single-blind design. The results were compared with those from six nonobese nondiabetic control subjects. Simvastatin therapy reduced serum TGs by 35% in the diabetic subjects. Compared with the control subjects, TRL-TG secretion was almost 2-fold higher in the diabetic subjects (45.4 +/- 4.9 vs. 24.4 +/- 1.9 micromol/min; P < 0.002) and was unaffected by simvastatin therapy. However, TRL-TG clearance was significantly increased in the diabetic subjects during simvastatin treatment compared with placebo (0.25 +/- 0.03 vs. 0.16 +/- 0.02 pools/h; P < 0.002). This change was accompanied by a 49% increase in preheparin plasma lipase activity (P < 0.03) and a 21% increase in postheparin LPL activity (P < 0.01). Together, these findings provide strong evidence that the effect of statins on serum TGs is related to an increase in LPL activity, resulting in accelerated delipidation of TRL particles. The effect of high-dose simvastatin on triglyceride-rich lipoprotein metabolism in patients with type 2 diabetes mellitus.