by Ronald L. Hoffman, MD, CNS
Although diets rich in marine lipids and fish-oil supplements have staked a claim to heart disease prevention, controversy remains. A recent study examining the role of omega-3-enriched margarine as a functional food for secondary prevention of heart attacks yielded negative results;1 publication of the study spawned skeptical and even derisive headlines in the popular press: Omega-3 Fats Offer No Protection Against Heart Disease—Study and Low Doses of Omega-3s Don't Help with Heart Disease: Say it Ain't So, Fish Oil!.
Are the results of this study (perhaps somewhat prematurely titled "The Alpha Omega Trial") definitive? How do clinicians reconcile these negative findings with the vast body of references that support the cardiovascular benefits of fish consumption and omega-3 supplementation? And most important, what are the implications for consumers and for potential advocacy by health-care professionals?
The current regulatory climate for fish-oil claims underscores this uncertainty. Lovaza (omega-3-acid ethyl esters), the only FDA-approved fish-oil supplement, is indicated only for its pharmacologic effect of lowering elevated triglycerides. FDA labeling specifically qualifies that "The effect of Lovaza on cardiovascular mortality and morbidity in patients with elevated triglyceride levels has not been determined."2
No comparable approval, either explicit or tacit, exists in the United States for the application of fish-oil supplements to primary or secondary prevention of cardiovascular disease (CVD). But a considerable proportion of the population consumes these supplements, and a high percentage of health-care providers embrace such recommendations—if not for their patients, then for themselves and their families.
While the exact proportions are not known, a recent investigation showed that 62% of U.S. doctors surveyed agreed that one of their roles as a health-care professional is to provide information to patients about appropriate dietary supplements.3 The most popular supplements among cardiologists were multivitamins, omega-3s/fish oil, and vitamin C.
Even regulatory language governing claims for cardioprotection via consumption of omega-3-rich foods remains highly circumscribed. In September 2004, the FDA announced permission for "qualified health claims on omega-3 fatty acids" to the effect that, "Supportive but not conclusive research shows that consumption of eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA] omega-3 fatty acids may reduce the risk of coronary heart disease."4
Compounding the controversy are international discrepancies vis a vis the prescription of fish-oil supplements. A front-page article in The New York Times pointed out that while 57% of family clinicians in Washington State acknowledged fish oil's potential benefits in preventing a second heart attack, only 17% identified themselves as frequent prescribers of fish oil.5
The situation in the United States stood in contrast with that in Europe where, according to Dr. Massimo Santini, chief of cardiology at Rome's San Filippo Neri hospital, a doctor's failure to prescribe fish oil to a heart patient "would be considered tantamount to malpractice."5
The article concluded that in the United States, "community doctors do not learn how to use [fish oil]," while insurers will not pay for cardiovascular prevention via fish-oil supplementation because it is not specifically approved by the FDA for that indication.5
In its scientific statement on fish consumption, fish oil, omega-3 fatty acids, and CVD, the American Heart Association (AHA) acknowledges that: "[Randomized controlled trials] have demonstrated that omega-3 fatty acid supplements can reduce cardiac events (e.g., death, nonfatal MI, nonfatal stroke) and decrease progression of atherosclerosis in coronary patients. However, additional studies are needed to confirm and further define the health benefits of omega-3 fatty acid supplements for both primary and secondary prevention."6
After some equivocation to the effect that dietary approaches to omega-3 supplementation are preferable, the AHA allows that certain individuals, whose dietary preferences preclude adequate intake from natural sources, "in consultation with their physician, could consider supplements for coronary heart disease [CHD] risk reduction."6
What lines of evidence support fish-oil supplementation for cardiovascular prevention? Early impetus came from epidemiologic studies. As early as 1944, Sinclair noted the decreased prevalence of CVD in Arctic Eskimos who subsisted largely on omega-3-rich fish and aquatic mammals.7 In the 1970s, Danish researchers noted improved cardiovascular profiles and lower MI mortality among Greenland Eskimos consuming a low-carbohydrate, fat-rich diet when compared with subjects consuming a Western diet on the Danish mainland.8
Three large randomized trials have documented the effects of omega-3 polyunsaturated fatty acid (PUFA) in primary and especially in secondary prevention of CHD.9 More than twenty years ago, the Diet and Reinfarction Trial demonstrated a 29% reduction in mortality—almost entirely attributable to decreased cardiovascular death—in subjects consuming high amounts of omega-3 from fish sources or supplements.10The reduction in cardiovascular events was particularly impressive in individuals consuming fish-oil capsules.
A subsequent prevention trial corroborated the benefits of fish-oil supplementation for secondary prevention in MI survivors, namely with one Lovaza capsule per day delivering
850 mg of EPA/DHA in a 1.2:1 ratio. Researchers demonstrated a 21% and a 30% reduction, respectively, in total death and cardiovascular death over the one-year duration of the study.11 These results were driven by an impressive 46% reduction in sudden cardiac death. Reductions in major clinical events continued to be demonstrated at 3.5-year follow-up.12
Additional evidence for the protective effects of omega-3 supplementation comes from the Japan EPA Lipid Intervention Study, published in 2007. In a mixed trial of primary and secondary prevention, 18,645 patients with hypercholesterolemia (70% women) were randomized to statin alone or statin and highly purified EPA 1,800 mg/day. At the end of the five-year study, those randomized to statin plus EPA had a 19% reduction in major cardiovascular events.13
How do clinicians reconcile these results with those of the Alpha Omega Trial, which demonstrated no secondary prevention benefits of supplementation with an omega-3-enriched margarine spread?
Researchers leading the Alpha Omega Trial assigned 4,837 MI survivors to one of four experimental groups. For 40 months, participants consumed either (1) placebo margarine, (2) margarine with a combined total of 400 mg of EPA/DHA, (3) margarine with 2 g of alpha-linolenic acid (ALA), a plant-derived precursor to EPA/DHA, or (4) a margarine containing a combination of EPA/DHA and ALA. State-of-the-art antihypertensive, antithrombotic, and lipid-modifying therapy was implemented in all four groups.
The results showed that neither the EPA/DHA nor the ALA (nor a combination of both) proved more beneficial than placebo. Does this constitute a repudiation of the omega-3 CVD prevention hypothesis?
Critics of the Alpha Omega Trial were quick to point out specific objections to its methodology. First, aggressive pharmacologic management may have masked some of the advantages conferred by fish oil. Additionally, it was thought that the results did not disqualify a putative benefit of omega-3-enriched margarines in primary prevention.
Some have claimed that the choice of a margarine-like spread as a delivery system might have compromised the efficacy of the active omega-3 component. The caloric load provided by daily consumption of 18.8 g of omega-6-rich vehicle and the attendant consumption of multiple slices of high glycemic index bread on which the margarine-like substance was spread, might have acted as confounders.
But most important, the low-dose of EPA/DHA (400 mg) employed in the trial is well below the threshold noted in some studies to influence cardiovascular outcome. The study might best be viewed as merely contravening the hoped-for cardio-protective role of a specific functional food—with inherent limitations to palatability attributable to fish oil's inability to being disguised in neutral vehicles. Therefore, it would be an overstatement to claim, as some have, that the Alpha Omega Trial is the death knell for omega-3 supplementation in CVD.
How then are omega-3 PUFAs thought to influence CVD progression and reduce mortality? Several pathways have been investigated.
Hypolipidemic. It is well established that omega-3s reduce lipid levels, and this is the basis for the approved indication for use of Lovaza in hypertriglyceridemia. It is generally acknowledged that doses >3-4 g/day are required. The mechanism of omega-3's triglyceride reduction relates to its favorable effects on reducing hepatic production and secretion of very-low-density lipoprotein (VLDL) and VLDL apo B particles, its favorable effects on plasma lipolytic activity through lipoprotein lipase-mediated clearance, and its ability to stimulate beta-oxidation of other fatty acids in the liver.14
Generally, omega-3s produce no significant improvements in LDL, except in patients with elevated triglycerides, in whom modest improvements in LDL and HDL are sometimes seen. Even when absolute LDL levels are not significantly impacted by fish-oil supplementation (or even when they increase, as has occasionally been reported), there is evidence that omega-3s may shift particle size to a more benign, "fluffier" LDL, thus favorably impacting the atherogenic profile.
Antithrombotic. Fish oils produce platelet inhibition and reduce fibrinogen. Although some experts contend that higher doses (>3-4 g/day) are required, others argue for a lower threshold. Epidemiologic evidence of fish oil's anti-clotting effects comes from several studies that link higher intake of fish oil to a reduction in ischemic strokes with coincident increased risk of hemorrhagic strokes. This leads to a net gain in protection, since ischemic strokes outnumber hemorrhagic strokes by 85% to 15%.
Antihypertensive. Fish oil is thought to exert an antihypertensive effect in several ways. It supports flow-mediated vasodilation, enhances vascular reactivity, and may balance autonomic tone by reducing adrenergic drive.
An analysis of randomized trials revealed that consumption of approximately 4.0 g/day of omega-3 fatty acid was associated with a significant 1.7- and 1.5-mm Hg reduction in systolic and diastolic BP, respectively; these reductions were more pronounced in older patients and in individuals with higher BP. Evidence suggests that lowering systolic BP by as little as 2 mm Hg can yield reductions of 4% in CAD mortality.15
Adiponectin. A hormone produced by fat cells, adiponectin plays a role in regulating lipids and glucose. Low levels of this hormone are associated with obesity, and higher levels have been shown to confer protection against heart disease. When administered to obese individuals, 1.8 g/day of EPA increased the level of adiponectin.16
Insulin regulator. To date, research in the area of insulin regulation has been inconclusive. While numerous studies support a role for EPA/DHA in attenuating insulin resistance, and omega-3 deficiency has been associated with the metabolic syndrome, the Agency for Healthcare Research and Quality reports, "Among 18 studies of type 2 diabetes or the metabolic syndrome, omega-3 fatty acids . . . had no effect on fasting blood sugar, or glycosylated hemoglobin, by meta-analysis. Omega-3 fatty acids had no effect on plasma insulin or insulin resistance in type 2 diabetics or patients with the metabolic syndrome, by qualitative analysis of four studies."17
Antiarrhythmic. The principle cause of sudden cardiac death (SCD) is sustained ventricular arrhythmia. Considerable evidence supports an antiarrhythmic role for omega 3s, probably via autonomic pathways. Studies show that EPA/DHA enhances sympatho-vagal tone, leading to slower heart rates and fewer arrhythmias, and in some studies, reduced incidence of SCD. Omega 3s have also been shown to favorably impact heart rate variability, a marker of autonomic adaptability.
Knowledge of fish oil's beneficial effects on arrhythmias recently prompted a trial of omega-3 supplementation in patients with implantable cardioverter defibrillators (ICDs). The results—which showed a higher incidence of ICD discharges in consumers of fish-oil supplements—and the consequent negative publicity have dampened enthusiasm over fish oil for this indication.
Some studies have shown impressive results for fish oil in prophylaxis of atrial fibrillation, particularly in patients at risk after coronary artery bypass grafting.18
Anti-inflammatory. Omega-3 fatty acids act as eicosanoid precursors to reduce inflammation. This may impact cardiovascular risk in several ways.
First, chronic inflammation is thought to play a critical role in endothelial damage that leads to compromised vascular reactivity and atherosclerotic changes. Additionally, inflammation promotes unstable plaque, a harbinger of thrombotic events.
Next, it is now commonly acknowledged that elevated high-sensitivity C-reactive protein (hs-CRP), a selective marker of intra-arterial inflammation, is a robust risk factor for CVD. Research on the potential for fish oil to lower this cardiospecific parameter has produced mixed results. One study found that atorvastatin (Lipitor), but not fish oil, reduced hs-CRP in obese, at-risk subjects.19 In other research, omega-3 supplements did not reduce hs-CRP in healthy subjects.20 Another set of findings showed that EPA/DHA reduced hs-CRP in patients on dialysis, a population known to be at higher risk for inflammation and CVD.21
Finally, fish oil has a less equivocal effect on other inflammatory mediators, specifically such cytokines as tumor necrosis factor-alpha, elevations of which are hallmarks of chronic congestive heart failure (CHF). Studies show improvement in cytokine levels in CHF patients taking EPA/DHA, with corresponding improvements in clinical outcomes.9
Prevention of restenosis. Accelerated restenosis following angioplasty or stenting is one of the thorniest problems in medicine, associated with numerous failed strategies. Because of fish oil's acknowledged antiatherosclerotic effects, it became a likely candidate for restenosis prevention. Early meta-analyses showed modest benefit and provided reason to be hopeful.22 Unfortunately, the Coronary Angioplasty Restenosis Trial demonstrated no reduction in restenosis rates with 5 g/day of Omacor, a precursor to Lovaza.23
Other benefits. Ancillary benefits of fish-oil supplementation are thought to be considerable. Therapeutic effects have been investigated, with various degrees of evidence, in such diverse conditions as bipolar disorder, depression, dementia, psychosis, certain cancers (including breast, colon, and prostate) cancer cachexia, rheumatoid arthritis, cystic fibrosis, inflammatory bowel disease, dysmenorrhea, IgA nephropathy, systemic lupus, asthma, chronic obstructive pulmonary disease, psoriasis, eczema, dry eye, macular degeneration, pre-eclampsia, and prevention of organ-transplant rejection.24
With regard to fish oil's role in secondary prevention of CVD, studies suggest potential amelioration of depression in heart attack survivors who take omega-3s. Depression is a known comorbidity following MI. It is also a negative prognostic, so, if proven, fish oil's antidepressant effects would be a valuable ancillary benefit.
A recent trial compared fish oil plus sertraline with sertraline alone in a group of post-MI patients. Mood improved in all the patients, but no difference was seen in depression scores between fish-oil-supplemented patients vs. controls.
"Whether higher doses of EPA, DHA, or sertraline, a longer duration of treatment, or the use of omega-3 as monotherapy can improve depression in patients with stable heart disease remains to be determined," the authors concluded.25
ADVERSE EFFECTS AND CONTRAINDICATIONS
The most commonly observed adverse effects of omega-3 PUFA supplementation are nausea, GI upset, and "fishy" burp. These problems can sometimes be ameliorated through the use of flavored, emulsified omega-3 formulations that are palatable even to children. Alternatively, enteric-coated forms of fish oil are designed to dissolve distal to the stomach, reducing the potential for oily reflux.
Because of fish oil's antiplatelet effects, concerns have been raised over the possibility that higher intakes will lead to an increase hemorrhagic complications. However, a comprehensive review concluded that no increased risk of clinically significant bleeding was noted with omega-3 PUFA doses of up to 7 g of combined DHA and EPA per day, even when coupled with antiplatelet therapy or warfarin.26 Results may vary in clinical practice, and such clotting disorders as Von Willebrand disease or thrombocytopenia may pose relative contraindications to high-dose fish-oil supplementation.
A recent study explored fish-oil supplements' synergy with aspirin and clopidogrel in post-stenosis patients. Resistance to platelet inhibition was overcome with no concomitant increase in adverse hemorrhagic events.27
To prevent excessive perioperative bleeding, patients undergoing elective surgery are typically advised to forgo fish-oil supplementation for several weeks, but some studies actually support a role for fish oil in improving surgical outcomes. Some researchers have credited fish oil with offering circulatory benefits, or alternatively with modulating immunity, perhaps by blunting the exaggerated cytokine response to surgery.28
Lately, concerns have been raised over fish oil's potential to down-regulate immunity in detrimental ways. One study observed that fish oil decreased resistance to influenza virus in a murine model,29 but the observation has not been extended to humans.
FISH-OIL SUPPLEMENTS ON THE MARKET
Fish-oil supplements come in a variety of forms. Cod-liver oil is the original article, but its disadvantages relative to omega-3 capsules include low ratio of EPA/DHA to overall calories, greater risk of mercury and PCB contamination, and potential for vitamin A and D toxicity with high levels of supplementation.
Most fish-oil capsules are free of significant mercury and PCB contamination, and industry standards for monitoring and disclosure are relatively stringent. Disparities exist, however, in the amount of active EPA and DHA delivered per capsule, and some ultra-discounted brands may be of dubious benefit due to the sheer number of capsules required to achieve clinically meaningful supplementation.
Manufacturers sometimes use the term "pharmaceutical grade" to imply greater potency or purity, but the claim is not regulated and almost meaningless. Also, controversy persists over relative advantages of two competing technologies for enhancing the EPA/DHA concentration: triglyceride substitution vs. ethyl esterification (used in Lovaza). Each camp claims superior bioavailability, but evidence remains inconclusive.
Finally, new formulation technology permits the offering of EPA/DHA in variable ratios designed to target specific clinical goals. EPA and DHA may have differing effects on desirable cardioprotective endpoints. The precise "optimal" ratio of EPA/DHA for heart disease prevention, if such a thing exists, has not yet been determined.
Uncertainty remains among health-care professionals as to the potential therapeutic application of fish-oil supplements in heart disease. A wide range of mechanistic, epidemiologic, and controlled-trial evidence substantiates a role for omega-3 fatty-acid supplementation in both primary and secondary cardiovascular prevention. An extensive margin of safety has been confirmed in most settings. More work remains to be done in studying the effects of fish oil in specific subpopulations and circumstances as well as in clarifying optimal dosage regimens and EPA/DHA ratios. Only then can specific guidelines for administration of omega-3 supplements coalesce into consistent and clear recommendations from health-care professionals.
Dr. Hoffman is founder and medical director of the Hoffman Center in New York City. The author has no relationships to disclose relating to the content of this article.
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All electronic documents accessed November 15, 2010.