Omega-3 Fatty Acids, Fish Oil, Alpha-Linolenic Acid

by MedlinePlus

Dietary sources of omega-3 fatty acids include fish oil and certain plant/nut oils. Fish oil contains both docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), while some nuts (English walnuts) and vegetable oils (canola, soybean, flaxseed/linseed, olive) contain alpha-linolenic acid (ALA).

There is evidence from multiple large-scale population (epidemiologic) studies and randomized controlled trials that intake of recommended amounts of DHA and EPA in the form of dietary fish or fish oil supplements lowers triglycerides, reduces the risk of death, heart attack, dangerous abnormal heart rhythms, and strokes in people with known cardiovascular disease, slows the buildup of atherosclerotic plaques ("hardening of the arteries"), and lowers blood pressure slightly. However, high doses may have harmful effects, such as an increased risk of bleeding. Although similar benefits are proposed for alpha-linolenic acid, scientific evidence is less compelling, and beneficial effects may be less pronounced.

Some species of fish carry a higher risk of environmental contamination, such as with methylmercury.

Should not be confused with omega-6 fatty acids.

Evidence

These uses have been tested in humans or animals. Safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider.

Uses based on scientific evidence

Hypertriglyceridemia (fish oil / EPA plus DHA)

There is strong scientific evidence from human trials that omega-3 fatty acids from fish or fish oil supplements (EPA + DHA) significantly reduce blood triglyceride levels. Benefits appear to be dose-dependent, with effects at doses as low as 2 grams of omega-3 fatty acids per day. Higher doses have greater effects, and 4 grams per day can lower triglyceride levels by 25-40%. Effects appear to be additive with HMG-CoA reductase inhibitor ("statin") drugs such as simvastatin, pravastatin, and atorvastatin. The effects of fish oil on hypertriglyceridemia are similar in patients with or without diabetes, and in those with kidney disease receiving dialysis. It is not clear how fish oil therapy compares to other agents used for hypertriglyceridemia, such as fibrates (like gemfibrozil or fenofibrate) or niacin/nicotinic acid.Fish oil supplements also appear to cause small improvements in high-density lipoprotein ("good cholesterol") by 1-3%. However, increases (worsening) in low-density lipoprotein levels (LDL/"bad cholesterol") by 5-10% are also observed. Therefore, for individuals with high blood levels of total cholesterol or low-density lipoprotein, significant improvements will likely not be seen, and a different treatment should be selected.It is not clear if alpha-linolenic acid significantly affects triglyceride levels, and there is conflicting evidence in this area.The American Heart Association, in its 2003 recommendations, reports that supplementation with 2-4 grams of EPA + DHA each day can lower triglycerides by 20-40%. Because of the risk of bleeding from omega-3 fatty acids (particularly at doses greater than 3 grams per day), a physician should be consulted prior to starting treatment with supplements.C-Reactive Protein (CRP) levels: The data on fish oils and CRP is mixed. While omega-3 fatty acids from both plants (ALA) and fish (EPA+DHA) have been shown to reduce CRP in some studies, others have failed to show an effect. There is growing evidence that reducing CRP is beneficial towards favorable cardiovascular outcomes, although additional research is pending in this area. Although statin drugs, weight reduction, smoking cessation, and COX-2 inhibitors all appear to reduce CRP, the evidence regarding fish oil remains equivocal.

Secondary cardiovascular disease prevention (fish oil / EPA plus DHA)

Several well-conducted randomized controlled trials report that in people with a history of heart attack, regular consumption of oily fish (200-400 grams of fish each week equal to 500-800mg of daily omega-3 fatty acids) or fish oil/omega-3 supplements (containing 850-1800mg of EPA + DHA) reduces the risk of non-fatal heart attack, fatal heart attack, sudden death, and all-cause mortality (death due to any cause). Most patients in these studies were also using conventional heart drugs, suggesting that the benefits of fish oils may add to the effects of other therapies. Benefits have been reported after 3 months of use, and after up to 3.5 years of follow-up. Benefits of supplements may not occur in populations that already consume large amounts of dietary fish.Multiple mechanisms have been proposed for the beneficial effects of omega-3 fatty acids. These include reduced triglyceride levels, reduced inflammation, slightly lowered blood pressure, reduced blood clotting, reduced tendency of the heart to develop abnormal rhythms, and diminished buildup of atherosclerotic plaques in arteries of the heart. Experiments suggest that omega-3 fatty acids may reduce platelet derived growth factor (PDGF), decrease platelet aggregation, inhibit the expression of vascular adhesion molecules, and stimulate relaxation of endothelial cells in the walls of blood vessels. The American Heart Association, in its 2003 recommendations, suggests that people with known coronary heart disease take in approximately 1 gram of EPA and DHA (combined) each day. This may be obtained from eating fish, or from fish oil capsule supplements. Because of the risk of bleeding from omega-3 fatty acids (particularly at doses greater than 3 grams per day), a physician should be consulted prior to starting treatment with supplements.

High blood pressure

Multiple human trials report small reductions in blood pressure with intake of omega-3 fatty acids. Reductions of 2-5 mmHg have been observed, and benefits may be greater in those with higher blood pressures. Effects appear to be dose-responsive (higher doses have greater effects). DHA may have greater benefits than EPA (29). However, intakes of greater than 3 grams of omega-3 fatty acids per day may be necessary to obtain clinically relevant effects, and at this dose level, there is an increased risk of bleeding. Therefore, a physician should be consulted prior to starting treatment with supplements. Other approaches are known to have greater effects on blood pressure, such as salt reduction, weight loss, exercise, or antihypertensive drug therapy. Therefore, although omega-3 fatty acids do appear to have effects in this area, their role in the management of high blood pressure is limited.

Primary cardiovascular disease prevention (fish intake)

Several large studies of populations ("epidemiologic" studies) report a significantly lower rate of death from heart disease in men and women who regularly eat fish (30-39). Other epidemiologic research reports no such benefits. It is not clear if reported benefits only occur in certain groups of people, such as those at risk of developing heart disease. Overall, the evidence suggests benefits of regular consumption of fish oil. However, well-designed randomized controlled trials which classify people by their risk of developing heart disease are necessary before a firm conclusion can be drawn.The American Heart Association, in its 2003 recommendations, suggests that all adults eat fish at least two times per week. In particular, fatty fish are recommended, including mackerel, lake trout, herring, sardines, albacore tuna, and salmon.

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