Omega-3 Choices: Fish or Flax?
By Alison J. Rigby, PhD, MPH, RD
Vol. 6 No. 1 p. 37

Fish consumption has been advocated on the basis of its lean high biological value protein, vitamin and mineral content, and “good” fat value from the omega-3 fatty acids found in fish. The health benefits of eating fish were essentially discovered by epidemiological studies of the Northern Inuit population, who were shown to have reduced rates of myocardial infarction as a result of their consumption of marine omega-3 fatty acids, compared with Western control subjects.1 European countries have been supplementing their baby formulas for some time with omega-3 fatty acids for brain health. A recent American Heart Association (AHA) Scientific Statement on fish consumption, fish oil, omega-3 fatty acids, and cardiovascular disease amplified the benefits of eating fish and recommended the AHA guidelines of at least two servings of fish per week and the use of omega-3 supplementation for patients with coronary heart disease (1 gram per day) and larger doses (2 grams per day to 4 grams per day) for those patients with hypertriglyceridemia.2

Review of the Omega-3 Fatty Acids
The omega-3 fatty acids are polyunsaturated fatty acids (missing many hydrogen atoms), with the last double bond located three carbons away from the methyl end. Eicosopentaenoic acid (EPA), or 20:5n-3, and Docosahexaenoic acid (DPA), or 22:6n-3, are the omega-3 fatty acids found in oily fish, with mackerel, salmon, trout, sardines, and herring being excellent sources. Approximately 1 gram of EPA/DPA can be obtained from 100 grams (3.5 ounces) of oily fish. Although this quantity of EPA/DPA can vary depending upon the degree of oiliness of the fish (Atlantic mackerel = 2.5 grams omega-3 per 100 grams; salmon = 1.2 grams omega-3 per 100 grams; tuna = 0.5 grams omega-3 per 100 grams; and red snapper = 0.2 grams omega-3 per 100 grams).

The average intake of total omega-3 fatty acids in the United States is approximately 1.6 grams per day (0.7% of energy intake), with actually only 0.1 grams per day to 0.2 grams per day coming from EPA/DHA (the rest from alpha-linolenic acid [ALA]).2 ALA (18:3n-3) from plant sources can desaturate and elongate in the human body to form EPA and DHA. Sources of ALA include oils from flaxseed, canola (rapeseed), soybean, walnut, and wheat germ, with flaxseed (linseed) being the most abundant source.

Health Benefits of Omega-3 Fatty Acids
A selection of epidemiological studies with important clinical trials have outlined the benefits of the omega-3 fatty acids.

The Diet and Reinfaction Trial supported the role of fish or fish oil in decreasing total mortality and sudden death in patients with myocardial infarction.3 The Lyon Diet Heart Study4 added canola oil as a source of ALA to the diet; the Singh study5 added fish or mustard oil; and the GISSI-Prevenzione trial6 added 850 grams to 882 grams of omega-3 fatty acids to a Mediterranean diet for a large 11,324-participant study, resulting in a decrease in mortality for these groups.

In the Cardiovascular Health Study,7 a population-based prospective cohort study among 3,910 adults, the consumption of tuna and other broiled or baked fish was associated with a lower risk of ischemic heart disease (IHD) death, especially arrthythmic IHD death. In the Nurses’ Health Study,8 the relative risk of total stroke was lower among women who regularly ate fish than among those who did not. A significant decrease in the risk of thrombotic stroke was observed in women who ate fish at least twice per week, compared with women who ate fish less than once per month, after adjustment for age, smoking, and other cardiovascular risk factors.

The omega-3 fatty acids have been associated with having anti-inflammatory, antithrombotic, antiarrhythmic, hypolipidemic, and vasodilatory properties.9 Some of the health benefits that have been associated with omega-3 fatty acids include the secondary prevention of chronic diseases and an association with the following:

• Inflammatory conditions: Improves rheumatoid arthritis, psoriasis, asthma, and some skin conditions
• Ulcerative colitis and Crohn’s disease: Reduces the severity of symptoms
•Cardiovascular disease: Lowers triglycerides and raises high-density lipoprotein cholesterol levels, improves blood circulation, reduces clotting, improves vascular function, and lowers blood pressure
• Type 2 diabetes mellitus: Reduces hyperinsulinemia and insulin resistance
• Renal disease: Preserves renal function in IgA nephropathy; potentially reduces vascular access thrombosis in hemodialysis patients and is cardioprotective
• Mental function: Reduces severity of several mental conditions such as Alzheimer’s disease, depression, and bipolar disorder; improvement in children with attention deficit hyperactivity disorder and dyslexia also noted
• Growth and development: Neurodevelopment and function of the brain and also the retina of the eye where visual function is affected

Fitting Omega-3 Fatty Acids Into the Diet
Western diets are characterized by low intakes of EPA and DHA relative to linoleic acid (LA; 18:2n-6) and arachidonic acid (AA; 20:4n-6). The high intake of trans fatty acids in our diets can interfere with the desaturation and elongation of LA and ALA. A high intake of LA leads to decreased production of AA and interferes with the desaturation and elongation of ALA to EPA and DHA. The high intake of LA also promotes a prothrombotic and proaggregatory state, characterized by increased blood viscosity and vasoconstriction, and potentially decreased bleeding time.

Therefore, the required intake of long-chain polyunsaturated omega-3 fatty acids needed for optimal effects depends on the intake of other fatty acids. The Western diet ratio of omega-6 to omega-3 fatty acids ranges from 20:1 to 30:19 and is probably even higher with the increased intake of vegetable oils, historically recommended as a substitute for saturated fat. The competing omega-6 vegetable oils include corn, safflower, cottonseed, sesame, and sunflower seed oils.

Fish vs. Flax
The optimal intake of LA compared with ALA appears critical for the metabolism of omega-3 fatty acids. An increase in AA, EPA, and DHA leads to an increase in membrane fluidity, alters the structure of the membrane receptors, and can have other beneficial effects associated with the omega-3 fatty acids. They also play a role in the regulation of cell surface expression, cell-cell interactions, and cytokine release.10 A ratio of 1:4 (LA:ALA) or less is recommended for conversion of ALA to longer chain metabolites (EPA and DHA).9 This is an important concept for vegetarians, whose diets are often much richer in LA. The intake of 3 grams per day to 4 grams per day of ALA is equivalent to 0.3 grams per day of EPA with optimal elongation.

The increased consumption of flaxseed, canola, soybean, walnut, and wheat germ oils should be supported. However, ALA does not appear to be comparable with its biological effects, compared with EPA and DHA found in fish oil. It appears that the EPA and DHA from marine oils are more rapidly incorporated into plasma and membrane lipids. Algae and some fungi are also capable of forming omega-3 fatty acids de novo, and the DHA from algae supplements needs to be explored further for the vegetarian.

The Methyl Mercury scare
A recent local survey in the Bay Area of California found several varieties of fish to contain toxic levels of mercury: swordfish (containing the highest concentration), Chilean sea bass, and ahi tuna. Mercury is the environmental pollutant largely from coal-fired power plants that is at the highest concentration in the large predator fish.

The FDA has an advisory warning that swordfish, shark, king mackerel, and tilefish consumption should be limited by pregnant women and women of childbearing age, and this warning is apparent in many fish markets. Mercury can damage the nervous, cardiovascular, immune, and reproductive systems, and symptoms include tremors, memory loss, and fatigue. Subtle symptoms of methyl mercury toxicity in adults have included numbness or tingling of the hands and feet or around the mouth.

According to the FDA, consumption of fish with methyl mercury levels of one part per million, such as shark and swordfish, should be limited to approximately 7 ounces per week. The FDA states that consumption advice is unnecessary for the top 10 seafood species, which makes up approximately 80% of the seafood market: canned tuna, shrimp, pollock, salmon, cod, catfish, clams, flatfish, crabs, and scallops. The methyl mercury levels in these species are at less than 0.2 parts per million, and not many people eat more than the suggested weekly limit of 2.2 pounds of fish. Canned tuna, which is composed of smaller pieces of tuna, such as skipjack and albacore, typically have lower levels of methyl mercury compared with large fresh tuna, sold as steaks or sushi.

A high dietary intake of mercury from the consumption of fish has been hypothesized to increase the risk of coronary heart disease. In a study that investigated the association between mercury levels in toenails and risk of coronary heart disease among male health professionals with no previous history of cardiovascular disease (40 to 75 years of age), there was no association between total mercury exposure and risk of coronary heart disease.11 Adjustment with the intake of omega-3 fatty acids did not substantially change the results from this study.

Recommending Omega-3 Supplements
A low rate of coronary heart disease has certainly been shown in fish-eating populations. Studies have highlighted reduced cardiovascular risk with a higher intake of ALA, and the omega-3 fatty acids have also consistently been shown to decrease serum triacylglycerol concentrations in studies in humans. A meta-analysis12 suggested that dietary and nondietary intake of omega-3 fatty acids reduces overall mortality, mortality due to myocardial infarction, and sudden death in patients with coronary heart disease.

Based on the AHA Scientific Statement, it seems reasonable to recommend at least two servings of fish per week in the diet and the use of omega-3 supplements for patients with coronary heart disease up to 1 gram per day and larger doses (2 grams per day to 4 grams per day) for those patients with hypertriglyceridemia.2 The exact ratio of EPA:DHA needs to be explored further in clinical trials.

As a cautionary safety note, a dose of 1.8 grams per day of EPA has not been documented as having any prolongation of bleeding time. The use of 4 grams per day has shown increased bleeding time and decreased platelet count, but no overall adverse effects.13

A fish oil supplement that is “clean” and has been processed by molecular distillation is important. A good place to start when deciding which brand of fish oil supplement to select is Consumer Reports magazine (www.consumerreports.org), which tests the top-selling brands of fish oil capsules. The 16 top-selling brands of fish oil capsules they tested found that the products “all contained roughly as much omega-3s as their labels claimed,” and none were contaminated with pollutants.

— Alison J. Rigby, PhD, MPH, RD, is a researcher at Stanford University, where she is currently investigating the use of fish oils in the diet. She also teaches nutrition/dietetics classes at San Francisco State University.

References
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11. Yoshizawa K, Rimm EB, Morris JS, et al. Mercury and the risk of coronary hear disease in men. N Eng J Med. 2002;347(22):1755-1760.
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