Abnormal rapid heart rhythms (ventricular arrhythmias). Population research suggests that eating a lot of fish has no effect on the risk for abnormal rapid heart rhythms. Clinical research is inconsistent. Some research shows that taking fish oil daily does not affect the risk for abnormal heart rhythms. But other research shows that taking fish oil for 11 months delays the development of the condition. However, overall, taking fish oil does not seem to reduce the risk of death in people with abnormal rapid heart rhythms.
Augood, C., Chakravarthy, U., Young, I., Vioque, J., de Jong, P. T., Bentham, G., Rahu, M., Seland, J., Soubrane, G., Tomazzoli, L., Topouzis, F., Vingerling, J. R., and Fletcher, A. E. Oily fish consumption, dietary docosahexaenoic acid and eicosapentaenoic acid intakes, and associations with neovascular age-related macular degeneration. Am J Clin Nutr 2008;88(2):398-406. View abstract.
For those who can’t or choose not to eat fatty fish, or who have certain health issues, supplementation is a way to increase omega-3 levels. “There are some conditions that might respond well to supplementation, such as depression or cardiovascular risk factors, including elevated triglycerides,” explains Kathie Madonna Swift, MS, RDN, LDN.  If you're ooking to increase your omega-3 levels, Click here for six tips to finding the right supplement.
Smithers, L. G., Collins, C. T., Simmonds, L. A., Gibson, R. A., McPhee, A., and Makrides, M. Feeding preterm infants milk with a higher dose of docosahexaenoic acid than that used in current practice does not influence language or behavior in early childhood: a follow-up study of a randomized controlled trial. Am J Clin Nutr 2010;91(3):628-634. View abstract.
The US National Institutes of Health lists three conditions for which fish oil and other omega-3 sources are most highly recommended: hypertriglyceridemia (high triglyceride level), preventing secondary cardiovascular disease, and hypertension (high blood pressure). It then lists 27 other conditions for which there is less evidence. It also lists possible safety concerns: "Intake of 3 grams per day or greater of omega-3 fatty acids may increase the risk of bleeding, although there is little evidence of significant bleeding risk at lower doses. Very large intakes of fish oil/omega-3 fatty acids may increase the risk of hemorrhagic (bleeding) stroke."[12]
Increased consumption of omega 3 fats is widely promoted globally because of a common belief that that it will protect against heart disease. There is more than one possible mechanism for how they might help prevent heart disease, including reducing blood pressure or reducing cholesterol. Omega 3 fats are readily available as over-the-counter supplements and they are widely bought and used.
Fish oil contamination even among “molecularly distilled” brands and those aimed at children is a widespread problem. One study in California tested 10 common brands and found PCBs — toxic industrial pollutants that have contaminated our oceans — in all of them. Some had 70 times the PCBs of other ones and 240x the toxicity. In another study, researchers tested 13 over-the-counter children’s dietary supplements containing fish oil for PCBs. PCBs were detected in all products. Our family takes algae-derived omega-3 (DHA/EPA) capsules, which are bioequivalent to fish oil capsules. Algae are actually the source where fish get their omega-3 content, so we skip the contaminated middle man (or, fish, in this case) and the neurotoxins that come with them given how polluted our oceans are now. I highly recommend parents do their research on what studies show about fish oil contamination and not just trust the labels, as well as consider algae-derived omega-3 capsules as more healthful bioequivalent to fish oil.
They also found that taking more long-chain omega 3 fats (including EPA and DHA), primarily through supplements probably makes little or no difference to risk of cardiovascular events, coronary heart deaths, coronary heart disease events, stroke or heart irregularities. Long-chain omega 3 fats probably did reduce some blood fats, triglycerides and HDL cholesterol. Reducing triglycerides is likely to be protective of heart diseases, but reducing HDL has the opposite effect. The researchers collected information on harms from the studies, but information on bleeding and blood clots was very limited. 
Thusgaard, M., Christensen, J. H., Morn, B., Andersen, T. S., Vige, R., Arildsen, H., Schmidt, E. B., and Nielsen, H. Effect of fish oil (n-3 polyunsaturated fatty acids) on plasma lipids, lipoproteins and inflammatory markers in HIV-infected patients treated with antiretroviral therapy: a randomized, double-blind, placebo-controlled study. Scand.J.Infect.Dis. 2009;41(10):760-766. View abstract.
Those foods provide enormous amounts of other nutrients that are good for you. nSo it is way better to eat those foods than to take fish oil. With that said, some people find it very difficult to get vitamin A or vitamin D, and particularly for vitamin A, cod liver oil may be a very important source of that vitamin. Cod liver oil is a form of fish oil that happens to be high in the fat-soluble vitamins. Vitamin A is best found in liver. It’s better in my opinion to eat liver once a week, but there are a lot of people out there who are not going to eat liver once a week. So if you are using cod liver oil to get the vitamins that you can’t get from food—and I should point out that vitamin A can also be derived from plant foods, but many people genetically or for other reasons don’t derive it very well from plant foods.
Attention deficit-hyperactivity disorder (ADHD) in children. Early research shows that taking fish oil improves attention, mental function, and behavior in children 8-13 years-old with ADHD. Other research shows that taking a specific supplement containing fish oil and evening primrose oil (Eye Q, Novasel) improves mental function and behavior in children 7-12 years-old with ADHD.
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