Weak bones (osteoporosis). Research suggests that taking fish oil alone or together with calcium and evening primrose oil slows the rate of bone loss and increases bone density at the thigh bone (femur) and spine in elderly people with osteoporosis. But taking fish oil does not slow bone loss in older people with osteoarthritis in the knee but without weak bones.
Irving, G. F., Freund-Levi, Y., Eriksdotter-Jonhagen, M., Basun, H., Brismar, K., Hjorth, E., Palmblad, J., Vessby, B., Vedin, I., Wahlund, L. O., and Cederholm, T. Omega-3 fatty acid supplementation effects on weight and appetite in patients with Alzheimer's disease: the omega-3 Alzheimer's disease study. J Am Geriatr Soc 2009;57(1):11-17. View abstract.
Cast about for healthy canned tuna. Think all tuna is created equal? Think again. Choose canned light tuna instead of tuna steaks or albacore tuna. It tends to have less mercury. Albacore may contain three times the mercury of chunk light tuna. Check fish guides for the latest information about foods low in toxins but high in omega-3. Two good online sources are:

“This idea has since been pretty discredited; we really don’t know if the Eskimos got heart disease or not,” said Malden C. Nesheim, emeritus professor of nutrition at Cornell University, who chaired an Institute of Medicine committee assessing the risks and benefits of seafood in the early 2000s. “I’ve been an omega-3 skeptic since doing this study.”
van der Meij, B. S., Langius, J. A., Smit, E. F., Spreeuwenberg, M. D., von Blomberg, B. M., Heijboer, A. C., Paul, M. A., and van Leeuwen, P. A. Oral nutritional supplements containing (n-3) polyunsaturated fatty acids affect the nutritional status of patients with stage III non-small cell lung cancer during multimodality treatment. J.Nutr. 2010;140(10):1774-1780. View abstract.

Warfarin (Coumadin) is used to slow blood clotting. Fish oil also might slow blood clotting. Taking fish oil with warfarin might slow blood clotting too much and increase the risk of bleeding. However, conflicting results suggests that fish oil does not increase the effects of warfarin. Until more is known, use cautiously in combination with warfarin. Have your blood checked regularly, as your dose of warfarin (Coumadin) might need to be changed.
Brand Names: Animi-3, Cardio Omega Benefits, Divista, Dry Eye Omega Benefits, EPA Fish Oil, Fish Oil, Fish Oil Ultra, Flex Omega Benefits, Icar Prenatal Essential Omega-3, Lovaza, Marine Lipid Concentrate, MaxEPA, MaxiTears Dry Eye Formula, MaxiVision Omega-3 Formula, Mi-Omega NF, Mom's Omega Advantage, Omega Essentials, Sea-Omega, Sea-Omega 30, TheraTears Nutrition, TherOmega, Vascazen
Moertl, D., Hammer, A., Steiner, S., Hutuleac, R., Vonbank, K., and Berger, R. Dose-dependent effects of omega-3-polyunsaturated fatty acids on systolic left ventricular function, endothelial function, and markers of inflammation in chronic heart failure of nonischemic origin: a double-blind, placebo-controlled, 3-arm study. Am.Heart J. 2011;161(5):915-919. View abstract.
Nine studies with 10 data sets used omega-3 PUFA dosages of less than 2000 mg/d.35,47,48,51,53,55,56,60,61 The main results revealed that there was no significant difference in the association of treatment with reduced anxiety symptoms between patients receiving omega-3 PUFA treatment and those not receiving it (k, 9; Hedges g, 0.457; 95% CI, –0.077 to 0.991; P = .09) (Figure 3B). Ten studies with 10 data sets used omega-3 PUFA dosages of at least 2000 mg/d.33,34,36,49,50,52,54,55,57-59 The main results revealed a significantly greater association of treatment with reduced anxiety symptoms in patients receiving omega-3 PUFA treatment than in those not receiving it (k, 11; Hedges g, 0.213; 95% CI, 0.031-0.395; P = .02) (Figure 3B). Furthermore, there was no significantly different estimated effect sizes between these 2 subgroups by the interaction test (P = .40).
Dangour, A. D., Allen, E., Elbourne, D., Fasey, N., Fletcher, A. E., Hardy, P., Holder, G. E., Knight, R., Letley, L., Richards, M., and Uauy, R. Effect of 2-y n-3 long-chain polyunsaturated fatty acid supplementation on cognitive function in older people: a randomized, double-blind, controlled trial. Am.J.Clin.Nutr. 2010;91(6):1725-1732. View abstract.

These low levels are especially bad when compared to the numbers from the Japanese population. In Japan, the average omega-3 index level is more than double that of the average American, with some surveys showing Japanese men consume over 100 g (approximately 3.5 oz) of fish every day. These radically different dietary habits help explain how even those with omega-3 indexes in the lowest 5th percentile of the Japanese population have higher omega-3 index averages than most Americans (8).

Children: Fish oil is POSSIBLY SAFE when taken by mouth appropriately. Fish oil has been used safely through feeding tubes in infants for up to 9 months. But young children should not eat more than two ounces of fish per week. Fish oil is also POSSIBLY SAFE when given in the vein by a health care professional to infants who cannot take food by mouth. Fish oil is POSSIBLY UNSAFE when consumed from dietary sources in large amounts. Fatty fish contain toxins such as mercury. Eating contaminated fish frequently can cause brain damage, mental retardation, blindness and seizures in children. 

Badia-Tahull, M. B., Llop-Talaveron, J. M., Leiva-Badosa, E., Biondo, S., Farran-Teixido, L., Ramon-Torrell, J. M., and Jodar-Masanes, R. A randomised study on the clinical progress of high-risk elective major gastrointestinal surgery patients treated with olive oil-based parenteral nutrition with or without a fish oil supplement. Br.J.Nutr. 2010;104(5):737-741. View abstract.
EPA and DHA  stand for eicosapentaenoic acid and docosahexaenoic acid respectively. These fatty acids are omega-3 fats, which are found in cold water fish. EPA DHA are highly unsaturated fats because they contain six and five double bonds on their long structural chains. These polyunsaturated fats play a very important role with the function of our bodies.
What about blood clotting? Circulating cells called platelets are critical in causing your blood to clot. When platelets are activated, they aggregate and cause clots. If these clots occur in particularly sensitive regions of your body, they can lead to a heart attack or stroke. EPA reduces platelet activation, an early step in platelet aggregation to help to reduce clotting. One study found that EPA was superior to DHA in decreasing platelet activation, a precursor to blood clotting.1
Egert, S., Somoza, V., Kannenberg, F., Fobker, M., Krome, K., Erbersdobler, H. F., and Wahrburg, U. Influence of three rapeseed oil-rich diets, fortified with alpha-linolenic acid, eicosapentaenoic acid or docosahexaenoic acid on the composition and oxidizability of low-density lipoproteins: results of a controlled study in healthy volunteers. Eur J Clin Nutr 2007;61(3):314-325. View abstract.
Bell, J. G., Miller, D., MacDonald, D. J., MacKinlay, E. E., Dick, J. R., Cheseldine, S., Boyle, R. M., Graham, C., and O'Hare, A. E. The fatty acid compositions of erythrocyte and plasma polar lipids in children with autism, developmental delay or typically developing controls and the effect of fish oil intake. Br J Nutr 2010;103(8):1160-1167. View abstract.
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Fish oil can be obtained from eating fish or by taking supplements. Fish that are especially rich in the beneficial oils known as omega-3 fatty acids include mackerel, herring, tuna, salmon, cod liver, whale blubber, and seal blubber. Two of the most important omega-3 fatty acids contained in fish oil are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Make sure to see separate listings on EPA and DHA, as well as Cod Liver Oil, and Shark Liver Oil.