Oe, H., Hozumi, T., Murata, E., Matsuura, H., Negishi, K., Matsumura, Y., Iwata, S., Ogawa, K., Sugioka, K., Takemoto, Y., Shimada, K., Yoshiyama, M., Ishikura, Y., Kiso, Y., and Yoshikawa, J. Arachidonic acid and docosahexaenoic acid supplementation increases coronary flow velocity reserve in Japanese elderly individuals. Heart 2008;94(3):316-321. View abstract.

For example, large predatory fish like shark, swordfish, king mackerel, tilefish and albacore tuna can contain high levels of methyl mercury, a toxin that would override any health benefit, especially for the developing brains of fetuses and young children as well as for adults, Dr. Nesheim and Marion Nestle, professor emerita of nutrition, food studies and public health at New York University, noted in 2014 in an editorial in the American Journal of Clinical Nutrition. (Levels of mercury and other contaminants in fish have since declined somewhat but are not negligible.)

People with metabolic syndrome (the combination of central obesity, high blood pressure, disturbed lipid profile, and impaired glucose tolerance) are at increased risk of death from cardiovascular disease, diabetes, cancer, and other apparently “age-related” disorders. Because metabolic syndrome is closely associated with chronic low-grade inflammation, the powerful anti-inflammatory effects of omega-3 fats are especially important as a means of slowing or stopping the progression of this deadly disorder.
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).
Interestingly, the results are also consistent with our recent findings that somatic anxiety is associated with omega-3 PUFA deficits and the genetic risks of PUFA metabolic enzyme cytosolic phospholipase A2 in major depressive disorder62,63 and interferon α–induced neuropsychiatric syndrome.63,64 Brain membranes contain a high proportion of omega-3 PUFAs and their derivatives and most animal and human studies suggest that a lack of omega-3 PUFAs in the brain might induce various behavioral and neuropsychiatric disorders,16,65-70 including anxiety-related behaviors.12,18,19,32,49,71 Emerging evidence suggests that omega-3 PUFAs interfere with and possibly control several neurobiological processes, such as neurotransmitter systems, neuroplasticity, and inflammation,12,72 which is postulated to be the mechanism underlying anxiety and depression.
Fish oil is a concentrated source of omega-3 fats, which are also called ω-3 fatty acids or n-3 fatty acids. To get more scientific, omega-3s are long-chain polyunsaturated fatty acids, or PUFAs. Our bodies are able to make most of the fats we need need, but that’s not true for omega-3 fatty acids. When it comes to these essential fats, we need to get them from omega-3 foods or supplements.

Samsonov, M. A., Vasil'ev, A. V., Pogozheva, A. V., Pokrovskaia, G. R., Mal'tsev, G. I., Biiasheva, I. R., and Orlova, L. A. [The effect of a soy protein isolate and sources of polyunsaturated omega-3 fatty acids in an anti-atherosclerotic diet on the lipid spectrum of blood serum and immunological indicators in patients with ischemic heart disease and hypertension]. Vopr.Med Khim. 1992;38(5):47-50. View abstract.
Humans are unable to place double bonds beyond position 9 on long chain polyunsaturated fatty acids (FA), making the omega-3 FA synthesized in plants and in marine microalgae essential elements to the human diet.1 Fish contain high levels of 2 omega-3 FA, eicosapentaenoic acid (EPA; C20:5 n-3), and docosahexaenoic acid [DHA]; C22:6 n-3)2,3 (Fig. 1). Many claims about the role of these omega-3 FA have been made in the prevention and treatment of cardiovascular disease. For instance, fish oil is seen as having a therapeutic role in coronary artery disease (CAD), heart failure, fatal and nonfatal arrhythmias, as well as offering an alternative or adjunct to the standard therapy for hypertriglyceridemia and diabetes. This review will highlight the potential mechanisms of fish oil on cardiovascular disease and provide an update of clinical trial results. The established uses in the treatment of hypertriglyceridemia and sources of omega-3 FA—both dietary and drug therapy—will be iterated, along with its potential application in combination with standard hypolipidemic agents. Finally, the limitations of current data will be addressed, as well as suggested recommendations for clinical use.

While I think the article is good, it does not tell the reader that most of fish oil capsules sold over the counter are unregulated, and contain widely different ingredients and potency levels. They are mostly a waste of money. If you have health concerns, you need to consult an MD or a Registered Dietitian. Not a naturopath, homeopath, or other pseudoscience practitioner. Eat a diet rich in whole grains, nuts, and some oily fish. I take a multivitamin supplement made by CVS, formulated for my gender and age. Not from the food supplement shelves, which are unregulated, and might contain anything at all, or nothing but vegetable oil or cornstarch.

It can be challenging to get the appropriate intake of EPA and DHA through diet alone, even though EPA and DHA are produced by water plants such as algae and are prevalent in marine animals. A shorter chain omega-3 fatty acid, α-linolenic acid (ALA),6 is a prominent component of our diet as it is found in many land plants that are commonly eaten, but it does not provide the health benefits seen with EPA and DHA. Although it is possible for the body to convert ALA to EPA and DHA by enlongase and desaturase enzymes, research suggests that only a small amount can be synthesized in the body from this process (8). For example, 1 study suggested that only ∼2 to 10% of ALA is converted to EPA or DHA (9), and other studies found even less: Goyens et al. (10) found an ALA conversion of ∼7% for EPA, but only 0.013% for DHA; Hussein et al. (11) found an ALA conversion of only 0.3% for EPA and <0.01% for DHA.

Fish oils rich in omega 3 fatty acids help improve fertility and cell division. Preliminary research conducted on animals has shown that when males are fed a diet containing fish oil, the quality of the sperm is enhanced. After ejaculation, the sperm has increased survival against lipid peroxidative attacks in the female genital tract, thereby increasing the chances of conception. On the other hand, similar animal studies have shown inhibition in the synthesis of prostaglandin E and prostaglandin F, which are produced in large quantities by human seminal vesicles. The research, however, found no impact on the count and mobility of sperm.

If you find yourself in a position where you are just not eating any of these foods, and you want to get enough omega-3 fatty acids, then I think fish oil is okay, but I would limit not the amount of fish oil but the amount listed on the label of EPA and DHA combined. I would limit that amount to around 250 milligrams per day because I don’t think most people need more than that. Some signs that you might not be getting enough omega-3 fatty acids include chronic low-grade inflammation, poor visual acuity, slower mental processing, trouble learning, and possibly Alzheimer’s disease and psychiatric conditions, like depression, anxiety, and attention deficit and hyperactivity disorder, ADHD.

A 2014 meta-analysis of eleven trials conducted respectively on patients with a DSM-defined diagnosis of major depressive disorder (MDD) and of eight trials with patients with depressive symptomatology but no diagnosis of MDD demonstrated significant clinical benefit of omega-3 PUFA treatment compared to placebo. The study concluded that: "The use of omega-3 PUFA is effective in patients with diagnosis of MDD and on depressive patients without diagnosis of MDD."[42]
RA causes chronic pain, swelling, stiffness, and loss of function in the joints. Some clinical trials have shown that taking omega-3 supplements may help manage RA when taken together with standard RA medications and other treatments. For example, people with RA who take omega-3 supplements may need less pain-relief medication, but it is not clear if the supplements reduce joint pain, swelling, or morning stiffness.

For slowing weight loss in patients with cancer: 30 mL of a specific fish oil product (ACO Omega-3, Pharmacia, Stockholm, Sweden) providing 4.9 grams of EPA and 3.2 grams of DHA daily for 4 weeks has been used. 7.5 grams of fish oil daily providing EPA 4.7 grams and DHA 2.8 grams has been used for about 6 weeks. In addition, two cans of a fish oil nutritional supplement containing 1.09 grams of EPA and 0.96 grams of DHA per can have been used daily for up to 7 weeks.
You “beat me to the punch.” despite labels, cured meats , aged fats, as well as those heated to a high enough temperature all have trans bonds. Fish that offer high amounts of Omega-3 also often are high in mercury. I was fortunate to have a very good teacher for experimental design. One should be careful to assume that a study actually measures what it claims to and without “confounders” Confounders are parts of the study that complicate the the “logic” of the design. Also, were other fat contents measured or controlled? It would be reasonable to suspect that those with higher levels of Omega-3 could have higher levels of Omega-6, fats in general , High levels of protein, higher levels of testosterone, or lower levels of certain hormones. In addition, statistical studies do not and have never indicated a causal relationship. I have a fear of how much we have begun to rely on statistical correlational studies which are at the end of the day”soft” science.
The effect of fish oil consumption on prostate cancer is controversial,[28][29] as one study showed decreased risk with higher blood levels of DPA, whereas another reported increased risk of more aggressive prostate cancer with higher blood levels of combined EPA and DHA.[30] Some evidence indicated an association between high blood levels of omega-3 fatty acids and an increased prostate cancer risk.[31]
In lab experiments, animals given krill showed improved navigation skills. What this means is that they achieved higher levels of cognition and memory required to navigate complex territory.28 In addition, research shows that animals supplemented with krill oil showed significantly fewer signs of depression and resignation. This improvement in mood was equivalent to the effect of the prescription anti-depressant drug imipramine (Tofranil®).29
In lab experiments, animals given krill showed improved navigation skills. What this means is that they achieved higher levels of cognition and memory required to navigate complex territory.28 In addition, research shows that animals supplemented with krill oil showed significantly fewer signs of depression and resignation. This improvement in mood was equivalent to the effect of the prescription anti-depressant drug imipramine (Tofranil®).29

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.

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.
Several large studies have linked higher blood levels of long-chain omega-3s with higher risks of prostate cancer. However, other research has shown that men who frequently eat seafood have lower prostate cancer death rates and that dietary intakes of long-chain omega-3s aren’t associated with prostate cancer risk. The reason for these apparently conflicting findings is unclear. 
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.