Fish oil is also used for diabetes, prediabetes, asthma, a movement and coordination disorder called dyspraxia, dyslexia, eczema, autism, obesity, weak bones (osteoporosis), rheumatoid arthritis (RA), osteoarthritis, psoriasis, an autoimmune disease called systemic lupus erythematosus (SLE), multiple sclerosis, HIV/AIDS, cystic fibrosis, gum disease, Lyme disease, sickle cell disease, and preventing weight loss caused by some cancer drugs.
Some research indicates that people who eat more seafood may have a reduced risk of cognitive decline. However, omega-3 supplements haven’t been shown to help prevent cognitive impairment or Alzheimer’s disease or to improve symptoms of these conditions. For example, a large NIH-sponsored study completed in 2015 indicated that taking EPA and DHA supplements did not slow cognitive decline in older adults. The people studied were participants in a larger eye disease study, and all of them had age-related macular degeneration (AMD). 
Henriksen, C., Haugholt, K., Lindgren, M., Aurvag, A. K., Ronnestad, A., Gronn, M., Solberg, R., Moen, A., Nakstad, B., Berge, R. K., Smith, L., Iversen, P. O., and Drevon, C. A. Improved cognitive development among preterm infants attributable to early supplementation of human milk with docosahexaenoic acid and arachidonic acid. Pediatrics 2008;121(6):1137-1145. View abstract.

To exclude the possible confounding effects of clinical variables on the Hedges g, metaregression analysis was conducted with an unrestricted maximum likelihood random-effects model of single variables when there were more than 10 data sets available. Specifically, the clinical variables of interest included mean age, female proportion, sample size, mean body mass index, daily omega-3 PUFA dosage, EPA to DHA ratio, treatment duration, dropout rate, and others. In addition, a subgroup meta-analysis was conducted to investigate potential sources of heterogeneity, specifically, a further subgroup meta-analysis focused on those trials that were placebo controlled or non–placebo controlled. To more clearly uncover the differences in the meta-analysis results among the recruited studies, a further subgroup meta-analysis was performed according to the presence of a specific clinical diagnosis or no specific clinical condition, mean omega-3 PUFA daily dosage, and mean age. In addition, in a previous study, the EPA percentage (ie, ≥60%) in the PUFA regimens had different effects on depression treatment.9 Therefore, we also arranged the subgroup meta-analysis based on the EPA percentage. Furthermore, we arranged subgroup meta-analysis procedures only when there were at least 3 data sets included.45 To investigate the potentially different estimated effect sizes between subgroups, we performed an interaction test and calculated the corresponding P values.46
Jump up ^ Talakoub, Lily; Neuhaus, Isaac M.; Yu, Siegrid S. (2008). "Chapter 2: Cosmoceuticals". In Alam, Murad; Gladstone, Hayes B.; Tung, Rebecca. Cosmetic Dermatology. Requisites in dermatology. Elsevier Health Sciences. p. 9. ISBN 9780702031434. Retrieved 2014-10-23. Other oils used as emollients include fish oil, petrolatum, shea butter, and sunflower seed oil.
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.

The FDA recommends that consumers do not exceed more than three grams per day of EPA and DHA combined, with no more than 2 grams from a dietary supplement.[56] This is not the same as 3000 mg of fish oil. A 1000 mg pill typically has only 300 mg of omega-3; 10 such pills would equal 3000 mg of omega-3. According to the European Food Safety Authority's (EFSA) Panel on Dietetic Products, Nutrition and Allergies, supplementation of 5 grams of EPA and DHA combined does not pose a safety concern for adults.[57] Dyerberg studied healthy Greenland Inuit and found an average intake of 5.7 grams of omega-3 EPA per day; among other effects these people had prolonged bleeding times, i.e., slower blood clotting.[58]
The question is whether the observed cardiovascular benefits often found among fish eaters is due solely to the oils in fish or to some other characteristics of seafood or to still other factors common to those who eat lots of fish, like eating less meat or pursuing a healthier lifestyle over all. Whatever the answer, it does not seem to be fish oil supplements.
Three omega−3 fatty acids are important in human physiology, α-linolenic acid (18:3, n-3; ALA), eicosapentaenoic acid (20:5, n-3; EPA), and docosahexaenoic acid (22:6, n-3; DHA).[67] These three polyunsaturates have either 3, 5, or 6 double bonds in a carbon chain of 18, 20, or 22 carbon atoms, respectively. As with most naturally-produced fatty acids, all double bonds are in the cis-configuration, in other words, the two hydrogen atoms are on the same side of the double bond; and the double bonds are interrupted by methylene bridges (-CH
DHA is especially vital for infant and child brain and nervous system development, as well as visual function. In older children, high DHA levels have been shown to improve learning ability, while deficiencies have been linked to learning problems and ADHD. And in adults, some studies have shown that DHA helps protect against cognitive decline and Alzheimer’s disease.
Human studies also confirm cognition and memory improvement with omega-3 supplementation. For example, a study showed that both fish oil and krill oil enhanced cognitive function in a group of older men by increasing oxygen delivery to their brains. Interestingly, for those taking krill oil this effect was more prominent than those taking fish oil, though both groups were significantly better than placebo.30 As we pointed out earlier, because the omega-3 DHA is bound to phospholipids in krill it may be more effectively incorporated into the critical cell membrane in brain cells.
Good points, Miroslav. Focusing on your 4th point, with so many different formulations on the market that contain various preservatives, only looking at the blood levels of omega-3’s as the flag for increased risk for prostate cancer tends to ignore the fact that certain populations in coastal regions maintain a diet high in omega fish oils and don’t have a marked increase level of prostate cancer, pointing to the fact that another agent may be to blame here.
One of the most well-known benefits of omega-3s are the way they positively affect risk factors associated with heart disease. That’s one reason the American Heart Association is very clear about encouraging people to get enough in their diets. (8) Heart disease and stroke are the leading causes of death worldwide, but communities who eat diets rich in fish have remarkably low instances of these diseases, which is at least partially due to their high omega-3 consumption. (9, 10)
Foods such as meat, eggs, fish and nuts contain omega-3 and omega-6 fatty acids, which the body converts into endocannabinoids – cannabinoids that the body produces naturally, said Aditi Das, a University of Illinois professor of comparative biosciences and biochemistry, who led the study. Cannabinoids in marijuana and endocannabinoids produced in the body can support the body’s immune system and therefore are attractive targets for the development of anti-inflammatory therapeutics, she said.
A, Subgroup meta-analysis of the anxiolytic effect of omega-3 polyunsaturated fatty acids (PUFAs) based on an underlying specific clinical diagnosis or not. The anxiolytic effect of omega-3 PUFAs was not significant in the subgroup of participants without specific clinical conditions (k, 5; Hedges g, –0.008; 95% CI, –0.266 to 0.250; P = .95) but was significant in the subgroup of participants with specific clinical diagnoses (k, 14; Hedges g, 0.512; 95% CI, 0.119-0.906; P = .01). Furthermore, the association of treatment with reduced anxiety symptoms of omega-3 PUFAs were significantly stronger in subgroups with specific clinical diagnoses than in subgroups without specific clinical conditions (P = .03). B, Subgroup meta-analysis of the anxiolytic effect of omega-3 PUFAs based on different mean omega-3 PUFA dosages. The anxiolytic effect of omega-3 PUFAs was not significant in subgroups of mean omega-3 PUFA dosages less than 2000 mg/d (k, 9; Hedges g, 0.457; 95% CI, –0.077 to 0.991; P = .09) but was significant in the subgroup of mean omega-3 PUFA dosage of at least 2000 mg/d (k, 11; Hedges g, 0.213; 95% CI, 0.031-0.395; P = .02).
Maternal nutrition guidelines have always stressed a diet including sufficient caloric and protein requirements, but recently fatty acids have also been deemed important (17). This is partially due to the fact that EPA and DHA supplementation during pregnancy has been associated with multiple benefits for the infant (Table 1). During pregnancy, the placenta transfers nutrients, including DHA, from the mother to the fetus (18). The amount of omega-3 fatty acid in the fetus is correlated with the amount ingested by the mother, so it is essential that the mother has adequate nutrition (19). The 2010 U.S. Department of Health and Human Services dietary guidelines recommend that women who are pregnant or breastfeeding should “consume 8 to 12 ounces of seafood per week from a variety of seafood types” (12). Ingesting 8–12 oz of seafood per week, depending on the type of fish, is equivalent to ∼300–900 mg EPA+DHA per day. Unfortunately, this amount is not being met by most mothers in the United States and Canada, which means that infants many not be receiving adequate amounts of these vital nutrients in the womb (20).
Cashew nuts are a versatile, creamy nut, eaten on their own as a snack or used as a base for many vegan cheese substitutes. RXBAR, a healthy alternative to the standard sugar-loaded snack bar, uses cashews for several of its flavor varieties. And with delicious (and kid-friendly!) flavors like gingerbread, chocolate chip, or “Berry Blast,” these bars are a tasty way to add more cashews to any diet.
So back to fish oil in general. The major fish oil benefits include decreasing the risk of heart disease and stroke while also helping reduce symptoms of depression, hypertension, attention deficit hyperactivity disorder (ADHD), joint pain, arthritis and chronic skin ailments like eczema. (2) Fish oil intake has also been associated with aiding the body in weight loss, fertility, pregnancy and increased energy. Prescription fish oil has even been approved by the FDA to lower unhealthy high triglyceride levels. (3)
The Japanese notably have the lowest levels of coronary heart disease mortality and atherosclerosis among developed nations — a phenomena that has been largely subscribed to diet. However, even within Japan, a 10-year study of over 41,000 people found that higher intakes of omega-3s were associated with lower risks of nonfatal coronary events (8). A more recent study also found that Japanese with higher omega-3 index levels (10%) had a lower risk of fatal coronary heart disease than those with a lower omega-3 index levels (8%) (9). The study begs the question of whether maybe even the Japanese have room to improve their omega-3 intake and whether 8% should be considered the lower limit of a desirable range.

Widenhorn-Müller  K, Schwanda  S, Scholz  E, Spitzer  M, Bode  H.  Effect of supplementation with long-chain ω-3 polyunsaturated fatty acids on behavior and cognition in children with attention deficit/hyperactivity disorder (ADHD): a randomized placebo-controlled intervention trial.  Prostaglandins Leukot Essent Fatty Acids. 2014;91(1-2):49-60. doi:10.1016/j.plefa.2014.04.004PubMedGoogle ScholarCrossref

In total, 19 articles with 19 data sets revealed the main results of the meta-analysis, namely that there was a significantly better association of treatment with reduced anxiety symptoms in patients receiving omega-3 PUFA treatment than in those not receiving it (k, 19; Hedges g, 0.374; 95% CI, 0.081-0.666; P = .01; Figure 2), with significant heterogeneity (Cochran Q, 178.820; df, 18; I2, 89.934%; P < .001) but no significant publication bias via Egger regression (t, 1.736; df, 17; P = .10) or inspection of the funnel plot (eFigure 2 in the Supplement). According to the trim-and-fill test, there was no need for adjustment for publication bias. The meta-analysis results remained significant after removal of any one of the included studies, which indicated that the significant results are not owing to any single study.

Schilling, J., Vranjes, N., Fierz, W., Joller, H., Gyurech, D., Ludwig, E., Marathias, K., and Geroulanos, S. Clinical outcome and immunology of postoperative arginine, omega-3 fatty acids, and nucleotide-enriched enteral feeding: a randomized prospective comparison with standard enteral and low calorie/low fat i.v. solutions. Nutrition 1996;12(6):423-429. View abstract.
However, the researchers do have some good news. They concluded that omega 3 fatty acids do appear to reduce the type of blood cholesterol known as triglycerides, but that supplements probably are not useful for preventing or improving heart and circulatory problems. And, upping your intake of plant-based omega 3s high in ALA (ie, walnuts, flaxseed and flax oil, chia seeds) may help your heart somewhat.2
Mercury and polychlorinated biphenyls (PCBs) are common toxins in seafood. Although the U.S. banned the use of PCBs and DDT in 1976, these and other chemicals are still used in half the world's commercial chemical processes. Substances like these can hang around in the air, soil, and water for many years. They end up in the bodies of fish and animals.

Hernandez, D., Guerra, R., Milena, A., Torres, A., Garcia, S., Garcia, C., Abreu, P., Gonzalez, A., Gomez, M. A., Rufino, M., Gonzalez-Posada, J., Lorenzo, V., and Salido, E. Dietary fish oil does not influence acute rejection rate and graft survival after renal transplantation: a randomized placebo-controlled study. Nephrol.Dial.Transplant. 2002;17(5):897-904. View abstract.
After a large number of lab studies found that omega-3 fatty acids may be effective in slowing or reversing the growth of hormonal cancers, namely prostate and breast cancer cells, animal and human epidemiological studies have been conducted to see whether this effect occurred in real-life scenarios. The evidence is somewhat conflicting in some reports, but there is some evidence to suggest breast and prostate cancers may be potentially slowed (or the risk reduced) in people who eat a lot of oily fish and possibly those who supplement with omega-3. (66, 67, 68)
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.