Ample evidence from animal studies supports regular supplementation with omega-3 oils as a means of lowering long-term cardiovascular risk. This may be due to omega-3 fatty acids’ effects on reducing inflammation, lowering triglycerides, reducing blood pressure, improving endothelial function, inducing new blood vessel formation after heart attack or stroke, and favorable modification of obesity-related inflammatory molecules.35-39

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)
^ Jump up to: a b Hooper L, Thompson RL, Harrison RA, Summerbell CD, Ness AR, Moore HJ, Worthington HV, Durrington PN, Higgins JP, Capps NE, Riemersma RA, Ebrahim SB, Davey Smith G (2006). "Risks and benefits of omega−3 fats for mortality, cardiovascular disease, and cancer: systematic review". BMJ. 332 (7544): 752–60. doi:10.1136/bmj.38755.366331.2F. PMC 1420708. PMID 16565093. Retrieved 2006-07-07.[permanent dead link]
All people need to consume omega-3 fats regularly. The recommended daily intake for adults is 1.6 grams for males  and 1.1 grams for females, according to the National Institutes of Health. The omega-3 family encompasses numerous fatty acids, but three primary forms are eicosapentaenoic acid, docosahexaenoic acid, and alpha-linolenic acid. The first two forms primarily occur in fish, such as salmon, mackerel, and tuna. The third can be found in plant oils, including flaxseed, soybean, walnut, and canola oils.
Sangiovanni, J. P., Agron, E., Meleth, A. D., Reed, G. F., Sperduto, R. D., Clemons, T. E., and Chew, E. Y. {omega}-3 Long-chain polyunsaturated fatty acid intake and 12-y incidence of neovascular age-related macular degeneration and central geographic atrophy: AREDS report 30, a prospective cohort study from the Age-Related Eye Disease Study. Am J Clin Nutr 2009;90(6):1601-1607. View abstract.
Typical Western diets provide ratios of between 10:1 and 30:1 (i.e., dramatically higher levels of omega−6 than omega−3).[92] The ratios of omega−6 to omega−3 fatty acids in some common vegetable oils are: canola 2:1, hemp 2–3:1,[93] soybean 7:1, olive 3–13:1, sunflower (no omega−3), flax 1:3,[94] cottonseed (almost no omega−3), peanut (no omega−3), grapeseed oil (almost no omega−3) and corn oil 46:1.[95]
Most leafy green vegetables have significant amounts of omega-3, and spinach is no exception. Despite its villainous reputation, raw spinach actually has a mild flavor, making it an ideal base for salads or a crunchy addition to sandwiches. Many people add spinach to eggs, soups, or pasta dishes without impacting flavor. If you’re dealing with a particularly picky eater, though, try some of the recipes in Jessica Seinfeld’s Deceptively Delicious — her spinach and carrot brownies are tasty, healthy, and chocolaty to boot!
56. Davidson MH, Stein EA, Bays HE, et al. COMBination of prescription Omega-3 with Simvastatin (COMBOS) Investigators. Efficacy and tolerability of adding prescription omega-3 fatty acids 4 g/d to simvastatin 40 mg/d in hypertriglyceridemic patients: an 8-week, randomized, double-blind, placebo-controlled study. Clin Ther. 2007;29:1354–1367. [PubMed]

Although results from studies regarding the disease processes of AD seem to be promising, there are conflicting data regarding the use of omega-3 fatty acids in terms of cognitive function. Neuropsychiatric symptoms accompany AD from early stages and tend to increase with the progression of the disease (55). An analysis of 174 patients randomized to a placebo group or to a group with mild to moderate AD (MMSE score ≥15) treated with daily DHA (1.7 g) and EPA (0.6 g) found that at 6 mo, the decline in cognitive function did not differ between the groups. Yet, in a subgroup with very mild cognitive dysfunction (n = 32, MMSE score >27), they observed a significant reduction in the MMSE decline rate in the DHA+EPA-supplemented group compared with the placebo group (47). Another study that looked at DHA supplementation in individuals with mild to moderate AD used the Alzheimer's Disease Assessment Scale–Cognitive subscale, which evaluates cognitive function on a 70-point scale in terms of memory, attention, language, orientation, and praxis. This study found that DHA supplementation had no beneficial effect on cognition during the 18-mo trial period for the DHA group vs. placebo (56).
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.
The three types of omega-3s are APA, EPA and DHA. The first is a medium-chain fatty acid and must be converted into EPA before being synthesized by the body, and only about 1 percent of the APA consumed is able to be converted. EPA and DHA are already in a form ready to be synthesized (and are the subject of most scientific research regarding omega-3s).

Irish AB, Viecelli AK, Hawley CM, et al; Omega-3 Fatty Acids (Fish Oils) and Aspirin in Vascular Access Outcomes in Renal Disease (FAVOURED) Study Collaborative Group. Effect of fish oil supplementation and aspirin use on arteriovenous fistula failure in patients requiring hemodialysis: A randomized clinical trial. JAMA Intern Med. 2017;177(2):184-193. View abstract.
Some studies reported better psychomotor development at 30 months of age in infants whose mothers received fish oil supplements for the first four months of lactation.[45] In addition, five-year-old children whose mothers received modest algae based docosahexaenoic acid supplementation for the first 4 months of breastfeeding performed better on a test of sustained attention. This suggests that docosahexaenoic acid intake during early infancy confers long-term benefits on specific aspects of neurodevelopment.[45]
The evidence linking the consumption of marine omega−3 fats to a lower risk of cancer is poor.[8][13] With the possible exception of breast cancer,[8][14][15] there is insufficient evidence that supplementation with omega−3 fatty acids has an effect on different cancers.[5][16] The effect of consumption on prostate cancer is not conclusive.[8][15] There is a decreased risk with higher blood levels of DPA, but an increased risk of more aggressive prostate cancer was shown with higher blood levels of combined EPA and DHA.[17] In people with advanced cancer and cachexia, omega−3 fatty acids supplements may be of benefit, improving appetite, weight, and quality of life.[18]
The most extensive data of the effect of fish oil on lipoprotein subfractions are based on trials performed before the widespread use of statins. This data were aggregated over a decade ago in a meta-analysis of 16 randomized trials including over 1500 patients.17 In this analysis, low-density lipoprotein (LDL) was increased by an average of 5% and high-density lipoprotein was marginally changed. Although a shift toward less atherogenic, larger and more buoyant LDL particle composition has been shown,74 this has been offset by the observation that the number of apolipoprotein B 100 particles increases and may be more susceptible to oxidation.75 Increased conversion of remnant particles (intermediate density lipoprotein) to LDL has also been observed.76

Research conducted by Professor Peter Howe at the University of South Australia has shown that fish oil improves the efficacy of exercise in attempts to reduce weight. Volunteers who were given fish oil in their diet showed greater weight loss as compared to those who did not regularly consume it. Fish oil contains omega-3 fatty acids, which help to promote the weight loss, so a combination of physical workout and intake of this oil helps in reducing body fat significantly faster.

Omega-3s have been studied for other conditions, with either inconclusive or negative results. These conditions include allergies, atopic eczema (an allergic skin condition), cystic fibrosis, diabetes, inflammatory bowel diseases (Crohn’s disease or ulcerative colitis), intermittent claudication (a circulatory problem), nonalcoholic fatty liver disease, and osteoporosis. 
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The number, location, and orientation of the double bonds determine the health effects of fatty acids on the body. One aspect of this is their effect on triglycerides and LDL and HDL types of cholesterol, which in turn affect how much cholesterol gets deposited on the inside of blood vessels. There are also subtypes of LDL and HDL which are also likely important to their health effects.
EPA and DHA are vital nutrients and may be taken to maintain healthy function of the following: brain and retina: DHA is a building block of tissue in the brain and retina of the eye. It helps with forming neural transmitters, such as phosphatidylserine, which is important for brain function. DHA is found in the retina of the eye and taking DHA may be necessary for maintaining healthy levels of DHA for normal eye function.
Scaly, itchy skin (eczema). Research shows that fish oil does not improve eczema. Most research also shows that taking fish oil during pregnancy doesn't PREVENT eczema in the child. Giving fish oil to an infant also doesn't seem to prevent eczema in children. But children who eat fish at least once weekly from the age of 1-2 years seem to have a lower risk of developing eczema.