In a 2009 letter on a pending revision to the Dietary Guidelines for Americans, the American Heart Association recommended 250–500 mg/day of EPA and DHA.[26] The Guidelines were revised again for 2015-2020; included is a recommendation that adults consume at least eight ounces of a variety of types of fish per week, equating to at least 250 mg/day of EPA + DHA.[citation needed] The Food and Drug Administration recommends not exceeding 3 grams per day of EPA + DHA from all sources, with no more than 2 grams per day from dietary supplements.[27]

Matsumura  K, Noguchi  H, Nishi  D, Hamazaki  K, Hamazaki  T, Matsuoka  YJ.  Effects of omega-3 polyunsaturated fatty acids on psychophysiological symptoms of post-traumatic stress disorder in accident survivors: a randomized, double-blind, placebo-controlled trial.  J Affect Disord. 2017;224:27-31. doi:10.1016/j.jad.2016.05.054PubMedGoogle ScholarCrossref
The Department of Ecology of the State of Washington has ranked various seafood based on its EPA and DHA concentrations. The highest-ranking seafood is mackerel, excluding King mackerel, that has a concentration of 1,790 milligrams of combined EPA and DHA per 100 grams, followed by salmon at 1,590; bluefin tuna has between 1173 and 1504 milligrams; sardines contain 980 milligrams; albacore tuna has 862 milligrams; bass has 640 milligrams; tuna has 630 milligrams; trout and swordfish have 580 milligrams; and walleye has 530 milligrams. Other seafood, which includes sea bass, clams, lobster, scallops, catfish, cod, pollock, crayfish and scallops contains between 200 and 500 milligrams of EPA and DHA per 100 grams. Breaded fish products rank lowest on the list with only 0.26 milligram per 100 grams.

EPA, which is eicosapentaenoic acid, and DHA, which is docosahexaenoic acid, are two types of omega-3 fatty acids that are most commonly found in seafood. These polyunsaturated fats are known to have preventative health benefits and have been studied for their role in treating certain chronic conditions. In addition to dietary sources, certain supplements such as fish oil, are also rich in EPA and DHA.
The 'essential' fatty acids were given their name when researchers found that they are essential to normal growth in young children and animals. The omega−3 fatty acid DHA, also known as docosahexaenoic acid, is found in high abundance in the human brain.[70] It is produced by a desaturation process, but humans lack the desaturase enzyme, which acts to insert double bonds at the ω6 and ω3 position.[70] Therefore, the ω6 and ω3 polyunsaturated fatty acids cannot be synthesized and are appropriately called essential fatty acids.[70]

Omega 3 is a type of fat. Small amounts of omega 3 fats are essential for good health, and they can be found in the food that we eat. The main types of omega 3 fatty acids are; alpha­linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).  ALA is normally found in fats from plant foods, such as nuts and seeds (walnuts and rapeseed are rich sources). EPA and DHA, collectively called long chain omega 3 fats, are naturally found in fatty fish, such as salmon and fish oils including cod liver oil.
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.
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.
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).
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)
"All these diseases have a common genesis in inflammation," says Joseph C. Maroon, MD, professor and vice chairman of the department of neurological surgery at the University of Pittsburgh School of Medicine. Co-author of Fish Oil: The Natural Anti-Inflammatory, Maroon says that in large enough amountsomega-3's reduce the inflammatory process that leads to many chronic conditions.
Marine and freshwater fish oil vary in contents of arachidonic acid, EPA and DHA.[15] The various species range from lean to fatty and their oil content in the tissues has been shown to vary from 0.7% to 15.5%.[16] They also differ in their effects on organ lipids.[15] Studies have revealed that there is no relation between total fish intake or estimated omega−3 fatty acid intake from all fish, and serum omega−3 fatty acid concentrations.[17] Only fatty fish intake, particularly salmonid, and estimated EPA + DHA intake from fatty fish has been observed to be significantly associated with increase in serum EPA + DHA.[17]
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.

A report by the Harvard Medical School studied five popular brands of fish oil, including Nordic Ultimate, Kirkland and CVS. They found that the brands had "negligible amounts of mercury, suggesting either that mercury is removed during the manufacturing of purified fish oil or that the fish sources used in these commercial preparations are relatively mercury-free".[66]
Preventing re-blockage of blood vessels after angioplasty, a procedure to open a closed blood vessel. Research suggests that fish oil decreases the rate of blood vessel re-blockage by up to 45% when given for at least 3 weeks before an angioplasty and continued for one month thereafter. But, when given for 2 weeks or less before angioplasty, it doesn't seem to have any effect.