Davidson, M. H., Stein, E. A., Bays, H. E., Maki, K. C., Doyle, R. T., Shalwitz, R. A., Ballantyne, C. M., and Ginsberg, H. N. 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(7):1354-1367. View abstract.

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).

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.
Most people get far too little omega-3s in their diet. In fact, research consistently indicates that the majority of Americans have just slightly more than half the amount of EPA and DHA in their tissues than they need for optimum brain and body health. This is partly due to a high dietary intake of unhealthy fats combined with an inadequate intake of EPA and DHA.
Your retina contains quite a bit of DHA, making it necessary for that fatty acid to function. (90) The National Eye Institute, part of the National Institutes of Health, concludes that there is “consistent evidence” suggesting long-chain polyunsaturated fatty acids DHA and EPA are necessary for retinal health and may help protect the eyes from disease. (91)

Omega-3 [(n-3)] long-chain PUFA, including EPA and DHA, are dietary fats with an array of health benefits (1). They are incorporated in many parts of the body including cell membranes (2) and play a role in antiinflammatory processes and in the viscosity of cell membranes (3, 4). EPA and DHA are essential for proper fetal development and healthy aging (5). DHA is a key component of all cell membranes and is found in abundance in the brain and retina (6). EPA and DHA are also the precursors of several metabolites that are potent lipid mediators, considered by many investigators to be beneficial in the prevention or treatment of several diseases (7).

In comparison, the omega-3s found in krill appear to be more rapidly incorporated into red blood cell phospholipids.7 This is important, because not only do scientists view the uptake of essential fatty acids in red blood cells as a biomarker for uptake into the brain,8 but additional research suggests that when omega-3 fatty acids such as DHA are bound to phospholipids as they are with krill, it increases their uptake to the brain.9 This is further supported by human clinical research, which suggests ingestion of phospholipid-bound EPA and DHA increase cognitive function scores to a greater degree compared with scores obtained when the fatty acids in the ingested oil were provided in the triglycerides storage form.10

Three randomized trials assessing more than 600 patients with known malignant ventricular arrhythmia were carried out under the protection of implanted cardioverter defibrillator (ICD) therapy.41–43 In all 3 of the trials, 75% of the patients had ischemic heart disease, survived ventricular tachycardia or ventricular fibrillation and were randomized to 1 to 3 g/d of fish oil. In the first trial of its kind, 402 patients with ICDs were randomized to either a fish oil or an olive oil supplement.41 Although statistical significance was not reached, after approximately 1 year the primary end-point of time to first ICD cardioversion for ventricular tachycardia or fibrillation or death from any cause was longer in the fish oil group. This finding was not replicated in a trial of 200 patients who were randomized to either fish oil or a placebo and followed for a median of approximately 2 years.42 In fact, time to first ICD cardioversion was not changed and the incidence of recurrent ventricular tachycardia and fibrillation was more common in the group assigned to fish oil. In the largest trial, 546 patients were randomized to supplemental fish oil or a placebo and were followed for a mean period of 1 year.43 The primary outcome of the rate of ICD cardioversion or all-cause mortality was not reduced. It was concluded in a recent meta-analysis of these trials that fish oil did not have a protective effect.44

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Those foods provide enormous amounts of other nutrients that are good for you. nSo it is way better to eat those foods than to take fish oil. With that said, some people find it very difficult to get vitamin A or vitamin D, and particularly for vitamin A, cod liver oil may be a very important source of that vitamin. Cod liver oil is a form of fish oil that happens to be high in the fat-soluble vitamins. Vitamin A is best found in liver. It’s better in my opinion to eat liver once a week, but there are a lot of people out there who are not going to eat liver once a week. So if you are using cod liver oil to get the vitamins that you can’t get from food—and I should point out that vitamin A can also be derived from plant foods, but many people genetically or for other reasons don’t derive it very well from plant foods.
They also found that taking more long-chain omega 3 fats (including EPA and DHA), primarily through supplements probably makes little or no difference to risk of cardiovascular events, coronary heart deaths, coronary heart disease events, stroke or heart irregularities. Long-chain omega 3 fats probably did reduce some blood fats, triglycerides and HDL cholesterol. Reducing triglycerides is likely to be protective of heart diseases, but reducing HDL has the opposite effect. The researchers collected information on harms from the studies, but information on bleeding and blood clots was very limited. 
The #1 Pharmacist Recommended Omega-3/Fish Oil brand,* Nature Made fish oil supply comes from deep ocean waters, not farm-raised fish. State-of-the-art purification processes remove mercury and ensure high levels of fish oil purity and concentration, guaranteed to pass the stringent standards of the Global Organization for EPA and DHA Omega-3 Voluntary Monograph.‡
“Lipid peroxidation induced by DHA enrichment modifies paracellular permeability in Caco-2 cells: protective role of taurine.” We conclude that hydrogen peroxide and peroxynitrite may be involved in the DHA-induced increase in paracellular permeability and that the protective role of taurine may be in part related to its capacity to counteract the effects of hydrogen peroxide.
Omega AD study, Freund-Levi et al. (47)	Double-blind, placebo-controlled, randomized	1741	DHA (1.7 g/d) and EPA (0.6 g/d)	Decline in cognitive function did not differ between supplemented group and placebo group at 6 mo. However, patients with very mild cognitive dysfunction (n = 32, MMSE score >27) in the EPA+DHA-supplemented group had a significant reduction in MMSE score decline rate at 6 mo

The American Heart Association (AHA) has made recommendations for EPA and DHA due to their cardiovascular benefits: individuals with no history of coronary heart disease or myocardial infarction should consume oily fish two times per week; and "Treatment is reasonable" for those having been diagnosed with coronary heart disease. For the latter the AHA does not recommend a specific amount of EPA + DHA, although it notes that most trials were at or close to 1000 mg/day. The benefit appears to be on the order of a 9% decrease in relative risk.[106] The European Food Safety Authority (EFSA) approved a claim "EPA and DHA contributes to the normal function of the heart" for products that contain at least 250 mg EPA + DHA. The report did not address the issue of people with pre-existing heart disease. The World Health Organization recommends regular fish consumption (1-2 servings per week, equivalent to 200 to 500 mg/day EPA + DHA) as protective against coronary heart disease and ischaemic stroke.

“This systematic review did find moderate evidence that ALA, found in plant oils (such as rapeseed or canola oil) and nuts (particularly walnuts) may be slightly protective of some diseases of the heart and circulation. However, the effect is very small, 143 people would need to increase their ALA intake to prevent one person developing arrhythmia. One thousand people would need to increase their ALA intake to prevent one person dying of coronary heart disease or experiencing a cardiovascular event.  ALA is an essential fatty acid, an important part of a balanced diet, and increasing intakes may be slightly beneficial for prevention or treatment of cardiovascular disease."
A certain kidney disease called IgA nephropathy. Some research shows that long-term but not short-term use of fish oil can slow the loss of kidney function in high-risk patients with IgA nephropathy. Fish oil might have greater effects when taken at higher doses. Also, it might be most effective in people with IgA nephropathy who have higher levels of protein in the urine.