Omega-3 fatty acids

Omega-3 fatty acids (also called ω-3 fatty acids or n-3 fatty acids) are polyunsaturated fatty acids (PUFAs) with a double bond (C=C) at the third carbon atom from the end of the carbon chain. The fatty acids have two ends, the carboxylic acid (-COOH) end, which is considered the beginning of the chain, thus "alpha", and the methyl (CH3) end, which is considered the "tail" of the chain, thus "omega." The way in which a fatty acid is named is determined by the location of the first double bond, counted from the methyl end, that is, the omega (ω-) or the n- end.
Health effects
Supplementation does not appear to be associated with a lower risk of all-cause mortality.
Cancer
The evidence linking the consumption of fish to the risk of cancer is poor. Supplementation with omega-3 fatty acids does not appear to affect this either.
A 2006 review concluded that there was no link between omega-3 fatty acids consumption and cancer. This is similar to the findings of a review of studies up to February 2002 that failed to find clear effects of long and shorter chain omega-3 fats on total risk of death, combined cardiovascular events and cancer. In those with advanced cancer and cachexia, omega-3 fatty acids supplements may be of benefit, improving appetite, weight, and quality of life. There is tentative evidence that marine omega-3 polyunsaturated fatty acids reduce the risk of breast cancer but this is not conclusive.
The effect of consumption on prostate cancer is not conclusive. There is a decreased risk with higher blood levels of DPA, but an increased risk of more aggressive prostate cancer with higher blood levels of combined EPA and DHA (found in fatty fish oil).
Cardiovascular disease
Evidence does not support a beneficial role for omega-3 fatty acid supplementation in preventing cardiovascular disease (including myocardial infarction and sudden cardiac death) or stroke.[3][17] However, omega-3 fatty acid supplementation greater than one gram daily for at least a year may be protective against cardiac death, sudden death, and myocardial infarction in people who have a history of cardiovascular disease. No protective effect against the development of stroke or all-cause mortality was seen in this population. Fish oil supplementation has not been shown to benefit revascularization or abnormal heart rhythms and has no effect on heart failure hospital admission rates. Eating a diet high in fish that contain long chain omega-3 fatty acids does appear to decrease the risk of stroke.
Evidence suggests that omega-3 fatty acids modestly lower blood pressure (systolic and diastolic) in people with hypertension and in people with normal blood pressure.[21] Some evidence suggests that people with certain circulatory problems, such as varicose veins, may benefit from the consumption of EPA and DHA, which may stimulate blood circulation and increase the breakdown of fibrin, a protein involved in blood clotting and scar formation. Omega-3 fatty acids reduce blood triglyceride levels but do not significantly change the level of LDL cholesterol or HDL cholesterol in the blood. ALA does not confer the cardiovascular health benefits of EPA and DHA.[26]
Large amounts may increase the risk of hemorrhagic stroke in women; lower amounts are not related to this risk.
Inflammation
Some research suggests that the anti-inflammatory activity of long-chain omega-3 fatty acids may translate into clinical effects. A 2013 systematic review found tentative evidence of benefit. Consumption of omega-3 fatty acids from marine sources lowers markers of inflammation in the blood such as C-reactive protein, interleukin 6, and TNF alpha.
For rheumatoid arthritis (RA), one systematic review found consistent, but modest, evidence for the effect of marine n-3 PUFAs on symptoms such as "joint swelling and pain, duration of morning stiffness, global assessments of pain and disease activity" as well as the use of non-steroidal anti-inflammatory drugs. However, the American College of Rheumatology (ACR) has stated that there may be modest benefit from the use of fish oils, but that it may take months for effects to be seen, and cautions for possible gastrointestinal side effects and the possibility of the supplements containing mercury or vitamin A at toxic levels. Due to the lack of regulations for safety and efficacy, the ACR does not recommend herbal supplements and feels there is an overall lack of "sound scientific evidence" for their use. The National Center for Complementary and Alternative Medicine has concluded that "[n]o dietary supplement has shown clear benefits for RA", but that there is preliminary evidence that fish oil may be beneficial, and called for further study.
Developmental disorders
Although not supported by current scientific evidence as a primary treatment for ADHD, autism spectrum disorders, and other developmental differences, omega-3 fatty acid supplements are being given to children with these conditions.
One meta-analysis concluded that omega-3 fatty acid supplementation demonstrated a modest effect for improving ADHD symptoms. A Cochrane review of PUFA (not necessarily omega-3) supplementation found "there is little evidence that PUFA supplementation provides any benefit for the symptoms of ADHD in children and adolescents", while a different review found "insufficient evidence to draw any conclusion about the use of PUFAs for children with specific learning disorders."[38] Another review concluded that the evidence is inconclusive for the use of omega-3 fatty acids in behavior and non-neurodegenerative neuropsychiatric disorders such ADHD and depression.
Fish oil has only a small benefit on the risk of early birth. A recent meta-analysis of the effect of Omega-3 supplementation during pregnancy did not demonstrate a decrease in the rate of preterm birth or improve outcomes in women with singleton pregnancies with no prior preterm births.
Mental health
There is some evidence that omega-3 fatty acids are related to mental health, including that they may tentatively be useful as an add-on for the treatment of depression associated with bipolar disorder. However, significant benefits due to EPA supplementation were only seen when treating depressive symptoms and not manic symptoms suggesting a link between omega-3 and depressive mood.[44] There is also preliminary evidence that EPA supplementation is helpful in cases of depression. The link between omega-3 and depression has been attributed to the fact that many of the products of the omega-3 synthesis pathway play key roles in regulating inflammation such as prostaglandin E3 which have been linked to depression. This link to inflammation regulation has been supported in both in vitro and in vivo studies as well as in meta-analysis studies. The exact mechanism in which omega-3 acts upon the inflammatory system is still controversial as it was commonly believed to have anti-inflammatory effects.
There is, however, significant difficulty in interpreting the literature due to participant recall and systematic differences in diets. There is also controversy as to the efficacy of omega-3 with many meta-analysis papers finding heterogeneity among results which can be explained mostly by publication bias. A significant correlation between shorter treatment trials was associated with increased omega-3 efficacy for treating depressed symptoms further implicating bias in publication.
There is some evidence to support the claim that omega-3 can help treat anxiety disorder symptoms as well but studies have been limited.
Cognitive aging
Epidemiological studies suggest that consumption of omega-3 fatty acids can reduce the risk of dementia, but evidence of a treatment effect in dementia patients is inconclusive. However, clinical evidence suggests benefits of treatment specifically in patients who show signs of cognitive decline but who are not sufficiently impaired to meet criteria for dementia.