Omega-3 fatty acids (also called ω−3 fatty acids or n−3 fatty acids) refers to a group of three fats called ALA (found in plant oils), EPA, and DHA (both commonly found in marine oils). Common sources of animal omega–3 EPA and DHA fatty acids include fish oils, algal oil, egg oil, squid oils, krill oil and some plant oils contain the omega 3 ALA fatty acid such as seabuckthorn seed and berry oils, flaxseed oil, Sacha Inchi oil, Echium oil, and hemp oil.
Omega-3 fatty acids are vital for normal metabolism but some of the potential health benefits of supplementation are controversial. Omega-3s are considered essential fatty acids, meaning that they cannot be synthesized by the human body -except that mammals have a limited ability, when the diet includes the shorter-chained omega-3 fatty acid ALA (α-linolenic acid, 18 carbons and 3 double bonds), to form the more important long-chain omega−3 fatty acids, EPA (eicosapentaenoic acid, 20 carbons and 5 double bonds) and then from EPA, the most crucial, DHA (docosahexaenoic acid, 22 carbons and 6 double bonds) with even much greater inefficiency. The ability to make the longer-chained omega-3 fatty acids from ALA may also be impaired in aging. In foods exposed to air, unsaturated fatty acids are vulnerable to oxidation and rancidity. And fish are much more efficient than mammals at converting the ALA to the EPA and DHA omega−3 fatty acids.
Omega-3 fatty acids are polyunsaturated fatty acids 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 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 nomenclature of the fatty acid is taken from the location of the first double bond, counted from the methyl end, that is, the omega (ω-) or the n- end.
Supplementation does not appear to be associated with a lower risk of all-cause mortality.
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. High blood levels of omega-3 fatty acid is linked to less prostate cancer.
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. Fish oil supplementation has not been shown to benefit revascularization or arrythmia and has no effect on heart failure admission rates. Eating a diet high in fish that contain long chain omega-3 fatty acids does appear to decrease the risk of stroke.
Large amounts may increase low-density lipoproteins (LDL) (see below), up to 46%, although LDL changes from small to larger, buoyant, less atherogenic particles.
Omega-3 fatty acids also have mild antihypertensive effects. When subjects consumed omega-3 fatty acids from oily fish on a regular basis, their systolic blood pressure was lowered by about 3.5–5.5 mmHg. The 18 carbon α-linolenic acid (ALA) has not been shown to have the same cardiovascular benefits that DHA or EPA may have.
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, increase the breakdown of fibrin, a compound involved in clot and scar formation, and, in addition, may reduce blood pressure. Evidently, omega−3 fatty acids reduce blood triglyceride levels, and regular intake may reduce the risk of secondary and primary heart attack. ALA does not confer the cardiovascular health benefits of EPA and DHA.
Large amounts may increase the risk of hemorrhagic stroke (see below); lower amounts are not related to this risk; 3 grams of total EPA/DHA daily are generally recognized as safe (GRAS) with no increased risk of bleeding involved and many studies used substantially higher doses without major side effects (for example: 4.4 grams EPA/2.2 grams DHA in 2003 study).
Although not confirmed as an approved health claim, current research suggests that the anti-inflammatory activity of long-chain omega−3 fatty acids may translate into clinical effects. For example, there is evidence that rheumatoid arthritis sufferers taking long-chain omega−3 fatty acids from sources such as fish have reduced pain compared to those receiving standard NSAIDs. Some potential benefits have been reported in conditions such as rheumatoid arthritis.
The brain and cognitive abilities
The DHA obtained through the consumption of polyunsaturated fatty acids is positively associated with cognitive and behavioral performance. In addition, DHA is vital for the grey matter structure of the human brain, as well as retinal stimulation and neurotransmission.
Though there is some evidence that omega-3 fatty acids are related to a variety of mental disorders, they may tentatively be useful as an add-on for the treatment of depression associated with bipolar disorder and there is preliminary evidence that EPA supplementation is helpful in cases of depression. There is, however, a significant risk of bias in the literature.
In a letter published October 31, 2000, the United States Food and Drug Administration Center for Food Safety and Applied Nutrition, Office of Nutritional Products, Labeling, and Dietary Supplements noted that known or suspected risks of EPA and DHA consumed in excess of 3 grams per day may include the possibility of:
The most widely available dietary source of EPA and DHA is cold water oily fish, such as salmon, herring, mackerel, anchovies, and sardines. Oils from these fish have a profile of around seven times as much omega−3 as omega−6. Other oily fish, such as tuna, also contain n−3 in somewhat lesser amounts. Consumers of oily fish should be aware of the potential presence of heavy metals and fat-soluble pollutants like PCBs and dioxins, which are known toaccumulate up the food chain. After extensive review, researchers from Harvard’s School of Public Health in the Journal of the American Medical Association(2006) reported that the benefits of fish intake generally far outweigh the potential risks. Although fish is a dietary source of omega−3 fatty acids, fish do not synthesize them; they obtain them from the algae (microalgae in particular) or plankton in their diets.
Marine and freshwater fish oil vary in content of arachidonic acid, EPA and DHA. They also differ in their effects on organ lipids. Not all forms of fish oil may be equally digestible. Of four studies that compare bioavailability of the glyceryl ester form of fish oil vs. the ethyl ester form, two have concluded the natural glyceryl ester form is better, and the other two studies did not find a significant difference. No studies have shown the ethyl ester form to be superior, although it is cheaper to manufacture.
Krill oil is a newly discovered source of omega−3 fatty acids. Various claims are made in support of krill oil as a superior source of omega−3 fatty acids. The effect of krill oil, at a lower dose of EPA + DHA (62.8%), was demonstrated to be similar to that of fish oil.
Flaxseed (or linseed) (Linum usitatissimum) and its oil are perhaps the most widely available botanical source of the omega−3 fatty acid ALA. Flaxseed oil consists of approximately 55% ALA, which makes it six times richer than most fish oils in omega−3 fatty acids. A portion of this is converted by the body to EPA and DHA, though the actual converted percentage may differ between men and women.
100 g of the leaves of Purslane contains 300–400 mg ALA.
Eggs produced by hens fed a diet of greens and insects contain higher levels of omega−3 fatty acids than those produced by chickens fed corn or soybeans. In addition to feeding chickens insects and greens, fish oils may be added to their diets to increase the omega-3 fatty acid concentrations in eggs.
Omega 3 fatty acids are formed in the chloroplasts of green leaves and algae. While seaweeds and algae are the source of omega 3 fatty acids present in fish, grass is the source of omega 3 fatty acids present in grass fed animals. When cattle are taken off omega 3 fatty acid rich grass and shipped to a feedlot to be fattened on omega 3 fatty acid deficient grain, they begin losing their store of this beneficial fat. Each day that an animal spends in the feedlot, the amount of omega 3 fatty acids in its meat is diminished.
The omega−6 to omega−3 ratio of grass-fed beef is about 2:1, making it a more useful source of omega−3 than grain-fed beef, which usually has a ratio of 4:1.
In a 2009 joint study by the USDA and researchers at Clemson University in South Carolina, grass-fed beef was compared with grain-finished beef. The researchers found that grass-fed beef is higher in moisture content, 42.5% lower total lipid content, 54% lower in total fatty acids, 54% higher in beta-carotene, 288% higher in vitamin E (alpha-tocopherol), higher in the B-vitamins thiamin and riboflavin, higher in the minerals calcium, magnesium, and potassium, 193% higher in total omega-3s, 117% higher in CLA (cis-9 trans-11, which is a potential cancer fighter), 90% higher in vaccenic acid (which can be transformed into CLA), lower in the saturated fats linked with heart disease, and has a healthier ratio of omega-6 to omega-3 fatty acids (1.65 vs 4.84). Protein and cholesterol content were equal.
In most countries, commercially available lamb is typically grass-fed, and thus higher in omega−3 than other grain-fed or grain-finished meat sources. In the United States, lamb is often finished (i.e., fattened before slaughter) with grain, resulting in lower omega−3.
The omega-3 content of chicken meat may be enhanced by increasing the animals’ dietary intake of grains high in omega−3, such as flax, chia, and canola.
Kangaroo meat is also a source of omega-3, with fillet and steak containing 74 mg per 100 g of raw meat.
Mammalian brains and eyes
The brains and eyes of mammals are extremely rich in DHA as well as other omega-3 fatty acids. DHA is a major structural component of the mammalian brain, and is in fact the most abundant omega-3 fatty acid in the brain.
Seal oil is a source of EPA, DPA, and DHA. According to Health Canada, it helps to support the development of the brain, eyes and nerves in children up to 12 years of age. However, like all seal products, it is not allowed for import into the European Union.