n −3 fatty acids (popularly referred to as ω−3 fatty acids or omega-3 fatty acids ) are a family of unsaturated fatty acids that have in common a final carbon–carbon double bond in the n −3 position; that is, the third bond from the methyl end of the fatty acid.
Important nutritionally essential n −3 fatty acids are: α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. The human body cannot synthesize n −3 fatty acids de novo , but it can form 20-carbon unsaturated n −3 fatty acids (like EPA) and 22-carbon unsaturated n −3 fatty acids (like DHA) from the eighteen-carbon n −3 fatty acid α-linolenic acid. These conversions occur competitively with n −6 fatty acids, which are essential closely related chemical analogues that are derived from linoleic acid. Both the n −3 α-linolenic acid and n −6 linoleic acid are essential nutrients which must be obtained from food. Synthesis of the longer n −3 fatty acids from linolenic acid within the body is competitively slowed by the n −6 analogues. Thus accumulation of long-chain n −3 fatty acids in tissues is more effective when they are obtained directly from food or when competing amounts of n −6 analogs do not greatly exceed the amounts of n −3.
History
Although omega-3 fatty acids have been known as essential to normal growth and health since the 1930s, awareness of their health benefits has dramatically increased in the past few years. New versions of ethyl esterized omega-3 fatty acids, such as E-EPA and combinations of E-EPA and E-DHA have drawn attention as highly purified and more effective products than the traditional ones. In the USA these novel versions are often called Prescription Omega-3s. In EU they are available as dietary supplements.
The heart-health benefits of the long-chain omega-3 fatty acids — DHA and EPA omega-3 — are the best known. These benefits were discovered in the 1970s by researchers studying the Greenland Eskimos. The Greenland Eskimos consumed large amounts of fat from seafood, but displayed virtually no cardiovascular disease. The high level of omega-3 fatty acids consumed by the Eskimos reduced triglycerides, heart rate, blood pressure, and atherosclerosis.
On September 8, 2004, the U.S. Food and Drug Administration gave "qualified health claim" status to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) n −3 fatty acids, stating that "supportive but not conclusive research shows that consumption of EPA and DHA fatty acids may reduce the risk of coronary heart disease." This updated and modified their health risk advice letter of 2001 (see below). Currently regulatory agencies do not accept that there is sufficient evidence for any of the other suggested benefits of DHA and EPA other than for cardiovascular health, and further claims should be treated with caution.
The Canadian Government has recognized the importance of DHA omega-3 and permits the following biological role claim for DHA: "DHA, an omega-3 fatty acid, supports the normal development of the brain, eyes and nerves."
As the importance of omega-3 fatty acids to health has received increasing awareness, the number of food products enriched in omega-3 fatty acids has increased. Many companies add fish oil or flax oil into their final product to enrich it in omega-3 fatty acids. Some animal products, such as milk and eggs, can be naturally enriched for omega-3 fatty acids by feeding the animals a diet that is rich in omega-3 fatty acids.
Chemistry
The term n −3 (also called ω−3 or omega-3 ) signifies that the first double bond exists as the third carbon-carbon bond from the terminal methyl end ( n ) of the carbon chain.
n −3 fatty acids which are important in human nutrition are: α-linolenic acid (18:3, n −3; ALA), eicosapentaenoic acid (20:5, n −3; EPA), and docosahexaenoic acid (22:6, n −3; DHA). These three polyunsaturates have either 3, 5 or 6 double bonds in a carbon chain of 18, 20 or 22 carbon atoms, respectively. All double bonds are in the cis -configuration, i.e. the two hydrogen atoms are on the same side of the double bond.
Most naturally-produced fatty acids (created or transformed in animalia or plant cells with an even number of carbon in chains) are in cis -configuration where they are more easily transformable. The trans -configuration results in much more stable chains that are very difficult to further break or transform, forming longer chains that aggregate in tissues and lacking the necessary hydrophilic properties. This trans -configuration can be the result of the transformation in alkaline solutions, or of the action of some bacterias that are shortening the carbonic chains. Natural transforms in plant or animal cells more rarely affect the last n −3 group itself. However, n −3 compounds are still more fragile than n −6 because the last double bond is geometrically and electrically more exposed, notably in the natural cis configuration.
List of n −3 fatty acids
This table lists several different names for the most common n −3 fatty acids found in nature.
Biological significances
A 1992 article by biochemist William E.M. Lands provides an overview of the research into n −3 fatty acids, and is the basis of this section.
The 'essential' fatty acids were given their name when researchers found that they were essential to normal growth in young children and animals. (Note that the modern definition of 'essential' is more strict.) A small amount of n −3 in the diet (~1% of total calories) enabled normal growth, and increasing the amount had little to no additional effect on growth.
Likewise, researchers found that n −6 fatty acids (such as γ-linolenic acid and arachidonic acid) play a similar role in normal growth. However, they also found that n −6 was "better" at supporting dermal integrity, renal function, and parturition. These preliminary findings led researchers to concentrate their studies on n −6, and it was only in recent decades that n −3 has become of interest.
In 1963 it was discovered that the n −6 arachidonic acid was converted by the body into pro-inflammatory agents called prostaglandins. By 1979 more of what are now known as eicosanoids were discovered: thromboxanes, prostacyclins and the leukotrienes. The eicosanoids, which have important biological functions, typically have a short active lifetime in the body, starting with synthesis from fatty acids and ending with metabolism by enzymes. However, if the rate of synthesis exceeds the rate of metabolism, the excess eicosanoids may have deleterious effects. Researchers found that n −3 is also converted into eicosanoids, but at a much slower rate. Eicosanoids made from n −3 fats are often referred to as anti-inflammatory, but in fact they are just less pro-inflammatory than those made from n −6 fats. If both n −3 and n −6 are present, they will "compete" to be transformed, so the ratio of n −3: n −6 directly affects the type of eicosanoids that are produced.
This competition was recognized as important when it was found that thromboxane is a factor in the clumping of platelets, which leads to thrombosis. The leukotrienes were similarly found to be important in immune/inflammatory-system response, and therefore relevant to arthritis, lupus, and asthma. These discoveries led to greater interest in finding ways to control the synthesis of n −6 eicosanoids. The simplest way would be by consuming more n −3 and fewer n −6 fatty acids.
In 1982 Dr. Charles Serhan's group at Harvard discovered that the omega-3 fatty acid EPA forms in the body potent antiinflamatory nanomolecules, called resolvins. Later his team found that omega-3s also turn into other antiinflammatory molecules called maresins and omega-3-oxylipins, which partly explain the versatile health effects of fish oil.
Health benefits
The 18 carbon α-linolenic acid has not been shown to have the same cardiovascular benefits as DHA or EPA. Currently there are many products on the market which claim to contain health promoting 'omega 3', but contain only α-linolenic acid (ALA), not EPA or DHA. These products contain mainly higher plant oils and must be converted by the body to create DHA and therefore considered less efficient. DHA and EPA are made by microalgae that live in seawater. These are then consumed by fish and accumulate to high levels in their internal organs. If a person has ethical concerns about killing fish, or is concerned about mercury and oceanborne contaminants in fish, DHA can be produced directly from microalgae as a vegetarian source. People with certain circulatory problems, such as varicose veins, benefit from such supplements containing EPA and DHA which stimulate blood circulation,
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