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Omega-3 fatty acid
Omega-3 fatty acids are polyunsaturated fatty acids which have a C=C double bond in the ω-3 position. (See Nomenclature for terms and discussion of ω (omega) nomenclature.)
Important omega-3 fatty acids in human nutrition are: α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). For a more complete list see List of omega-3 fatty acids. The human body cannot synthesize omega-3 fatty acids from scratch, but can synthesize all the other necessary omega-3 fatty acids from the omega-3 fatty acid Alpha-linolenic acid. Therefore Alpha-linolenic acid is an Essential nutrient which must be obtained from food, and the other omega-3 fatty acids which can be either synthesized from it within the body or obtained from food are sometimes also referred to as essential nutrients.
Chemistry
For detail on omega (ω) nomenclature and numbering, see EFA Nomenclature.
Chemical structure of alpha-linolenic acid (ALA), an essential omega-3 fatty acid, (18:3Δ9c,12c,15c). Although chemists count from the carbonyl carbon (blue numbering), physiologists count from the omega (ω) carbon (red numbering). Note that from the omega end (diagram right), the first double bond appears as the third carbon-carbon bond (line segment), hence the name "omega-3"
The term omega-3 (aka "n-3", "ω-3") signifies that the first double bond exists as the third carbon-carbon bond from the terminal methyl end (ω) of the carbon chain.
Omega-3 fatty acids which are important in human nutrition are: alpha-linolenic acid (18:3, ALA), eicosapentaenoic acid (20:5, EPA), and docosahexaenoic acid (22:6, 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.
Biological significance
The biological effects of the ω-3 fatty acids are largely mediated by their interactions with the ω-6 fatty acids, see Essential fatty acid interactions for detail.
A 1992 paper by biochemist William E.M. Lands provides an overview of the research into omega-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 omega-3 in the diet (~1% of total calories) enabled normal growth, and increasing the amount had little to no additional benefit.
Likewise, researchers found that omega-6 fatty acids (such as γ(gamma)-linolenic acid and arachidonic acid) play a similar role in normal growth. However they also found that omega-6 is "better" at supporting dermal integrity, renal function, and parturition. This led researchers to concentrate study on omega-6, and it is only in recent decades that omega-3 has become of interest.
In 1963 it was discovered that the omega-6 arachidonic acid is 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 omega-3 is also converted into eicosanoids, but at a much slower rate. Eicosanoids made from omega-3 fats often have opposing functions to those made from omega-6 fats (ie, anti-inflammatory rather than inflammatory). If both omega-3 and omega-6 are present, they will "compete" to be transformed, so the ratio of omega-3:omega-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 omega-6 eicosanoids. The simplest way is by consuming more omega-3 and fewer omega-6 fatty acids.
Cardiovascular health benefits
On September 8, 2004, the U.S. Food and Drug Administration gave "qualified health claim" status to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) omega-3 fatty acids, stating that "supportive but not conclusive research shows that consumption of EPA and DHA omega-3 fatty acids may reduce the risk of coronary heart disease."
A 2006 report in the Journal of the American Medical Association concluded that their review of literature covering cohorts from many countries with a wide variety of demographic characteristics failed to demonstrate a link between omega-3 fatty acids and cancer prevention. This is similar to the findings of a review by the British Medical Journal of studies up to February 2002 that failed to find clear effects of long and shorter chain omega-3 fats on total mortality, combined cardiovascular events and cancer.
In April 2006, a team led by Lee Hooper at the University of East Anglia in Norwich, UK, published a review of almost 100 separate studies into omega-3 fatty acids, found in abundance in oily fish. It concluded that they do not have a significant protective effect against cardiovascular disease.
The above stands in stark contrast with two different reviews also performed in 2006 by the American Journal of Clinical Nutrition and a second JAMA review that both indicated decreases in total mortality and cardiovascular incidents (i.e. myocardial infarctions) associated with the regular consumption of fish and fish oil supplements. Of particular importance is that no or very few complications were documented.
Research in 2005-06 has suggested that the in-vitro anti-inflammatory activity of omega-3 acids translates into clinical benefits. Cohorts of neck pain patients and of rheumatoid arthritis sufferers have demonstrated benefits comparable to those receiving standard NSAIDs.
Those who follow a Mediterranean-style diet tend to have higher HDL ("good") cholesterol levels. Similar to those who follow a Mediterranean diet, Arctic-dwelling Inuit - who consume high amounts of omega-3 fatty acids from fatty fish - also tend to have increased HDL cholesterol and decreased triglycerides (fatty material that circulates in the blood). In addition, fish oil supplements containing EPA and DHA have been shown to reduce LDL ("bad") cholesterol and triglycerides. Finally, walnuts (which are rich in ALA) have been shown to lower total cholesterol and triglycerides in people with high cholesterol.
Omega-3 fatty acids and Autism
According to an internet survey approximately 30% of parents use omega-3 supplements as a therapy for autistic children. There are currently only a few studies on the effectiveness of essential fatty acid supplementation as a treatment of autism and none of these have been well-controlled. Bell and colleagues reported that parents of 18 autistic children who had been supplemented with fish oil for six months described improvements in overall health, cognition, sleep patterns, social interaction, and eye contact. Another case report found that an autistic child given 540 mg of EPA per day over a for week period experienced a complete elimination of his previous anxiety about everyday events as reported by his parents and clinician.
Daily values
As macronutrients, fats are not assigned recommended daily allowances. Macronutrients have AI (Acceptable Intake) and AMDR (Acceptable Macronutrient Distribution Range) instead of RDAs. The AI for n-3 is 1.6 grams/day for men and 1.1 grams/day for women while the AMDR is 0.6% to 1.2% of total energy.
"A growing body of literature suggests that higher intakes of α-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) may afford some degree of protection against coronary heart disease. Because the physiological potency of EPA and DHA is much greater than that for α-linolenic acid, it is not possible to estimate one AMDR for all n-3 fatty acids. Approximately 10 percent of the AMDR can be consumed as EPA and/or DHA.
There was insufficient evidence as of 2005 to set a UL (upper intake limit) for n-3 fatty acids.
Researchers believe the ideal omega-6 intake should be no more than 4-5 times that of our omega-3 intake. The National Institutes of Health recently published recommended daily intakes of fatty acids, specific recommendations include 650 mg of EPA and DHA, 2.22 g/day of alpha-linolenic acid and 4.44 g/day of linoleic acid.
The greatest risk of fish oil omega-3 supplementation is heavy metal poisoning by the body's accumulation of traces of heavy metals, in particular mercury, lead, nickel, arsenic and cadmium as well as other contaminants (PCBs, furans, dioxins), which may be found especially in less refined fish oil supplements.[citation needed] For these reasons, the FDA recommends that total dietary intake of omega-3 fatty acids from fish be limited to 3 grams per day, of which no more than 2 grams per day are from nutritional supplements.
Historically, the Council for Responsible Nutrition (CRN) and the World Health Organization (WHO) have published acceptable standards regarding contaminants in fish oil. The most stringent current standard is the International Fish Oils Standard (IFOS). Fish oils that typically make this highest grade are those that are molecularly distilled under vacuum, and have virtually no measurable level of contaminants (measured parts per billion and parts per trillion).
Fish
The most widely available source of EPA and DHA is cold water oily fish such as salmon, herring, mackerel, anchovies and sardines. The oil 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 omega-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 may accumulate up the food chain. Some supplement manufacturers remove heavy metals and other contaminants from the oil through various means, such as molecular distillation (see above), which increases purity, potency and safety.
Even some forms of fish oil may not be optimally digestible. Of four studies that compare
bioavailability of the triglyceride form of fish oil vs. the ester form, two have concluded that the natural triglyceride form is better, and the other two studies did not find a significant difference. No studies have shown the ester form to be superior although it is cheaper to manufacture.
Although fish is a dietary source of omega-3 fatty acids, fish do not synthesize them; they obtain them from the algae in their diet. For this reason, there is often a significant difference in EPA and DHA concentrations in farmed vs wild caught fish.
Flax
Flax (aka linseed) (Linum usitatissimum) and its oil are perhaps the most widely available botanical sources of omega-3. Flaxseed oil consists of ca. 55% ALA (alpha-linolenic acid). Flax, like chia, contains approximately three times as much omega-3 as omega-6.
15g of flaxseed oil provides ca. 8g of ALA, which is converted in the body to EPA and then DHA at an efficiency of (5%-10%), and (2%-5%) respectively.
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