Omega-3 fatty acid

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Omega-3 fatty acids are polyunsaturated fatty acids found in oil from oily fish and vegetable sources such as the seeds of chia, perilla, flax, walnuts, purslane, lingonberry, seabuckthorn, and hemp. Omega-3 fatty acids are classified as essential because they cannot be synthesized in the body; they must be obtained from food.

Important omega-3 fatty acids in human nutrition are: α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).

Contents

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Chemistry

For detail on omega (ω) nomenclature and numbering, see EFA Nomenclature.

Image:ALAnumbering.png
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.

Structurally, omega-3 fatty acids are helically twisted, because every cis- double bond, separated by a methylene group, changes the carbon chain's direction. This configuration may explain a host of biological phenomena observed in structures that are rich in polyunsaturated fatty acids, especially the lipid bilayer of the cell membrane.

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Daily values

As fats are macronutrients they 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 1.2 grams/day for women [1] while the AMDR is 0.6% to 1.2% of total energy [2].

"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." [3]

There was insufficient evidence as of 2005 to set a UL (upper intake limit) for n-3 fatty acids.[4]

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 and other contaminants (PCBs, furans, dioxins), which may be found in less refined fish oil supplements. For these reasons, the FDA recommends that total dietary intake of omega-3 fatty acids be limited to 3 grams per day, of which no more than 2 grams per day are from nutritional supplements.[5]

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 the (highest) grade oil are those that are molecularly distilled under vaccuum, and have virtually no measurable level of contaminants (measured parts per billion and parts per trillion).

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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. LandsTemplate:Ref label 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. If both omega-3 and omega-6 are present, they will "compete" to be transformed, so the amount of omega-3 present is directly related to a decrease in the rate of eicosanoid production.

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; one way being, of course, the consumption of greater amounts of omega-3.

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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."Template:Ref label

A January 25, 2006 report in the Journal of the American Medical Association concludes:

A large body of literature spanning numerous cohorts from many countries and with different demographic characteristics does not provide evidence to suggest a significant association between omega-3 fatty acids and cancer incidence. Dietary supplementation with omega-3 fatty acids is unlikely to prevent cancer.

An article by British Medical Journal in April 1, 2006, based on a systematic review of multiple studies concludes: Long chain and shorter chain omega 3 fats do not have a clear effect on total mortality, combined cardiovascular events, or cancer.

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Health risks

In its letter dated from the 1970s entitled Letter Regarding Dietary Supplement Health Claim for omega-3 Fatty Acids and Coronary Heart Disease the U. S. Food and Drug Administration Center for Food Safety and Applied Nutrition, Office of Nutritional Products, Labeling, and Dietary Supplements noted that the known or suspected risks of omega-3 fatty acids may include:

  • Increased bleeding can occur if overused (normally over 3 grams per day)
  • The possibility of hemorrhagic stroke,
  • Oxidation of omega-3 fatty acids forming biologically active oxidation products,
  • Increased levels of low density lipoproteins (LDL) cholesterol or apoproteins associated with LDL cholesterol among diabetics and hyperlipidemics,
  • Reduced glycemic control among diabetics.
  • Suppression of immune and inflammation responses, and consequently, to decreased resistance to infections and increased susceptibility to opportunistic bacteria
  • Inducing Omega-6 deficiency, which could worsen the condition for which it is used.
  • A study suggest that those with a history of ventricular tachycardia or ventricular fibrillation should limit their intake of fish oil since it might be proarrhythmic in some patients.

A significant potential risk is the possibility of vitamin poisoning from taking large doses of supplements which contain large quantities of vitamins (particularly vitamin A) in addition to omega-3 fatty acids. For this reason, the primary source of omega-3, if taken as a supplement, should be from fish body oil and not from a fish liver based oil.

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Research frontiers

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Psychological disorders

Template:References Omega-3's are definitively known to have membrane-enhancing capabilities in brain cells.Template:Fact. One medical explanation is that omega-3's play a role in the fortification of the myelin sheaths. Not coincidentally, omega-3 fatty acids comprise approximately eight percent of the average human brain according to the late Dr. David Horrobin, a pioneer in fatty acid researchTemplate:Fact. Ralph Holman of the University of Minnesota, another major researcher in studying essential fatty acids, surmised how omega-3 components are analogous to the human brain by stating that "DHA is structure, EPA is function."Template:Fact

Consequently, the past decade of omega-3 fatty acid research has procured some Western interest in omega-3's as being a legitimate 'brain food.' Still, recent claims that intelligence quota and verbal reasoning skills are increased on account of omega-3's consumed by pregnant mothers remain unreliable and controversial. An even more significant locus of research, however, lies in the role of omega-3's as a non-prescription treatment for certain psychiatric and mental diagnoses and has become a topic of much research and speculation.

Dr. Andrew Stoll and his colleagues at Harvard University were among the first to accomplish the testing of such hypothetical research through a 1999 double-blind placebo study done with thirty patients diagnosed with bi-polar disorder. This experiment was designed for nine months. He rendered his results by affording olive oil capsules to fifteen placebo control subjects and nine grams of pharmaceutical-quality EPA and DHA supplements to the fifteen others. In doing so he was able to make the general distinction between the placebo group failing to improve while the Omega-3 group experienced a noticeable degree of recovery. Though Stoll believes that the 1999 experiment was not as optimal as it could have been and has accordingly pursued further research, the foundation has been laid for more researchers to explore the theoretical association between absorbed omega-3's and signal transduction inhibition in the brain.Template:Fact

Should enough research that is currently underway come to confirm the legitimacy of this association, then a debate and reassessment will of course be necessitated between Omega-3's and such over-the-counter prescription bipolar treatments as Eskalith, or brand Lithium Carbonate. Some physicians and psychiatric specialists in the United States do allow willing bipolar patients to use Omega-3 supplements as complementary or conditional treatments.Template:Fact Omega-3's, unlike Eskalith, are less expensive and do not commonly induce such side effects as diarrhea, drowsiness, and fatigue. More time is needed, however, for Omega-3-induced signal transduction inhibition in the brain to become a thoroughly proven association. Until then, the present application of what some may consider speculation remains verifiable.

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Dietary sources

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Fish

Image:Fish high in omega-3.jpg

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.[6] 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 as yet have shown the ester form to be superior although it is cheaper to manufacture. Template:Ref labelTemplate:Ref label

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.[7]

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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)Template:Ref label 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. Template:Ref label

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Botanical sources

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Eggs

A P Simopoulos[8] found that chickens which wandered freely in the countryside, eating rich sources of omega-3 such as purslane, produced eggs with 20 times more omega-3 than eggs of grain-fed chickens. [9] [10]

In the early 21st century, improving the ALA content of feed provided to egg-laying hens have increased the DHA concentration of the resulting eggs. Unlike traditional eggs which are very high in pro-inflammatory omega-6, the increased DHA eggs produce a healthier omega-6 to omega-3 ratio. Dr. Bruce Holub is a Canadian research scientist at the University of Guelph, who has been instrumental in the development of increased DHA content in eggs.

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Other sources

Krill, a small, shrimp-like zooplankton, is a less-known source of omega-3. A patented process has been developped at Université de Sherbrooke to extract its oil and exclusive rights have been acquired by the Canadian company Neptune Technologies & Bioressources [11]. This "krill oil", commercialized as "Neptune Krill Oil", naturally contains omega-3 fatty acids, such as EPA and DHA, and antioxidants, such as astaxanthin and a novel flavonoid (the first from an animal source), both carried by phospholipids. It is these phospholipids that bring a higher bioavailability of the omega-3s than triglycerides, because triglycerides have to be metabolized by the organism to be attached to phospholipids and enter the cell wheter it is not the case with omega-3 already attached to phospholipids (see that article [12]). Krill being at the bottom of the food chain, heavy metals levels are very low, even without molecular distillation.

Meat from grass-fed animals is often higher in omega-3 than meat from the corresponding grain-fed animal. [13][14] The n6:n3 ratio of grass-fed beef is about 2:1, making it a more useful source of omega-3 than grain-fed beef (ca. 4:1)[15]. Commercially available lamb is almost always grass-fed, and subsequently higher in omega-3 than other common meat sources.

Milk and cheese from grass-fed cows may be good sources of omega-3. [16] Milk from non-organic, grain-fed cows may contain about two-thirds less omega-3. One UK study showed that half a pint of milk provides 10% of the recommended daily intake (RDI) of ALA, while a "matchbox sized piece of organic cheese will give you up to 88%". [17]

The microalgae Crypthecodinium cohnii and Schizochytrium are rich sources of DHA (22:6 ω-3), produced commercially in bioreactors. Oil from brown algae (kelp) is a source of EPA.

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The omega-6 to omega-3 ratio

Clinical studiesTemplate:Ref labelTemplate:Ref labelTemplate:Ref label indicate that the ingested ratio of omega-6 to omega-3 (especially Linoleic vs Alpha Linolenic) fatty acids are important to maintaining cardiovascular health.

Both omega-3 and omega-6 fatty acids are essential, i.e. humans must consume them in the diet. Omega-3 and omega-6 compete for the same metabolic enzymes, thus the omega-6:omega-3 ratio will significantly influence the ratio of the ensuing eicosanoids (hormones), (e.g. prostaglandins, leukotrienes, thromboxanes etc.), and will alter the body's metabolic function. Metabolites of omega-6 are significantly more inflammatory (esp. arachidonic acid) than those of omega-3. This necessitates that omega-3 and omega-6 be consumed in a balanced proportion; the ideal ratio of omega-6:omega-3 being from 3:1 to 5:1. Studies suggest that the evolutionary human diet, rich in seafood, nuts and other sources of omega-3, may have provided such a ratio. Template:Ref label

Simopoulos, et alTemplate:Ref label recommend daily intakes of three omega-3 forms: 650 mg of EPA and DHA, and 2.22 g of ALA, and one omega-6 form: 4.44 g of LA. This translates to a 3:2 omega-6 to omega-3 ratio. (i.e. 1.5:1)

Typical Western diets provide ratios of between 10:1 and 30:1 - i.e., dramatically skewed toward omega-6. Examples of oils rich in omega-6 compared to omega-3 are soy, sunflower, cottonseed, canola, peanut, grapeseed oil and corn oil. There is no compelling reason that a healthy person consuming a typical western diet should ever need to supplement their diet with omega-6.

Sources: [18], [19]

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See also

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Notes and references

  1. Template:Note label FDA press release: FDA Announces Qualified Health Claims for omega-3 Fatty Acids
  2. Template:Note labelTemplate:Note label W.E.M. Lands (1992). Biochemistry and physiology of n-3 fatty acids. FASEB J 6: 2530-2536.
  3. Template:Note labelTemplate:Note labelTemplate:Note labelTemplate:Note labelTemplate:Note labelTemplate:Note labelTemplate:Note labelTemplate:Note labelSeed Oil Fatty Acids - SOFA Database Retrieval
  4. Template:Note labelTemplate:Note labelSimopoulos AP, Cleland LG (eds): "omega-6/omega-3 Essential Fatty Acid Ratio: The Scientific Evidence." World Rev Nutr Diet. Basel, Karger, 2003, Vol 92.
  5. Template:Note labelLawson LD and Hughes BG. "Absorption of EPA and DHA from fish oil triacylglycerols or fish oil esters co-ingested with a high fat meal." Biochem. Biophys Res. Commun., 156:960-963, 1988.
  6. Template:Note labelBeckermann B, et al. "Comparative bioavailability of EPA and DHA from triglycerides, free fatty acids and ethyl esters in volunteers." Arzneimittelforschung, 40: 700-704, 1990.
  7. Template:Note label Vegetarian Society, Infromation Sheet on Omega 3 fats
  8. Template:Note label Okuyama H. High n-6 to n-3 ratio of dietary fatty acids rather than serum cholesterol as a major risk factor for coronary heart disease. Eur J Lipid Sci Technol. 2001; 103:418-22.
  9. Template:Note labelSimopoulos AP, Leaf A, Salem Jr N. Statement on the essentiality of and recommended dietary intakes for omega-6 and omega-3 fatty acids. Prostaglandins, Leukotrienes and Essential Fatty Acids 2000;63:119-121.
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Further reading

  • Erasmus U. Fats that heal, fats that kill. 3rd ed. Burnaby (BC): Alive Books; 1993.
  • Chow, Ching Kuang. Fatty Acids in Foods and Their Health Implications. Routledge Publishing. New York, New York. 2001.
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External links

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