Omega-3 fatty acid

Omega-3 fatty acids (also called ω-3 fatty acids or n-3 fatty acids^[1]) 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 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 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.

The three types of omega-3 fatty acids involved in human physiology are 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, egg oil, squid oils, krill oil, while some plant oils contain the omega 3 ALA fatty acid such as seabuckthorn seed and berry oils, clary sage seed oil, algal oil, 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 greater inefficiency. The ability to make the longer-chained omega-3 fatty acids from ALA may also be impaired in aging.^[2][3] In foods exposed to air, unsaturated fatty acids are vulnerable to oxidation and rancidity.^[4] Fish are much more efficient than mammals at converting the ALA to the EPA and DHA omega-3 fatty acids.^{[citation needed]}

Health effects

Supplementation does not appear to be associated with a lower risk of all-cause mortality.^[5]

Cancer

The evidence linking the consumption of fish to the risk of cancer is poor.^[6] Supplementation with omega-3 fatty acids does not appear to affect this either.^[7]

A 2006 review concluded that there was no link between omega-3 fatty acids consumption and cancer.^[8] 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.^[9][10] In those with advanced cancer and cachexia, omega-3 fatty acids supplements may be of benefit, improving appetite, weight, and quality of life.^[11] There is tentative evidence that marine omega-3 polyunsaturated fatty acids reduce the risk of breast cancer but this is not conclusive.^[12][13]

The effect of consumption on prostate cancer is not conclusive.^[13] 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).^[14]

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.^[5][15] Fish oil supplementation has not been shown to benefit revascularization or arrythmia and has no effect on heart failure admission rates.^[16] Eating a diet high in fish that contain long chain omega-3 fatty acids does appear to decrease the risk of stroke.^[17]

Large amounts may increase low-density lipoproteins (LDL) (see below), up to 46%, although LDL changes from small to larger, buoyant, less atherogenic particles.^[18]

Omega-3 fatty acids may have a modest effect on systolic blood pressure, though studies have produced inconsistent results.^[19] The 18 carbon α-linolenic acid (ALA) has not been shown to have the same cardiovascular benefits that DHA or EPA may have.^[20]

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.^[21][22] Evidently, omega-3 fatty acids reduce blood triglyceride levels,^[23][24][25] and regular intake may reduce the risk of secondary and primary heart attack.^[26] ALA does not confer the cardiovascular health benefits of EPA and DHA.^[27]

Large amounts may increase the risk of hemorrhagic stroke; lower amounts are not related to this risk.^[28]

Inflammation

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.^[29]

For rheumatoid arthritis (RA), one systematic review found a 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.^[30] 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.^[31] The National Center for Complementary and Alternative Medicine has concluded there is "no 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.^[32]

Developmental disorders

Although not supported by current scientific evidence as a primary treatment for ADHD, autism spectrum disorders, and other developmental differences,^[33][34] omega-3 fatty acids have gained popularity for children with these conditions.^[33]

A Cochrane review found "there is little evidence that PUFA supplementation provides any benefit for the symptoms of ADHD in children and adolescents",^[35] while a different review found "insufficient evidence to draw any conclusion about the use of PUFAs for children with specific learning disorders."^[36] 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.^[37] A different systematic review concluded there is a modest effect for omega-3 fatty acids in ADHD, but its effect is less than more traditional pharmaceutical medications.^[38]

Fish oil has only a small benefit on the risk of early birth.^[39][40]

Mental health

There is some evidence that omega-3 fatty acids are related to mental health,^[41] including that they may tentatively be useful as an add-on for the treatment of depression associated with bipolar disorder^[42] and there is preliminary evidence that EPA supplementation is helpful in cases of depression.^[43] There is, however, a significant difficulty in interpreting the literature due to participant recall and systematic differences in diets.^[44]

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.^[45] 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.^[46]

Chemistry

Chemical structure of alpha-linolenic acid (ALA), an essential omega-3 fatty acid, (18:3Δ9c,12c,15c, which means a chain of 18 carbons with 3 double bonds on carbons numbered 9, 12, and 15). Although chemists count from the carbonyl carbon (blue numbering), biologists count from the n (ω) carbon (red numbering). Note that, from the n end (diagram right), the first double bond appears as the third carbon-carbon bond (line segment), hence the name "n-3". This is explained by the fact that the n end is almost never changed during physiological transformations in the human body, as it is more energy-stable, and other compounds can be synthesized from the other carbonyl end, for example in glycerides, or from double bonds in the middle of the chain.

Chemical structure of eicosapentaenoic acid (EPA).

Chemical structure of docosahexaenoic acid (DHA).

Omega-3 fatty acids that are important in human physiology 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. As with most naturally-produced fatty acids, all double bonds are in the cis-configuration, in other words, the two hydrogen atoms are on the same side of the double bond; and the double bonds are interrupted by methylene bridges (-CH
2-), so that there are two single bonds between each pair of adjacent double bonds.^{[citation needed]}

List of omega-3 fatty acids

This table lists several different names for the most common omega-3 fatty acids found in nature.

Mechanism of action

The 'essential' fatty acids were given their name when researchers found that they are essential to normal growth in young children and animals, though 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 effect on growth.^[47]

Likewise, researchers found that omega-6 fatty acids (such as γ-linolenic acid and arachidonic acid) play a similar role in normal growth. However, they also found that omega-6 was "better" at supporting dermal integrity, renal function, and parturition. These preliminary findings led researchers to concentrate their studies on omega-6, and it is only in recent decades that omega-3 has become of interest.^[47]

In 1964, it was discovered that enzymes found in sheep tissues convert omega-6 arachidonic acid into the inflammatory agent called prostaglandin E₂,^[48] which both causes the sensation of pain and expedites healing and immune response in traumatized and infected tissues.^[47] By 1979, more of what are now known as eicosanoids were discovered: thromboxanes, prostacyclins, and the leukotrienes.^[47] 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.^[47] Researchers found that certain omega-3 fatty acids are also converted into eicosanoids, but at a much slower rate. Eicosanoids made from omega-3 fatty acids are often referred to as anti-inflammatory, but in fact they are just less inflammatory than those made from omega-6 fats. If both omega-3 and omega-6 fatty acids are present, they will "compete" to be transformed,^[47] so the ratio of long-chain omega-3:omega-6 fatty acids directly affects the type of eicosanoids that are produced.^[47]

This competition was recognized as important when it was found that thromboxane is a factor in the clumping of platelets, which can both cause death by thrombosis and prevent death by bleeding. Likewise, the leukotrienes were found to be important in immune/inflammatory-system response, and therefore relevant to arthritis, lupus, asthma, and recovery from infections. These discoveries led to greater interest in finding ways to control the synthesis of omega-6 eicosanoids. The simplest way would be by consuming more omega-3 and fewer omega-6 fatty acids.^[47]

They are required during the prenatal period for the formation of synapses and cell membranes. These processes are also essential in postnatal human development for injury response of the central nervous system and retinal stimulation.^[49]

Interconversion

Conversion efficiency of ALA to EPA and DHA

Men's bodies convert short-chain omega-3 fatty acids to long-chain forms (EPA, DHA) with an efficiency below 5%.^[50][51] The omega-3 conversion efficiency is greater in women, but less well-studied.^[52]

These conversions occur competitively with omega-6 fatty acids, which are essential closely related chemical analogues that are derived from linoleic acid. Both the omega-3 α-linolenic acid and omega-6 linoleic acid must be obtained from food. Synthesis of the longer omega-3 fatty acids from linolenic acid within the body is competitively slowed by the omega-6 analogues. Thus, accumulation of long-chain omega-3 fatty acids in tissues is more effective when they are obtained directly from food or when competing amounts of omega-6 analogs do not greatly exceed the amounts of omega-3.^{[citation needed]}

The conversion of ALA to EPA and further to DHA in humans has been reported to be limited, but varies with individuals.^[53][54] Women have higher ALA conversion efficiency than men, which is presumed ^{[citation needed]} to be due to the lower rate of use of dietary ALA for beta-oxidation. This suggests that biological engineering of ALA conversion efficiency is possible. Goyens et al. argue that it is the absolute amount of ALA, rather than the ratio of omega-3 and omega-6 fatty acids, that controls the conversion efficiency.^[55]

The omega-6 to omega-3 ratio

Some older clinical studies^[47][56] indicate that the ingested ratio of omega-6 to omega-3 (especially linoleic vs alpha-linolenic) fatty acids is important to maintaining cardiovascular health. However, three studies published in 2005, 2007 and 2008, including a randomized controlled trial, found that while omega-3 polyunsaturated fatty acids are extremely beneficial in preventing heart disease in humans, the levels of omega-6 polyunsaturated fatty acids (and therefore the ratios) did not matter.^[57][58][59]

Both omega-6 and omega-3 fatty acids are essential; i.e., humans must consume them in their diet. Omega-6 and omega-3 eighteen-carbon polyunsaturated fatty acids compete for the same metabolic enzymes, thus the omega-6:omega-3 ratio of ingested fatty acids has significant influence on the ratio and rate of production of eicosanoids, a group of hormones intimately involved in the body's inflammatory and homeostatic processes which includes the prostaglandins, leukotrienes, and thromboxanes, among others. Altering this ratio can change the body's metabolic and inflammatory state.^[60] In general, grass-fed animals accumulate more omega-3 than do grain-fed animals, which accumulate relatively more omega-6.^[61] Metabolites of omega-6 are more inflammatory (esp. arachidonic acid) than those of omega-3. This necessitates that omega-6 and omega-3 be consumed in a balanced proportion; healthy ratios of omega-6:omega-3, according to some authors, range from 1:1 to 1:4 (an individual needs more omega-3 than omega-6).^[62] Other authors believe that ratio 4:1 (when the amount of omega-6 is only 4 times greater than that of omega-3) is already healthy.^[63][64] Studies suggest the evolutionary human diet, rich in game animals, seafood, and other sources of omega-3, may have provided such a ratio.^[65][66]

Typical Western diets provide ratios of between 10:1 and 30:1 (i.e., dramatically higher levels of omega-6 than omega-3).^[67] The ratios of omega-6 to omega-3 fatty acids in some common vegetable oils are: canola 2:1, hemp 2-3:1,^[68] soybean 7:1, olive 3–13:1, sunflower (no omega-3), flax 1:3,^[69] cottonseed (almost no omega-3), peanut (no omega-3), grapeseed oil (almost no omega-3) and corn oil 46:1 ratio of omega-6 to omega-3.^[70]

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 since the 1980s.^[71][72]

On September 8, 2004, the U.S. Food and Drug Administration gave "qualified health claim" status to EPA and DHA omega-3 fatty acids, stating, "supportive but not conclusive research shows that consumption of EPA and DHA [omega-3] fatty acids may reduce the risk of coronary heart disease."^[73] This updated and modified their health risk advice letter of 2001 (see below). As of this writing, regulatory agencies do not accept that there is sufficient evidence for any of the 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."^[74]

Dietary sources

Daily values

As macronutrients, fats are not assigned Dietary Reference Intakes. Macronutrients have acceptable intake (AI) levels and acceptable macronutrient distribution ranges (AMDRs) instead of RDAs. The AI for omega-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.^[78]

A growing body of literature suggests that higher intakes of α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) may afford some degree of protection against coronary disease. Because the physiological potency of EPA and DHA is much greater than that of ALA, it is not possible to estimate one AMDR for all omega-3 fatty acids. Approximately 10 percent of the AMDR can be consumed as EPA and/or DHA."^[78] There was insufficient evidence as of 2005 to set an upper tolerable limit for omega-3 fatty acids.^[78]

Heavy metal poisoning by the body's accumulation of traces of heavy metals, in particular mercury, lead, nickel, arsenic, and cadmium, is a possible risk from consuming fish oil supplements.^{[medical citation needed]} Also, other contaminants (PCBs, furans, dioxins, and PBDEs) might be found, especially in less-refined fish oil supplements.^{[citation needed]} In reality, however, heavy metal toxicity from consuming fish oil supplements is highly unlikely, because heavy metals selectively bind with protein in the fish flesh rather than accumulate in the oil. An independent test in 2005 of 44 fish oils on the US market found all of the products passed safety standards for potential contaminants.^[79]

The FDA has advised that adults can safely consume a total of 3 grams per day of combined DHA and EPA, with no more than 2 g per day coming from dietary supplements.^[80]

Throughout their history, the Council for Responsible Nutrition and the World Health Organization have published acceptable standards regarding contaminants in fish oil. The most stringent current standard is the International Fish Oils Standard.^[81] Fish oils that are molecularly distilled under vacuum typically make this highest-grade, and have measurable levels of contaminants (measured parts per billion and parts per trillion).^{[citation needed]}

A recent trend has been to fortify food with omega-3 fatty acid supplements. Global food companies have launched omega-3 fatty acid fortified bread, mayonnaise, pizza, yogurt, orange juice, children's pasta, milk, eggs, popcorn, confections, and infant formula.^{[citation needed]}

The American Heart Association has set up dietary recommendations for EPA and DHA due to their cardiovascular benefits: Individuals with no history of coronary heart disease or myocardial infarction should consume oily fish or fish oils two times per week; those having been diagnosed with coronary heart disease after infarction should consume 1 g EPA and DHA per day from oily fish or supplements; those wishing to lower blood triglycerides should consume 2–4 g of EPA and DHA per day in the form of supplements.^[82]

Fish

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 to accumulate 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 are 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.^[83]

Fish oil

Marine and freshwater fish oil vary in content of arachidonic acid, EPA and DHA.^[84] They also differ in their effects on organ lipids.^[84] 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.^[85][86]

Krill

Krill oil is a newly discovered source of omega-3 fatty acids. Various claims are made in support of krill oil as a superior^{[citation needed]} 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.^[87]

Calamari oil

Calamari oil (also known as Squid oil) is another source of omega-3 fatty acid.^[88] Calamari is considered to be more environmentally friendly than fish or krill oil, due to it being prepared from the largely unused portions of calamari catches.^[89]

Plant sources

Flax seeds produce linseed oil, which has a very high ALA content

These tables are incomplete.

Table 1. ALA content as the percentage of the seed oil.^[90]

Table 2. ALA content as the percentage of the whole food.^[92][94]

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.^[95] A portion of this is converted by the body to EPA and DHA, though the actual converted percentage may differ between men and women.^[96]

100 g of the leaves of Purslane contains 300–400 mg ALA.^[97]

In 2013 Rothamsted Research in the UK reported they had developed a genetically modified form of the plant Camelina that produced EPA and DHA. Oil from the seeds of this plant contained on average 11% EPA and 8% DHA in one development and 24% EPA in another.^[98][99]

Eggs

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.^[100] 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.^[101]

The addition of flax and canola seeds to the diets of chickens, both good sources of alpha-linolenic acid, increases the omega-3 content of the eggs, predominantly DHA.^[102]

The addition of green algae or seaweed to the diets boosts the content of DHA and EPA content, which are the forms of omega-3 approved by the FDA for medical claims. A common consumer complaint is "Omega-3 eggs can sometimes have a fishy taste if the hens are fed marine oils."^[103]

Meat

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.^[104] 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.^[105]

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.^[61]

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-finished 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.^[61]

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.^[106]

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.^[107]

Kangaroo meat is also a source of omega-3, with fillet and steak containing 74 mg per 100 g of raw meat.^[108]

Mammalian brains and eyes

The brains and eyes of mammals are extremely rich in DHA as well as other omega-3 fatty acids.^[109] DHA is a major structural component of the mammalian brain, and is in fact the most abundant omega-3 fatty acid in the brain.^[110]

Seal oil

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.^[111] However, like all seal products, it is not allowed for import into the European Union.^[112]

Other sources

The microalgae Crypthecodinium cohnii and Schizochytrium are rich sources of DHA, but not EPA, and can be produced commercially in bioreactors.^{[citation needed]}

Oil from brown algae (kelp) is a source of EPA.^[113]

In 2006 the Journal of Dairy Science published a study entitled, "The Linear Relationship between the Proportion of Fresh Grass in the Cow Diet, Milk Fatty Acid Composition, and Butter Properties". The study found that butter made from the milk of grass fed cows contains substantially more CLA, vitamin E, beta-carotene, and omega-3 fatty acids than butter made from the milk of cows that have limited access to pasture. It was also found that the more fresh pasture in the cow’s diet, the softer the butter.^[114]

References

Further reading

External links

• University of Maryland Medical Center, omega-3 Fatty Acids
• MedlinePlus Herbs and Supplements: Omega-3 fatty acids, fish oil, alpha-linolenic acid