omega-3 fatty acid

Omega 3 fatty acids (popularly referred to as ω−3 fatty acids or n-3 fatty acids^{[citation needed]}) are fats commonly found in marine and plant oils. They are polyunsaturated fatty acids with a double bond (C=C) starting after the third carbon atom from the end of the carbon chain. The fatty acids have two ends—the acid (COOH) end and the methyl (CH3) end. The location of the first double bond is counted from the methyl end, which is also known as the omega (ω) end or the n end.

N-3 fatty acids have health benefits and are considered essential fatty acids, meaning that they cannot be synthesized by the human body but are vital for normal metabolism. Though mammals cannot synthesize n−3 fatty acids, they have a limited ability to form the long-chain n−3 fatty acids including eicosapentaenoic acid (EPA, 20 carbons and 5 double bonds), docosahexaenoic acid (DHA, 22 carbons and 6 double bonds) and α-linolenic acid (ALA, 18 carbons and 3 double bonds).

Common sources of n–3 fatty acids include fish oils, algal oil, squid oil and some plant oils such as echium oil and flaxseed oil.

Chemistry

Chemical structure of alpha-linolenic acid (ALA), an essential n−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), physiologists 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 physiologic transformations in the human body, as it is more energy-stable, and other carbohydrates 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).

N−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.

List of n−3 fatty acids

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

Significance to human nutrition and health

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 1990s.^[1]

The health benefits of the long-chain omega-3 fatty acids — primarily EPA and DHA are the best known. These benefits were discovered in the 1970s by researchers studying the Greenland Inuit Tribe. The Greenland Inuit people consumed large amounts of fat from fish, but displayed virtually no cardiovascular disease. The high level of omega-3 fatty acids consumed by the Inuit reduced triglycerides, heart rate, blood pressure, and atherosclerosis.^[2]

On September 8, 2004, the U.S. Food and Drug Administration gave "qualified health claim" status to EPA and DHA n−3 fatty acids, stating that "supportive but not conclusive research shows that consumption of EPA and DHA [n−3] fatty acids may reduce the risk of coronary heart disease."^[3] This updated and modified their health risk advice letter of 2001 (see below). As of this writing, regulatory agencies^[who?] 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."^[4]

Biological significance

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 stricter. 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.^[5]

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 is only in recent decades that n−3 has become of interest.^[5]

In 1964, it was discovered that enzymes found in sheep tissues convert n−6 arachidonic acid into the inflammatory agent called prostaglandin E₂,^[6] which both causes the sensation of pain and expedites healing and immune response in traumatized and infected tissues.^[5] By 1979, more of what are now known as eicosanoids were discovered: thromboxanes, prostacyclins, and the leukotrienes.^[5] 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.^[5] Researchers found that certain n−3 fatty acids are also converted into eicosanoids, but at a much slower rate. Eicosanoids made from n−3 fatty acids are often referred to as anti-inflammatory, but in fact they are just less inflammatory than those made from n−6 fats. If both n−3 and n−6 fatty acids are present, they will "compete" to be transformed,^[5] so the ratio of long-chain n−3:n−6 fatty acids directly affects the type of eicosanoids that are produced.^[5]

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 n−6 eicosanoids. The simplest way would be by consuming more n−3 and fewer n−6 fatty acids.^[5]

Interconversion

Conversion efficiency of ALA to EPA and DHA

The short-chain n−3 fatty acids are converted to long-chain forms (EPA, DHA) with an efficiency below 5%^[7][8] in men, and at a greater percentage in women which may be due to the importance for meeting the demands of the fetus and neonate for DHA.^[9]

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 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.^{[citation needed]}

The conversion of ALA to EPA and further to DHA in humans has been reported to be limited, but varies with individuals.^[10][11] Women have higher ALA conversion efficiency than men, it is presumed 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 n−3 and n−6 fatty acids, that controls the conversion efficiency.^[12]

The n−6 to n−3 ratio

Some clinical studies^[5][13][14] indicate that the ingested ratio of n−6 to n−3 (especially linoleic vs alpha-linolenic) fatty acids is important to maintaining cardiovascular health. However, two studies published in 2005 and 2007 found that while n−3 polyunsaturated fatty acids are extremely beneficial in preventing heart disease in humans, the levels of n−6 polyunsaturated fatty acids (and therefore the ratios) were insignificant.^[15][16]

Both n−3 and n−6 fatty acids are essential; i.e., humans must consume them in the diets. N−3 and n−6 eighteen-carbon polyunsaturated fatty acids compete for the same metabolic enzymes, thus the n−6:n−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.^[17] In general, grass-fed animals accumulate more n−3 than do grain-fed animals, which accumulate relatively more n−6.^[18] Metabolites of n−6 are more inflammatory (esp. arachidonic acid) than those of n−3. This necessitates that n−3 and n−6 be consumed in a balanced proportion; healthy ratios of n−6:n−3 range from 1:1 to 1:4 (an individual needs more n−3 than n−6).^[19] Studies suggest the evolutionary human diet, rich in game animals, seafood, and other sources of n−3, may have provided such a ratio.^[20][21]

Typical Western diets provide ratios of between 10:1 and 30:1 (i.e., dramatically higher levels of n−6 than n-3).^[22] The ratios of n−6 to n−3 fatty acids in some common vegetable oils are: canola 2:1, soybean 7:1, olive 3-13:1, sunflower (no n−3), flax 1:3,^[23] cottonseed (almost no n−3), peanut (no n−3), grapeseed oil (almost no n−3) and corn oil 46:1 ratio of n−6 to n−3.^[24]

Potential health benefits

The 18 carbon α-linolenic acid (ALA) has not been shown to have the same cardiovascular benefits that DHA or EPA may have.^[25] Currently, there are many products on the market that claim to contain health-promoting "omega 3", but contain only ALA, not EPA or DHA. These products contain mainly plant oils and must be converted by the body to create DHA. DHA and EPA are made by marine microalgae. These are then consumed by fish and accumulate to high levels in their internal organs. The United States Environmental Protection Agency issues fish consumption advisories to empower Americans to avoid toxic mercury levels in certain fish and shellfish while still reaping the possible health benefits of consuming fish and shellfish.^[26]

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.^[27][28] Evidently, n−3 fatty acids reduce blood triglyceride levels,^{[29][30][31][32]} and regular intake may reduce the risk of secondary and primary heart attack.^{[33][34][35][36]} A systematic review of studies prior to 2005 showed that ALA does not confer the cardiovascular health benefits of EPA and DHA.^[37]

Some potential benefits have been reported in conditions such as rheumatoid arthritis^[38][39] and cardiac arrhythmias.^[40][41][42]

There is preliminary evidence that EPA supplementation, either with DHA or medication, is helpful in cases of depression^[43][44][45] There is also limited evidence that supplementation with n-3 fatty acids, alone or in combination with n-6 fatty acids, may reduce anxiety,^[46] however, the only live study to suggest an anxiety reducing effect involved α-Linolenic acid (as opposed to EPA or DPA).^[47] The New York Times,^[48] however, reports that at least one study^[49] has not found a connection between depression in heart patients taking Sertraline and daily supplements containing two grams total of EPA and DHA during a ten-week period.

Some research suggests that fish oil intake may reduce the risk of ischemic and thrombotic stroke,^[50][51][52] although large amounts may actually increase the risk of hemorrhagic stroke (see below): Lower amounts are not related to this risk;^[52] 3 grams of total EPA/DHA daily are Generally Recognized As Safe (GRAS) with no increased risk of bleeding involved^[53] and many studies used substantially higher doses without major side effects (for example: 4.4 grams EPA/2.2 grams DHA in 2003 study).^[43]

Cancer

Several studies report possible anti-cancer effects of n−3 fatty acids (in particular, breast, colon, and prostate cancer).^[54][55][56] Omega-3 fatty acids reduced prostate tumor growth, slowed histopathological progression, and increased survival in mice.^[57] Among n-3 fatty acids, neither long-chain nor short-chain forms were consistently associated with breast cancer risk. High levels of DHA, however, the most abundant n-3 polyunsaturated fatty acid in erythrocyte membranes, were associated with a reduced risk of breast cancer.^[58] Conversely, a 2011 study, the largest ever done involving 3,400 men, examined the association of dietary fats and prostate cancer risk, and found that men with the highest blood percentages of DHA have two-and-a-half-times the risk of developing aggressive, high-grade prostate cancer compared to men with the lowest DHA levels.^[59][60]

A 2006 report in the Journal of the American Medical Association, in their review of literature covering cohorts from many countries with a wide variety of demographics, concluded that there was no link between n−3 fatty acids and cancer.^[61] 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 n−3 fats on total mortality, combined cardiovascular events and cancer.^[62]

A 2007 systematic review of n-3 fatty acids and cachexia found evidence that oral n-3 fatty acid supplements benefit cancer patients, improving appetite, weight, and quality of life.^[63] A 2009 trial found that a supplement of eicosapentaenoic acid helped cancer patients retain muscle mass.^[64]

Cardiovascular disease

In 1999, the GISSI-Prevenzione Investigators reported in The Lancet the results of major clinical study in 11,324 patients with a recent myocardial infarction. Treatment 1 gram per day of n−3 fatty acids reduced the occurrence of death, cardiovascular death, and sudden cardiac death by 20%, 30%, and 45%, respectively.^[65] These beneficial effects were seen from three months onwards.^[66]

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 of n−3 fatty acids found in abundance in oily fish. It concluded that they do not have a significant protective effect against cardiovascular disease.^[67] This meta-analysis was controversial and stands in stark contrast with two different reviews also performed in 2006 by the American Journal of Clinical Nutrition^[68] and a second JAMA review;^[69] both indicated decreases in total mortality and cardiovascular incidents (i.e., myocardial infarctions) associated with the regular consumption of fish and fish oil supplements.

In the March 2007 edition of the journal Atherosclerosis, 81 Japanese men with unhealthy blood sugar levels were randomly assigned to receive 1.8 g daily of eicosapentaenoic acid (EPA), with the other half being a control group. The thickness of the carotid arteries and certain measures of blood flow were measured before and after supplementation. This went on for approximately two years. A total of 60 patients (30 in the E-EPA group and 30 in the control group) completed the study. Those given the EPA had a statistically significant decrease in the thickness of the carotid arteries, along with improvement in blood flow. The authors indicated that this was the first demonstration that administration of purified EPA improved the thickness of carotid arteries and improved blood flow in patients with unhealthy blood sugar levels.^[70]

In a study published in the American Journal of Health-System Pharmacy March 2007, patients with high triglycerides and poor coronary artery health were given 4 grams a day of a combination of EPA and DHA along with some monounsaturated fatty acids. Those patients with very unhealthy triglyceride levels (above 500 mg/dl) reduced their triglycerides on average 45% and their VLDL cholesterol by more than 50%. VLDL is a "bad" type of cholesterol, and elevated triglycerides can also be deleterious for cardiovascular health.^[71]

A study on the benefits of EPA published in The Lancet in March 2007 involved over 18,000 patients with unhealthy cholesterol levels. The patients were randomly assigned to receive either 1.8 g a day of E-EPA with a statin drug or a statin drug alone. The trial lasted five years. At the end of the study, those patients in the E-EPA group had superior cardiovascular function and nonfatal coronary events were also significantly reduced. The authors concluded that EPA is a promising treatment for prevention of major coronary events, especially nonfatal coronary events.^[72]

Similar to those following a Mediterranean diet, Arctic-dwelling Inuit - who consume high amounts of n−3 fatty acids from fatty fish - also tend to have higher proportions of n−3, increased HDL cholesterol and decreased triglycerides (fatty material that circulates in the blood), and less heart disease. Eating walnuts (the ratio of n−6 to n−3 is circa 4:1^[73]) was reported to lower total cholesterol by 4% relative to controls when people also ate 27% less cholesterol.^[74]

A study of 465 women showed that serum levels of EPA are inversely related to levels of anti-oxidized-LDL antibodies. Oxidative modification of LDL is thought to play an important role in the development of atherosclerosis.^[75]

Survivors of past heart attacks are less likely to die from an arrhythmic event if they are consuming high levels of n-3.^[76] These antiarrhythmic effects are thought to be due to n-3 fatty acids' ability to increase the fibrillation threshold of the heart tissue.^[77]

N-3 fatty acids also have mild antihypertensive effects. When subjects consumed n-3 from oily fish on a regular basis, their systolic blood pressure was lowered by about 3.5-5.5 mmHg.^[78]

Immune function

In a study regarding fish oil published in the Journal of Nutrition in April 2007, sixty-four healthy Danish infants from nine to twelve months of age received either cow's milk or infant formula alone or with fish oil. Those infants supplemented with fish oil were found to have improvement in immune function maturation, with no apparent reduction in immune activation.^[79]

Neurology

Limited evidence suggests that long-chain n-3 fatty acids may delay or prevent the progression of certain psychotic disorders in high-risk children and adolescents.^[80] The individuals diagnosed with schizophrenia exhibited reduced levels of both n-6 and n-3 polyunsaturated fatty acids, and the results of a study in which the treatment of high-risk children with a dietary supplement containing both eicosapentaenoate and docosahexaenoate produced a statistically significant (95% confidence, but not 97.5% confidence) decrease in progression to schizophrenia.

Consumption of ethyl eicosapentaenoate (E-EPA) partially countered memory impairment in a rat model of Alzheimer's disease^[81] and produced a statistically insignificant decrease in human depression.^[82]

Studies looking at the effects of omega-3 fatty acids on cognitive performance have seen mixed results. A study published in 2005 showed beneficial effects of omega-3 fatty acids in the cognitive performance of healthy subjects.^[83] However, a later study found that fish oil had no effect on cognitive performance in individuals 65 years of age or older without dementia.^[84]

Omega-3 fatty acid supplementation, particularly with higher doses of eicosapentaenoic acid, was modestly effective in the treatment of ADHD.^[85]

There is also some evidence that the consumption of omega-3 fatty acids not only results in faster regeneration of peripheral nerves which have been injured, but also causes decreased levels of muscle wasting after nerve damage.^[86]

Inflammation

Although not confirmed as an approved health claim, current research suggests that the anti-inflammatory activity of long-chain n−3 fatty acids may translate into clinical effects.^[87] For example, there is evidence that rheumatoid arthritis sufferers taking long-chain n−3 fatty acids from sources such as fish have reduced pain compared to those receiving standard NSAIDs.^[88]

Risks

Noncardiac health risks

In a letter published October 31, 2000,^[89] 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:

• Increased incidence of bleeding
• 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

Subsequent advice from the FDA and national counterparts have permitted health claims associated with heart health.

Cardiac risk

Persons with congestive heart failure, chronic recurrent angina pectoris, or evidence that their heart is receiving insufficient blood flow are advised to talk to their doctors before taking n−3 fatty acids.^[90]

In a recent large study, n−3 fatty acids on top of standard heart failure therapy produced a small but statistically significant benefit in terms of mortality and hospitalization.^[91] In congestive heart failure, cells that only barely receive enough blood flow become electrically hyperexcitable. This can lead to increased risk of irregular heartbeats, which, in turn, can cause sudden cardiac death. Certain n−3 fatty acids seem to stabilize the rhythm of the heart by effectively preventing these hyperexcitable cells from functioning, thereby reducing the likelihood of sudden cardiac death. For most people, this is beneficial and could account for most of the large reduction in the likelihood of sudden cardiac death. Nevertheless, for people with congestive heart failure, the heart is barely pumping blood well enough to keep them alive. In these patients, n−3 fatty acids may eliminate enough of these few pumping cells that the heart would no longer be able to pump sufficient blood to live, causing an increased, rather than decreased, risk of cardiac death.^[90]

Research frontiers

Developmental differences

Although not supported by current scientific evidence as a primary treatment for ADHD, autism spectrum disorders, and other developmental differences,^[92][93] omega-3 fatty acids have gained popularity for children with these conditions.^[92] A 2004 Internet survey found that 29% of surveyed parents used essential fatty acid supplements to treat children with autistic spectrum disorders.^[94]

Omega-3 fatty acids offer a promising complementary approach to standard treatments for ADHD and developmental coordination disorder.^[93] Fish oils appear to reduce ADHD-related symptoms in some children.^[93] Double blind studies have shown "medium to strong treatment effects of omega 3 fatty acids on symptoms of ADHD" after administering amounts around 1 gram for three to six months.^[95][96][97]

A 2009 survey concluded that there is not enough scientific evidence to support the effectiveness of 'n-3 fatty acids for autism spectrum disorders.^[98] One randomized controlled trial found that n-3 fatty acids did not significantly affect aberrant behavior in autistic children, and although the investigators noted reduced hyperactivity,^[99] their later reanalysis reported that the reduction was not statistically significant.^[100]

Low birth weight

In a study of nearly 9,000 pregnant women, researchers found that women that ate fish once a week during their first trimester had 27% less risk of low birth weight and premature birth than those that ate no fish. Low consumption of fish was a strong risk factor for preterm delivery and low birth weight,^[101][102] but attempts by other groups to reverse this increased risk by encouraging increased prenatal consumption of fish were unsuccessful.^[103]

Psychiatric disorders

Though there is some evidence that n-3 fatty acids are connected to a variety of symptoms and disorders, there is not enough evidence to support including them as part of a treatment program for any mental disorders.^[104]

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 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.^[105]

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 n−3 fatty acids. Approximately 10 percent of the AMDR can be consumed as EPA and/or DHA."^[105] There was insufficient evidence as of 2005 to set an upper tolerable limit for n−3 fatty acids.^[105]

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. Also, other contaminants (PCBs, furans, dioxins, and PBDEs) might be found, especially in less-refined fish oil supplements. 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.^[106] The FDA recommends that the total dietary intake of n−3 fatty acids from fish not exceed 3 grams per day, with no more than 2 grams per day from nutritional supplements.^[3]

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.^[107] 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).

A recent trend has been to fortify food with n−3 fatty acid supplements. Global food companies have launched n−3 fatty acid fortified bread, mayonnaise, pizza, yogurt, orange juice, children's pasta, milk, eggs, popcorn, confections, and infant formula.

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.^[108]

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 n−3 as n−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 is a dietary source of n−3 fatty acids, fish do not synthesize them; they obtain them from the algae (microalgae in particular) or plankton in their diets.^[109]

Fish oil

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.^[113][114]

Krill

Krill oil is a newly^[when?] discovered source of n−3 fatty acids. Various claims are made in support of krill oil as a superior^{[citation needed]} source of n−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.^[115]

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.^[116]

Table 2. ALA content as the percentage of the whole food.^[117][118]

Flaxseed (or linseed) (Linum usitatissimum) and its oil are perhaps the most widely available botanical source of the n−3 fatty acid ALA. Flaxseed oil consists of approximately 55% ALA, which makes it six times richer than most fish oils in n−3 fatty acids.^[119] A portion of this is converted by the body to EPA and DHA, though this may differ between men and women.^[120]

100 g of the leaves of Purslane contains 300-400 mg ALA.^[121]

Eggs

Eggs produced by hens fed a diet of greens and insects contain higher levels of n−3 fatty acids than those produced by chickens fed corn or soybeans.^[122] In addition to feeding chickens insects and greens, fish oils may be added to their diets to increase the n-3 fatty acid concentrations in eggs.^[123]

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.^[124]

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."^[125]

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 meats.^[126] 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.^[127]

The n−6 to n−3 ratio of grass-fed beef is about 2:1, making it a more useful source of n−3 than grain-fed beef, which usually has a ratio of 4:1.^[18]

In a 2009 study which was a joint effort between the USDA and researchers at Clemson University in South Carolina grass-fed beef was compared with grain-fed beef and researchers found that grass-fed beef is: lower in total fat, higher in beta-carotene, higher in vitamin E (alpha-tocopherol), higher in the B-vitamins thiamin and riboflavin, higher in the minerals calcium, magnesium, and potassium, higher in total omega-3s, higher in CLA (cis-9 trans-11) which is a potential cancer fighter, 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).^[18]

In most countries, commercially available lamb is typically grass-fed, and thus higher in n−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 n−3.^[128]

The omega-3 content of chicken meat may be enhanced by increasing the animals' dietary intake of grains high in n−3, such as flax, chia, and canola.^[129]

Kangaroo meat is also a source of n−3, with fillet and steak containing 74 mg per 100g of raw meat.^[130]

Mammalian brains and eyes

The brains and eyes of mammals are extremely rich in DHA as well as other n-3 fatty acids.^[131] DHA is a major structural component of the mammalian brain, and is in fact the most abundant (n-3) fatty acid in the brain.^[132]

Seal oil

Seal oil is a source of EPA, DPH, and DPA. According to Health Canada, it helps to support the development of the brain, eyes and nerves in children up to 12 years of age.^[133] However, like all seal products, it is not allowed for import into the European Union^[134]

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.^{[citation needed]}

In 2006 a study was published in the Journal of Dairy Science entitled "The Linear Relationship between the Proportion of Fresh Grass in the Cow Diet, Milk Fatty Acid Composition, and Butter Properties". It was found that grass fed butter contains substantially more CLA, vitamin E, beta-carotene, and omega-3 fatty acids than butter from cows raised in factory farms or that have limited access to pasture. It was also found that the softer the butter, the more fresh pasture in the cow’s diet. Cows that get all their nutrients from grass have the softest butterfat of all.^[135]

See also

Food portal

Notes and references

Further reading

External links