Omega 3 Fatty Acid Metabolism And Human Health Biology Essay

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In our endless hunt for thinness, we have twisted fat into a villain. And when we try to eat low-fat diets and avoid saturated fats and cholesterol, we usually make the mistake of avoiding all fats. But not all fats are bad and, in fact some fats can be good, even essential. We definitely require certain fats for health - lipids, sterols, and essential fatty acids. Of the many "good" fats that play vital roles in the body, two families of polyunsaturated fatty acids; the omega-3 and omega-6 polyunsaturated fatty acids, have received much notice in the scientific community in the last three decades for their beneficial effects. Alpha-linolenic acid (ALA) and linoleic acid (LA) are considered as the parent n-3 and n-6 fatty acids in each series, respectively.

Although the importance of omega-3 fatty acids has been known since 1930's, the awareness of their health benefits has noticeably increased in the past few years [1]. Omega -3 fatty acids (also known as n-3 fatty acids) are polyunsaturated fats (PUFA) and are often categorized as "essential fatty acids," meaning that the human body is unable to produce them and they can only be obtained by the diet. In addition, these fats are essential for normal growth and cell function and nutritionally significant omega-3 fatty acids include α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). This essay will focus on the basic structure and metabolism of the omega-3 fatty acids, and the role that these fats play in normal human physiology.

The chemical structure of fatty acids

Basic structure of different fatty acids

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About 90% of our dietary fats come in the form of triglycerides, which are made up of fatty acids and glycerol. Fatty acids are the building blocks from which fats (lipids) are produced. Fats stored in the body are primarily present in the form of triacylglycerols (TAGs). Triacylglycerols (fig.1) is a polymer which contains a glycerol molecule and 3 fatty acid molecules. The fatty acid molecules can be found at any of three positions on the glycerol molecule (termed sn -1; sn -2 and sn -3) and the characteristics (i.e. melting point and digestibility) of the TAGs will also influenced by the presence of different fatty acids at different positions on the glycerol molecule.

Fatty acids are made up of a backbone of carbon atoms, with a carboxyl group (COOH) at one end [the delta (Δ) end] and a methyl group (CH 3) [the omega (ω) or n -end] at the other end. Hydrogen atoms are attached to the string of carbon atoms, forming a hydrocarbon chain[2].

Figure 1: The structure of triacylglycerol. The fatty acids in the sn -1, sn -2 and sn -3 positions (F 1, F 2 and F 3)[2].

Fatty acids vary in length from 2 to 80 carbons and the carbon chain length affects the characteristics of a fatty acid, as does the presence or absence of double bonds between carbon atoms. The fatty acid is said to be a saturated fatty acid if all of the carbons in the fatty acid chain are connected by single bonds and the fatty acid is said to be an unsaturated fatty acid if one or more double bonds are present in the fatty acid chain. It is said to be a monounsaturated fatty acid (MUFA) if there is only one double bond present in an unsaturated fatty acid and if there is more than one double bond present, the fatty acid is said to be a polyunsaturated fatty acid (PUFA). Based on the position of the first double bond in the fatty acid chain PUFAs can be further classified as either n -3 (omega 3) or n -6 (omega 6) PUFAs(fig.2). All members of the n-3 family of fatty acids have their first double bond between the third and fourth carbon atoms while all members of the n-6 family of fatty acids contain their first double bond between the sixth and seventh carbon atoms from the terminal methyl group.

n-3 PUFA

n-6 PUFA

Figure 2. n-3 and n-6 fatty acids. PUFA, Polyunsaturated fatty acids

Cis and trans fatty acids

In unsaturated fatty acids, double bonds can be arranged in one of two configurations, cis or trans form (fig. 3). Double bonds found in foods are mainly in the cis form, where both hydrogen atoms are located on the same side of the fatty acid. It is the presence of cis bond that lowers the melting point of the fatty acid, making it more likely to be liquid at room temperature. Trans -fatty acids, where the hydrogen atoms are located on opposite sides of the fatty acid, are less common in nature and are normally found in the fat of ruminant meats in small amounts and in milk being formed in the rumen during digestion and later absorbed as an energy source for the animal.

Cis fatty acid Trans fatty acid

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Figure 3. cis- and trans-configurations.

The Omega-3 Polyunsaturated Fatty Acids

The omega-3 family of fatty acids is derived from the parent fatty acid α -linolenic acid (ALA). Starting with alpha linolenic acid (ALA) and ending at docosahexaenoic acid (DHA) there are a total of eight omega-3 fatty acids in the omega-3 family. Some play significant individual roles while others take part in supporting function and all are involved in and members of the body's usual fatty acid biochemistry. The 8 members of this fatty acid family include (arranged in order of occurrence);

1. ALA - alpha-linolenic acid

2. SDA - stearadonic acid

3. ETrA - eicosatrienoic acid

4. ETA - eicosatetraenoic acid

5. EPA - eicosapentaenoic acid

6. HPA - heneicosapentaenoic acid

7. DPA - docosapentaenoic acid

8. DHA - docosahexaenoic acid

Among these polyunsaturated fatty acids ALA is considered essential as the human body cannot produce this fatty acid, and it must be derived from the diet. All other omega-3 fatty acids are described as being "conditionally essential" because although the body can synthesise these by metabolising ALA, the body cannot produce them fast enough to keep up with demand. Though humans can lengthen or produce dietary α -linolenic acid to the long chain n- 3 PUFAs eicosapentaenoic acid and docosahexaenoic acid, the rate of production may not be adequate to meet the needs, and for that reason, it is suggested that good sources of these fatty acids are also included in the diet.

Synthesis of Long Chain Omega- 3 Fatty Acids from ALA

Digestion and Absorption

The most important biological function of ALA is as a precursor for the synthesis of longer-chain n-3 polyunsaturated fatty acids (PUFA). It is the efficiency of dietary fat absorption across the gastrointestinal tract, uptake and partitioning towards beta-oxidation and incorporation into structural and storage pools, which determine the bioavailability of dietary α-LNA for conversion to longer-chain PUFA. In order to be properly digested and absorbed, dietary fats have to first be emulsified by bile secreted into the small intestine from the gallbladder. As the fat droplets form small micelles are dispersible in water it can be acted on by pancreatic lipases cleaving the fatty acids at the sn-1 and sn-3 positions of the TAGs. The end-products from the digestion of fat (free fatty acids, a 2-monoacylglycerol, and very limited amounts of glycerol) are absorbed by diffusion across the gut wall into the cells of the intestine. The mode of transport away from the gut is dependent on chain length; the short to medium chain fatty acids (2-12 carbon atoms) are transported bound to the carrier protein albumin via the hepatic portal vein to the liver. However, dietary fat mostly consists of the longer chain fatty acids (>12 carbon atoms), which are reassembled into triacylglycerols in the intestinal cell. Triacylglycerols are then packaged into chylomicrons, the lipoprotein particles, and are transported via the lymph system into the peripheral circulation. The rate of lipolysis (breakdown of fat), absorption of fatty acids and transport is also influenced by the degree of unsaturation of the fatty acids [3]. Once in the bloodstream chylomicrons acquire apolipoprotein C (apoC) from high density lipoproteins (HDL). At the target cell (e.g. muscle), the apoC subunit activates the enzyme lipoprotein lipase, which is found within the endothelial cell membrane. This results in cleavage of the fatty acids, which are then transported into tissues for storage or metabolism [3].

Metabolism

Fatty acids destined for metabolism are transported into the mitochondria of the target cells. In the mitochondria, fatty acids undergo β-oxidation to release energy. Beta- oxidation is a metabolic pathway whereby a fatty acid is broken down step by step, two carbon atoms at a time. The chemical energy locked up in the carbon compounds is released in a controlled manner and used to generate ATP (Adenosine triphosphate), which is used to fuel cellular processes. Depending on the chain length and the number of double bonds in the molecule, the exact energy yield of the different fats is varies, however 1 g of fat is believed to release ~37 kJ (9 kcal) of energy irrespective of its precise structural properties. Almost all unsaturated fatty acids can be synthesized in the body after a series of fatty acid chain elongation and desaturation steps and an enzyme complex, fatty acid synthase, catalyses this process. Desaturases present in plants are capable of inserting double bonds at positions 3, 6 and 9 from the terminal methyl group, but animal and human desaturases are only capable of inserting double bonds at specific positions beyond carbon 6. For this reason, there are several fatty acids, termed essential fatty acids (EFA), which cannot be produced in the body but are nevertheless necessary for normal physiological function in humans. The n-3 essential fatty acid family is derived from the parent essential fatty acid, Cis n-3 PUFA alpha (α)-linolenic acid; through a series of enzyme catalysed desaturation and elongation reactions that usually take place in the cell cytosol or in the mitochondria.

Alpha-linoleic Acid Metabolism

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α -LNA is a substrate for β-oxidation in humans, and the ratio of ingested [13C] α - LNA use in β-oxidation is estimated from the appearance of labelled CO2 in the breath[4]. The values reported so far for the amount of labelled α -LNA which undergoes β-oxidation is through to represent an approximately 30% underestimate of the actual proportion of ingested α-LNA used in energy production, due to the trapping of 13CO2 in bicarbonate pools, which are not included in the calculations [4]. The degree of partitioning of α-LNA towards β-oxidation is almost twice that of palmitic, stearic and oleic acid [5]. The extent of partitioning of α -LNA towards β-oxidation (fig.4) is comparatively stable over short periods of time and changing the amount of either α -LNA or long-chain n-3 PUFA in the diet does not considerably alter this process. Besides the conversion to CO2 by the activity of the Krebs cycle, carbon in acetyl-CoA generated by fatty acid β-oxidation can also be recycled and used in de novo fatty acid synthesis[6].

Figure 4. Summary of the relative partitioning of α -LNA between desaturation/elongation and β-oxidation in human liver [6].

Gender differences in partitioning of ALA to β-oxidation and conversion to omega-3 LCPUFA

According to the reports from previous studies, the concentrations of saturated fatty acids (SFA) and mono saturated fatty acids (MUFA) in plasma are higher in triacylglycerols (TAG) than phosphatidylcholine (PC) and these data suggest the channelling of SFA and MUFA synthesised by the recycling pathway into phospholipids by the liver in contrast to the molecular partitioning of the bulk of the hepatic SFA and MUFA pools towards TAG[6]. The total concentration of labelled SFA and MUFA in plasma lipids was 20% higher in men compared to women[6]. This is in agreement with greater partitioning of α-LNA towards β-oxidation in men compared to women. One general implication is that the extent of partitioning of α-LNA towards β-oxidation and carbon recycling may be significant in the regulation of the availability of α-LNA for conversion to longer-chain PUFA[6]. There are currently only two reports which have directly compared the rate of α-LNA conversion in women and men of reproductive age. Burdge and Wootton [7] showed that the conversion of α-LNA to EPA and DHA was considerably greater (2.5-fold and >200-fold, respectively) in women aged about 28 years than in a similar study of men of same age [8]. This finding is supported by a kinetic analysis, which demonstrated that the rate constant coefficient for the conversion of DPAn-3 to DHA was about four-fold more in women compared to men [9]. This difference may reflect the greater availability of α-LNA in women than in men, due in part to lower partitioning towards β-oxidation in females. Besides the differences between men and women in the extent of partitioning of α-LNA towards β-oxidation, it is likely that there is a gender-related difference in the activity of the desaturation/elongation pathway. One possible explanation for the greater synthesis of EPA and DHA from α-LNA in women compared to men is the action of oestrogen. It is suggested that oestrogen may increase the activity of the desaturation/elongation pathway, and this is supported by data showing that oestrogen based hormone replacement in postmenopausal therapy resulted in greater plasma dihomo-γ-linolenic and arachidonic acid concentrations than before treatment [10].

Conversion of ALA to n-3 LCPUFA

A general pathway for the conversion of α -LNA into longer-chain n-3 PUFA is described below (fig.5). Alpha (α)- linolenic acid is metabolised to docosahexaenoic acid (DHA; 22:6n-3) via eicosapentaenoic acid (EPA; 20:5n-3) and docosapentaenoic acid (DPA; 22:5n-3), However, conversion is not 100% efficient. Desaturation of DPA at the Δ-4 position to make DHA does not occur via a single step catalysed by Δ-4 desaturase, but also which involves elongation and desaturation of DPA and tetracosahexenoic acid in microsomes followed by a chain shortening step to DHA in peroxisomes, the so called 'Sprecher Pathway'[11]. The capacity of humans to produce DHA from α-LNA is found to be very limited. It has been estimated that less than 8% of ALA is metabolised to EPA, and that only between 0.02% and 4% of ALA is metabolised further to DHA [8,12,13], with women having a greater ability for DHA synthesis than men[7].

Figure 5. Metabolism of omega- 3 essential fatty acid (Adapted from Napier & Sayanova, 2005[14]).

Eicosanoids- LCPUFA derived factors

The metabolites of the EFAs have several important physiological functions. The oxidised metabolites of fatty acids are known as eicosanoids and they act as important regulatory signals in the body and can bring about inflammatory effects. Eicosanoids are produced by cells to act in their immediate environment in response to extracellular stimuli e.g. blood vessel damage, and the carbon metabolites of essential fatty acids function as the basis for these important regulatory signals. Eicosanoids are hormone-like compounds that function similar to thermostats throughout the body, either increasing or lowering an extensive range of bodily activities and as their action is restricted rather than originating from a specific gland such as the pancreas or adrenals they have been noticed or discovered only recently. Eicosanoids are made up entirely of omega-3 and -6 fatty acids and they are usually come in pairs to regulate the bodily functions: one to increase and the other to decrease. Once omega-3s are deficient in the diet, the body make a lesser amount of one of the pairs and as a result these internal thermostats don't work properly.

There are different families of eicosanoids (fig. 6): the prostaglandins (which regulate muscle contraction, immune response and inflammation), prostacyclins (which inhibit platelet aggregation) and thromboxanes (which bring about platelet aggregation) and these are all produced by the action of cyclo-oxygenase enzymes; the leukotrienes (which affect microvascular and bronchial constriction or dilatation) and hydroxyfatty acids (which regulate cell adhesion) are produced by the action of lipoxygenases. The activity of the different eicosanoids produced from essential fatty acids is different; eicosanoids derived from the cis n-6 PUFAs usually exert pro-inflammatory effects, which are opposed by those originated from n-3. Thus the diet has an important role to play in determining the final mix and strength of eicosanoids.

Figure 6. Mechanism of eicosanoid formation (adapted from Sanders & Emery 2003[15])

How omega-3 fatty acids exert health benefits or how it functions?

Roles of omega-3 fatty acids

Though fats in general have various uses in the human body, their most important roles involve the cell membranes, brain, and a host of hormone-like substances that work like thermostats in the body, switching on or off a number of different metabolic functions so as to maintain energy balance and promote good health. Fatty acids are used in the body as a means of energy storage, to provide energy, and to provide stable cellular membranes. The importance and beneficial effects of omega-3 fatty acids are well publicised. They are important structural components of the phospholipid membranes of tissues all over the body and are enriched in the retina, brain, and spermatozoa, where docosahexaenoic acid (DHA) makes up 36.4% of total fatty acids [16,17]. Many substances in the body which play a role in the regulation of blood pressure and inflammatory responses are derived from omega-3 (ω-3) fatty acids which are present in the cell membrane. Omega-3 fatty acids are also an important component of nerve cell membranes, and are essential for maintaining integrity of nerve cells. This ensures that the nerve cells can communicate with each other, which is an essential step in maintaining optimal mental health. A further well-described function of omega-3 fatty acids is its effects on the development and maintenance of retinal function. Omega-3 fatty acids also reduce inflammation and the anti-inflammatory effects of the oemga-3 fats have particular importance for its protective effect against many diseases.

As mentioned earlier omega-3 fats also play a significant role in the production of prostaglandins which are effective hormone-like substances. Prostaglandins help to regulate the production of other hormones and the functions of the kidneys and gastrointestinal tract. Also prostaglandins help out to normalize large numbers of major physiological functions including blood clotting, blood pressure, nerve transmission, inflammatory and allergic responses. Basically, all prostaglandins carry out important physiological functions. But, the production of prostaglandins may be different depending on the type of fat in the diet, sometime it may be produced in large quantities, whereas other time it may not be produced at all. This will lead to an imbalance all the way through the body that can lead to disease.

Effect of omega-3 LCPUFA on cell function

Each cell in our body is bounded by a cell membrane composed primarily of fatty acids. The cell membrane makes sure that waste products are promptly removed from the cell and allows the right amounts of nutrients to enter the cell. However, the cell membrane should keep up its integrity and fluidity to properly carry out these functions favourably. Cells in which the content of n-3 fatty acids is sub-optimal are unable to communicate effectively with other cells and also fail in their capacity to retain water and essential nutrients. As cell membranes are composed of fat, the integrity and fluidity of our cell membranes is influenced, to a great extent, by the type of fats we consume. Unlike saturated fats, polyunsaturated fats are liquid at room temperature and remain as liquid when refrigerated or frozen and so omega-3 fats are liquid at room temperature. Diets with large amounts of saturated or hydrogenated fats make cell membranes that are hard with less fluidity. In contrast, diets rich in omega-3 fats make cell membranes with a high degree of fluidity. These differences in the properties of the cell membrane affect cell function.

Health Benefits of Omega-3 Fatty Acids and Its Importance in Human Nutrition

Health benefits of Omega-3s

Omega-3 fatty acids are very important for our health in a variety of ways. It has been publicized that the intake of sufficient amount of omega-3s may help to attain the following health-promoting benefits.

Reduce risk of heart disease including CHD (coronary heart disease) and atherosclerosis.-It has been proven in many clinical studies that omega-3 fatty acids benefit heart health, mainly through their anti-atherogenic (preventing the formation of artery-blocking atherosclerotic plaques) and vasodilatory (widening of blood vessels) effects [18-21]. American Heart Association also support these findings.

Reduce risk of unwanted blood clots-Omega-3s help to prevent thrombosis (formation of blood clots) by stopping platelets (thrombocytes) from sticking together and forming blood clots[22]. Blood clots may possibly result in heart attack or pulmonary embolism (PE) and stroke which are the main cause of death in Western world. However most of them are preventable by adding omega-3 fatty acids and supplements and other anti-clotting foods into one's diet.

Lower blood pressure-Omega-3 fatty acids have been made known to lower mild hypertension when it is due to cardiovascular disease, specially atherosclerosis[23] (hardening of the artery walls, formation of arterial plaques, and the resulting narrowing of the arteries).

Reduce inflammatory diseases- As omega-3s are natural anti-inflammatory agents they act to prevent or ease the symptoms of arthritis, menstrual cramps, migraine headaches, and asthma [24-26].

Vision-Omega-3s are important both for the retina and blood supply through the tiny capillaries in the eyes[27,28].

Brain and mood-Omega-3s are a major constituent of the brain, especially DHA. Dietary n-3 polyunsaturated fatty acids have positive effects on neural and brain development [27,29,30]. Omega-3 supplementation may also benefit persons with mood disorders. Lack of omega-3 fatty acids has been linked to depression by researchers.

Helping patients with attention deficit/hyperactivity disorder (ADHD), dyslexia and dyspraxia - Omega-3 supplementation may help persons who undergo ADHD, dyspraxia and dyslexia (lack of ability to perform coordinated skilled movements) [31,32].

Helping to improve memory and learning skills, and prevent Alzheimer's disease - Research have found that omega-3 intake is associated with a lowered risk of developing Alzheimer's disease and the supplement of Omega-3 will also have protecting effect on the nervous system and improve brain function [33].

Cancer-Omega-3s lower the risk of cancer. They make the body's immune system stronger to protect against the appearance of new cancerous cells. It also make it difficult for a tumour to metastasize to other areas of the body[2].

Reduce risk of osteoporosis-Bones are living tissue, which can be broken down continually and rebuilt. Eicosanoids help to normalize the balance between osteoclasts(which break down bone) and osteoblasts(which rebuild). Research indicates that healthy omega-3 levels play a role in rebuilding bone rather than losing it[34].

Reduce the risk of becoming obese- Omega 3s enhance body's ability to respond to insulin by stimulating the secretion of leptin, a hormone that helps to control food intake, body weight and metabolism and reduce the risk of becoming obese.

Importance of Omega-3 Fatty Acids in Human Nutrition

There is little doubt that n-3 fatty acids are important in human nutrition. n-3 Fatty acids are necessary fatty acids, required from conception through pregnancy and infancy and, certainly throughout life. There are two most important periods for gaining these essential n-3 fatty acids: throughout fetal development and after birth until the biochemical development in the brain and retina is completed. During pregnancy, there is a great importance for both maternal stores and dietary intake of n-3 fatty acids in insuring that the fetus has sufficient amounts of n-3 fatty acids and all the polyunsaturated fatty acids, including DHA, are transferred across the placenta into fetal blood at the time of birth [35]. It has been evidenced in a number of studies in monkeys that the infant at birth is deficient in n-3 fatty acids, when the maternal diet is deficient in n-3 fatty acids, as evidenced by low DHA concentrations in infant's plasma and red blood cells [36]. It has been shown in humans that giving fish oil or sardines to pregnant women led to higher DHA concentrations in both maternal red blood cells and blood plasma and in cord red blood cells and blood plasma at the time of birth[37]. In addition, the adult risk for chronic diseases may be influenced by intrauterine nutrition [38], signifying that early nutrition has an imprinting consequence on later life. This further highlights the importance of an adequate supply of essential PUFA during pregnancy, lactation, and infancy.

Infants are at risk for developing vision and nerve problems if they do not get an adequate supply of omega-3 fatty acids from their mothers during pregnancy. The biochemical evidence in human studies, mainly in red blood cells, plasma, and, seldom, in tissues from autopsied infants has demonstrated that lower n-3 fatty acid concentrations in plasma and red blood cells are associated with reduced concentrations in the brain and retina[16]. Lower concentrations of brain DHA are found in formula-fed infants than infants fed human milk [39,40] and formula-fed infants also have lower intelligence quotients[41]. DHA is a most important constituent of membrane phospholipids, particularly in the retina and brain and its deficiency can cause decreased vision and abnormal record of electrical changes in retina [16]. In adults, symptoms of omega-3 fatty acid deficiency include poor memory, heart problems, fatigue, mood swings or depression, dry skin, brittle nails and hair, constipation, frequent colds, accelerated aging and poor circulation. Hence there is no doubt that n-3 fatty acids are essential in human nutrition, and need to be consumed as part of a healthy balanced diet. Consumption of recommended amounts of omega-3 fatty acids can contribute to improvement of general health and welfare of entire population, especially young people.

Recommended intakes of Omega-3 Fatty acids

Prehistoric humans evolved on a diet that consisted primarily of fresh fruits, leafy vegetables and animals and all these foods provided a fairly good balance of omega-6 and omega-3 PUFA upon which physiological and metabolic processes were established. For millions of years the approximate 1: 1 ratio of omega-6 to omega-3 remained unchanged [42-44]. Significant changes in the composition of the food supply of western societies over the past 150 years resulted in an increase in the consumption of omega-6 PUFA and a corresponding decrease in the intake of omega-3 PUFA. Today, in North America the ratio of omega-6 to omega-3 is estimated to be in the range of 10:1 to 25:1 [42-45]. These changes in the food supply and consequent shift in the ratio of omega6 to omega3 PUFA is attributed to a number of factors. Modern food production methods decreased the omega3 fatty acid content of many foods. The current western diet which is high in long chain omega6 polyunsaturated fatty acids (LC PUFA) and low in long chain Omega3 PUFA may not supply sufficient omega3 LC PUFA or an appropriate balance between these critical nutrients. This imbalance is believed to cause many varied diseases.

Nutrition experts hold opposing views both on how much we require, and the optimum ratio between omega-3 and omega-6 fatty acids. Some suggest consuming equal quantities (a 1:1 ratio), while others suggest no more than 10 omega-6s to each omega-3. It looks as if the ratio between the two is what is really important. Intake of too much omega-6 compared to omega-3 may increase the risk of numerous health problems. Even though there is no Recommended Daily Amount of Omega 3 that we should have daily, recently European Food Safety Agency has proposed a Recommended Nutrient Intake (RNI) of 2g per day of Alpha Linolenic Acid. Small quantities of Alpha Linolenic Acid are present in many plant foods but plants do not supply EPA and DHA, which are so useful to our bodies. A suggestion of the level of EPA and DHA needed is provided by some authorities such as The International Society for the Study of Fatty Acids who recommend a combined EPA and DHA of 500mg per day and the UK government's Scientific Advisory Committee on Nutrition in 2004 who suggested a minimum of 450mg per day which can be obtained from consuming 2 portions of fish a week (one of which oily). However the intakes of Alpha Linolenic Acid, EPA and DHA are typically lower than either of these recommendations in many EU populations. As the typical EPA+DHA intake is around 250mg per day in UK many people find it beneficial to supplement their diet through the consumption of fish oils as they find it easier. In 1990, separate dietary recommendations for omega6 and omega3 PUFA have been issued in Canada[46]. The Department of Health in the United Kingdom, advise an omega3 PUFA intake of a minimum of 0.2 percent of energy [47]. Also, the Task Force of the British Nutrition Foundation proposes a daily omega3 PUFA consumption ranging from 0.5 to 2.5 percent of energy in the form of Alpha Linolenic Acid. The IOM (U.S. Institute of Medicine) set an Adequate Intake for ALA, based on the median daily intake of healthy Americans who are not likely to be deficient in this nutrient [48]. The Adequate Intakes of ALA are shown in Table 1. The Adequate Intake is 1.6 g ALA per day for men and 1.1 g ALA per day for women.

Table 1. Adequate Intakes of ALA for children, adolescents, adults and pregnant and lactating women

Source: Institute of Medicine[48]

Food Sources of Omega-3 Fatty Acids

There are a lot of dietary sources both animal and vegetable, available to us for omega 3 and omega 6 fatty acids but the utmost activity comes from fish for omega- 3 and meat for omega 6 as animals will provide the preformed long chain members of these fatty acid families. Interestingly it is because of the fact that fish consume algae which themselves are rich sources of EPA and DHA that fish is a much better source of omega 3 than land animals.

EPA and DHA

EPA and DHA are found mainly in fatty fish such as mackerel, sardine, salmon, tuna, herring, lake trout, capelin (codefish) and anchovy (Table 2.) As far as supplementation is concerned the two primary sources of EPA and DHA are currently either Cod Liver Oil or Fish Body Oil from fatty fish such as Tuna, Anchovy or Sardines. Either is acceptable as sources of EPA and DHA but on the whole Cod Liver Oil tends to be less concentrated in the fatty acids while also providing vitamins A and D. Although fish are commonly seen as the most important source of omega 3 fatty acids they are not so good at providing the parent essential fatty acid Alpha Linolenic Acid (ALA).

Table 2. Dietary sources of EPA and DHA

Eicosapentaenoic acid(20:5 n3) Docosahexaenoic acid(22:6 n3)

Freshwater fish (5-13%) Sardine (9-13%)

Pacific anchovy (18%) Pacific anchovy (11%)

Capelin (codfish) (9%) Mackerel (8%)

Mackerel (8%) Capelin (codfish) (3%)

Herring (3-5%) Herring (2-3%)

Sardine (3%) Freshwater fish (1-5%)

Note: Values in parentheses represent percent of total fatty acid, Lee[43].

Table 3. Total EPA and DHA content of selected fish

Fish gram/100gram*

Atlantic mackerel 2.5

Atlantic salmon,farmed 1.8

Pacific herring 1.7

Atlantic herring 1.6

Lake trout 1.6

Bluefin tuna 1.6

Sturgeon 1.5

Anchovy 1.4

Sprat 1.3

Sardines, canned,drained 1.1

Source: Nettleton[49]

ALA

ALA is found in plants, animals, plankton and marine species[50]. Up to 80% of the fatty acids in leafy green plants are in the form of ALA; but leafy plants do not contribute significant amounts of ALA to our diets as their overall fat content is low[51]. Flax is the richest source of ALA in the North American diet. ALA is also found in canola oil, walnut oil, olive oil, and soybean oil; in nuts such as butternuts and walnuts; in soybeans and pumpkin seeds; in omega-3-enriched eggs; and in purslane. Fish contain only trace amounts of ALA. With a recommended intake of 2g ALA per day it is clear to see that even with a consumption of 5 portions of fruit and vegetables a day (typical portion 80g) it is not possible to reach a 2g intake without incorporating fish or a more concentrated ALA source such as flaxseed oil. Fruits tend to have a lower ALA content than vegetables [52].

Table 4. Food Sources of ALA

Source: Institute of Medicine[48]

Conclusion

Like vitamins and minerals, essential fatty acids are also essential to us; in reality some people even refer them as Vitamin F. If we take a good balanced diet containing 2 portions of fresh fish with at least one being oily fish it will help to support our omega 3 intake but if we do not eat fresh fish or are planning to increase our intake of omega 3 fatty acids then we have to select a high quality fish oil supplement that is protected from oxidation and supplies a good intake of the long chain omega 3 fatty acids to help support positive health of the body and mind. As the typical diet contains on average half of the daily requirement of omega- 3 fatty acids and often too much omega- 6 we should concentrate on the intake of omega- 3. This has been supported highly by many researchers recently. With an average diet providing 10 times as much omega 6 as omega 3, our fatty acids are out of balance so we have to focus on the intake of both our omega 3 and 6 to make sure we not only get enough but a balanced intake of both. With this in mind our body then has the right balance of fats to keep it healthy. For most people a normal dietary intake will be safe and beneficial. However some people may experience problems with high-dose supplementation of omega- 3 fatty acids.