Chemistry Essays - Essential Fatty Acids

A discussion on the role of essential fatty acids in human health.

Introduction: Fatty acids are important components of lipids (fat-soluble components of living cells) in plants, animals, and microorganisms.  Generally, a fatty acid consists of a straight hydrocarbon chain and a terminal carboxyl (-COOH) group.  It is the carboxyl group that makes it an acid.  If the carbon-to-carbon bonds are all single, the acid is saturated; if any of the bonds is double or triple, the acid is unsaturated and is more reactive.  The physical properties of fatty acids are determined by chain length, degree of unsaturation, and chain branching.

This essay focuses on the role of essential fatty acids, i.e. fatty acids that must normally be present in the diet of certain animals including man.  These acids occur in body structures, especially the different membranes inside and around cells, and cannot be synthesised in the body from other fats.  Essential fatty acids all possess double bonds at the same two positions along their hydrocarbon chain and so can act as precursors of prostaglandins, thromboxanes and leukotrines.  Deficiency of essential fatty acids can cause dermatosis, weight loss, irregular oestrus etc.  Table 1 shows the most important natural fatty acids.

Table 1: The most important natural fatty acids.

Omega-3 (w3) & Omega-6 (w6) fatty acids: The Western Diet

There is an absolute requirement for a dietary intake of w3 and w6 polyunsaturated fatty acids (PUFA), because of their role in constructing cell membranes, and also as precursors for the prostaglandins and other eicosanoids.  In fact, w3 fatty acids are particularly prevalent in the brain, retina, and spermatozoa, in which docosahexanoic acid (see table 1) constitutes approximately 36.4% of total fatty acids. 

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The two parent compounds, which are considered to be dietary essentials, are linoleic acid (w6) and a-linolenic acid (w3).  These two can undergo chain elongation and further desaturation to yield arachidonic acid (w6) and docosahexaenoic acid (w3) respectively, which are both precursors to two families of prostaglandins.  There is no requirement for a dietary source of the longer chain PUFA, but they are potentially desirable in the diet, since their formation from linoleic or linolenic acids is a rate-limiting step. 

So, what is the desirable level of w3 and w6 fatty acid intake and does the western diet meet this level?  Ideally, the body requires a near balanced intake of both w3 and w6 fatty acids since the eicosanoids derived from w3 fatty acids have opposing metabolic properties to those derived from w6 fatty acids.  Omega-3 fatty acids, on the one hand, have anti-inflammatory, antithrombotic, antiarrhythmic, hypolipidemic, and vasodilatory properties.  For example, they have the ability to make blood thinner by reducing platelet aggregation (this is particularly beneficial in reducing blood clots and stroke).  They also play an important role in the modulation and prevention of human diseases, particularly coronary heart disease, type 2 diabetes, and renal disease, and there is strong evidence that w3 fatty acids are essential for human development from infancy onwards.  On the other hand, w6 fatty acids promote platelet aggregation and vasoconstriction, as well as increasing blood viscosity and bleeding time.

The western diet is far richer in w6 fatty acids than w3.  This is due to the increased intake of w6-rich foods such as cereals, fried foods, baked foods etc.  In fact, in today’s western diet, the ratio of w6 to w3 fatty acids ranges from @20-30:1 instead of the traditional, and optimum, range of 1-2:1.  This means that the eicosanoids produced from the metabolism of w6 fatty acids are formed in greater amounts than those derived from the w3 fatty acids, specifically eicosapentaenoic acid.  The result is an increase in allergies, inflammation, type 2 diabetes, and the formation of thrombi and atheromas.  This explains why cardiovascular diseases are far more prevalent in the west.  Consequently, people should be encouraged to eat more w3-rich foods (such as cold-water fish, flaxseed, rapeseed etc) in order to make the ratio more even. 

Figure 1 shows the relationship between the ratio of w6 to w3 fatty acids in dietary lipids in the Indian diet and the prevalence of type 2 diabetes.  It is clear that, as the ratio of w6 PUFAs to w3 PUFAs increases, the prevalence of type 2 diabetes also increases.

Figure 2: Data obtained from Raheja BS, Sadikot SM, Phatak RB, Rao MB. Significance of the n-6/n-3 ratio for insulin action in diabetes. Ann N Y Acad Sci 1993, 683, 258-7.

As far as cardiovascular diseases are concerned, a more even w6 to w3 ratio (i.e. greater w3 fatty acid intake or decreased w6 intake) has been shown to contribute to keeping cholesterol low, stabilising arrhythmia, and keeping blood pressure low.

Fatty Acids, Inflammation & Autoimmune Diseases

Omega-3 PUFAs, in particular eicosapentaenoic acid and docosahexaenoic acid, have been shown to have potent immunomodulatory effects.  Some of the effects of w3 PUFA are induced by modulation of the amount and types of eicosanoids made, and other effects are triggered by alternative mechanisms that do not involve eicosanoids, for example actions upon intracellular signalling pathways, transcription factor activity and gene expression.

Eicosapentaenoic acid (w3) competes with arachidonic acid (w6) for the enzymatic metabolism (by cyclooxygenases) that results in prostaglandin formation.  The eicosanoid metabolites of eicosapentaenoic acid produce fewer inflammatory and chemotactic derivatives than the arachidonic acid metabolites.  Therefore, increasing the dietary intake of w3 fatty acids reduces the likelihood of inflammation.

Omega-3 fatty acids have also been shown to slow down the production of interleukin 1 (IL-1b) by suppressing the IL-1b mRNA, as well as the Cox2 (cytooxygenase) mRNA that is induced by IL-1b.  Cox2 is overexpressed in colon cancer cells.  Furthermore, research also suggests that a high dose of a-linolenic acid can suppress the formation of Tumor Necrosis Factor (TNF), which is implicated in causing some of the pathological responses that occur in inflammatory conditions.  Hence, it appears that by improving diet management, and ingesting more w3 (as opposed to w6) fatty acids, we could significantly reduce inflammatory disorders and the 40% of cancers that are caused by diet alone.  Managing autoimmune and inflammatory conditions in this way is clearly beneficial since it eliminates many of the side effects associated with the available drug therapies.

Fatty Acids and Mental Health

Given the fact that around 60% of the brain is composed of fat, it is not surprising that the organ needs w3 fatty acids to function properly.  There is evidence that links mood disorders with low concentrations of w3 fatty acids in the body.  According to certain theories of depression, w3 fatty acids help to regulate mental health problems by improving the ability of the brain’s serotonin (5-HT) receptors to understand mood-related signals from other neurones.  These theories are being investigated further.

Conclusion

Essential fatty acids have justifiably been linked with a plethora of health issues affecting society at large today.  What is most evident is that, in the western world, we are failing to ensure that we make optimum use of the benefits that w3 fatty acids have to offer.  In order to benefit more, we must take action to balance our intake of w3 and w6 fatty acids.  At present, the balance is tipped too greatly towards w6 fatty acids.  Consequently, heart disease, hypertension, type 2 diabetes, depression, inflammatory disorders, cancer etc are on the increase.  That is not to say that w6 fatty acids are totally bad for us: the body requires reasonable amounts for the purposes of vasoconstriction etc.  The ratio of w6/w3 essential fatty acids can be improved by reducing our intake of w6 fatty acids from vegetable oils and increasing our intake of w3 fatty acids from foods such as oily fish, leafy green vegetables, wild game etc.

What is clear is that new benefits of w3 fatty acids are being discovered every day.  This is evident from the masses of literature on the subject.  But it is one thing to learn about the benefits of w3 fatty acids and a completely other to use this information effectively.  This is the area that needs to be addressed.      

REFERENCES

Journal Articles Referred to in the text:

• Neuringer M et al., Biochemical & functional effects of prenatal and postnatal omega-3 fatty acid deficiency on retina and brain in rhesus monkeys, Proc Natl Acad USA 1986, 83, 4021-5
• Connor W.E., Importance of n-3 fatty acids in health and disease, American Journal of Clinical Nutrition, 2000, 71(1), 171-5
• Simopoulos A.P, Essential fatty acids in health and chronic disease, American Journal of Clinical Nutrition, 1999, 70(3), 560-569
• Raheja BS, Sadikot SM, Phatak RB, Rao MB. Significance of the n-6/n-3 ratio for insulin action in diabetes. Ann N Y Acad Sci 1993, 683, 258-7
• Harris WS et al., Effect of fish oil on VLDL triglyceride kinetics in man, J Lipid Res 1990, 31: 1549-58
• McLennan PL, Relative effects of dietary saturated, monounsaturated, and polyunsaturated fatty acids on cardiac arrhythmias in rats, Am J Clin Nutr, 1993, 57, 207-12
• Charnock, JS, Antiarrhythmic effects of fish oils, World Rev Nutr Diet, 1991, 66: 278-91
• Berry EM, Hirsch J., Does dietary linolenic acid influence blood pressure? Am J Clin Nutr, 1986, 44: 336-40
• Simopoulos, AP., Omega-3 Fatty acids in inflammation and autoimmune diseases, American Journal of Clinical Nutrition, 2002, vol 21(6), 495-505
• Maes M et al., Fatty acid composition in major depression: decreased omega-3 fractions in cholesteryl esters…, J Affect Disord, 1996, 38, 35-46