Polyunsaturated Fatty Acids (PUFAs) – The Essential Fats
A very well know proverb stated by a German Philosopher says, “A man is what he eats”. This proverb rightly sums up the importance of food in providing vital nutrients for life and in disease prevention. There is increasing scientific evidence that supports the role of a healthy diet in the prevention of various chronic diseases such as diabetes, heart diseases, stroke, cancer, etc. (Liu, 2013). A very crucial part of a healthy diet is the consumption of adequate amounts of dietary fats, particularly “fatty acids and fat-soluble vitamins” (Roche, 1999, pp.397–401). These are usually present in the form of triglycerides in oils, dairy products, etc., cholesterol esters in animal-derived products, and phospholipids (Laposata, 1995, pp.172–179).
Fatty acids can be divided into two major groups: saturated fatty acids (SFA) and unsaturated fatty acids. Unsaturated fatty acids can be further broken down into two groups based on the number of double bonds present: (i) monounsaturated fatty acids (MUFA) and (ii) polyunsaturated fatty acids (PUFA) (Rustan and Drevon, 2005). Even though it is often believed that fats are not good for health and thus should be consumed in limited quantities, this does not stand true for all types of fats. PUFAs have been found to play an essential role in maintaining human health. Food and Agriculture Organisation of the United States concluded that “fatty acids should now be considered as key nutrients that affect early growth and development and nutrition-related chronic disease later in life.” (FAO, 2010, p. 3) Keeping these key points in mind, this paper will attempt to discuss the structure, role and optimum levels of PUFAs required in maintaining human health and in the prevention of various diseases.
Structure of fatty acids
A fatty acid is composed of two parts: a hydrocarbon chain with a methyl group and an acidic carboxylic group. When all the carbons in the hydrocarbon chain are linked to each other through single bonds so that the molecule holds as many hydrogen atoms as possible, the fatty acid is saturated. However, if the carbon atoms in the chain contain one or more double bonds, the fatty acid is unsaturated. Based on the position of the hydrogen atoms on the double bonded carbon atoms, there can be two different geometric configurations of the unsaturated fatty acid. Cis-unsaturated bonds have the hydrogen atoms on the same side which leads to a kink in the chain whereas trans-fatty acids have the hydrogen atoms on opposite sides of the double bond resulting in a straight configuration. When only one double bond is present, it is a monounsaturated fatty acid (MUFA) whereas when two or more double bonds are present is it a polyunsaturated fatty acid (PUFA) (Roche, 1999, pp.397–401).
Depending on the position of the first carbon double bond, PUFAs can be further grouped into two types: omega-3 (ω-3) and omega-6 (ω-6). They are also represented as n-3 and n-6 respectively. Human beings cannot synthesize their precursors, linoleic acid (18:2n-6; LA) and α-linolenic acid (18:3n-3; ALA), making them essential fatty acids, and thus need to be obtained from the diet. These fatty acids can be desaturated into their longer-chain derivatives, arachidonic acid (20:4n-6), eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) (Roche, 1999, pp.397–401).
Importance of omega-3 fatty acids and its dietary sources
n-3 fatty acids include ALA, stearidonic acid (18:4n-3; SDA), EPA, docosapentaenoic acid (22:5n-3; DPA) and DHA. In theory, ALA can be converted to DHA and EPA, thus the body should have no direct need of these fatty acids from the diet. However, this is not the case. An enzyme called d-6-desaturase is required for the biosynthetic pathways of n-3 and n-6 fatty acids. Due to the presence of high levels of plasma LA, the action of this enzyme is shifted towards the n-6 pathway (Budowski, 1988, pp.214–274). This results in low plasma levels of DHA and EPA. Moreover, the efficiency of the conversion of ALA to DHA has also been questioned. A study performed by Sanders and Roshanai (1983) comparing supplementation using linseed oil (ALA) and fish oil (EPA and DHA) showed that the consumption of linseed oil led to a moderate increase in platelet EPA while fish oil resulted in a rise in both platelet EPA and DHA. These two reasons show the importance of having an exogenous supply of these fatty acids to avoid clinical deficiency.
Plants are the primary source of ALA, however, flaxseeds, chia seeds, walnut, canola and echium seed oils are also known to be good sources of ALA. Salmon, sardine, halibut, and herring oil contain high amounts of EPA and DHA (Shahidi and Ambigaipalan, 2018, pp.345–381). It is important to consume these fatty acids regularly and in moderate amounts as they provide numerous health benefits. DHA was found to be the primary component of membrane phospholipids in the parts of the brain such as cerebral cortex, synaptic vesicles and mitochondria (Connor, 2000, pp. 171s–175s). Studies have also shown that n-3 PUFA helps in the prevention of various diseases. This has been represented in table 1.
Type of Disease Findings Reference
Cardiovascular
an intake of 5.5 g of n-3 fatty acids per month was associated with a 50% reduction in the risk of primary cardiac arrest. Siscovick et al. (1995)
The risk of heart failure reduced with an increase in consumption of baked/broiled fish in postmenopausal women. Belin et al. (2011)
Diabetes Type-2 diabetic patients saw an increased sensitivity for insulin with an increase in n-3 PUFA intake. Farzi et al. (2014)
Cancer n-3 PUFA exert an anticarcinogenic effect Prener et al. (1996)
Improves the efficacy of chemotherapy Mocellin et al. (2017)
Table 1: Summary of research findings on the beneficiary effects of n-3 on various diseases
The above table shows the importance of adequate consumption of omega-3 PUFAs to lead a healthy life.
Effects of omega-6 fatty acids and recommended dietary intake
The optimal dietary fat profile suggests a low intake of both saturated and n-6 PUFAs along with a moderate intake of n-3 PUFAs. n-6 fatty acids include LA and arachidonic acid and are precursors to endocannabinoids, lipoxins, and eicosanoids. Vegetable oils such as corn, grape seed and sunflower oil are good sources of LA whilst and arachidonic acid is mainly present in meat. Several chronic diseases such as cancer, asthma, depression, etc. are associated with an increased consumption of n-6 PUFAs and decreased by an increased consumption of n-3 PUFAs (Ristić-Medić et al., 2013, pp.1269–1289).
While LA has been associated with a reduced risk of atherosclerosis, cardiovascular heart diseases and type-2 diabetes (Ristić-Medić et al., 2013, pp.1269–1289), studies have shown the negative health effects of n-¬6 fatty acids such as inflammatory effects. It was found that a group of rats with a diet high in n-3 PUFAs was better protected from myocardial preconditioning as compared to rats with a dietary intake of high n-6 PUFAs and saturated fatty acids (Abdukeyum, Owen and McLennan, 2008, pp.1902–1909).
In the last few decades, the intake of LA has significantly increased, causing the n-6 to n-3 ratio to increase. In most western populations, this ratio is typically between 5-20. Since n-6 PUFAs have inflammatory effects whereas n-3 PUFAs have anti-inflammatory effects, the ratio of n-6 to n-3 is recommended to be lower (Ristić-Medić et al., 2013, pp.1269–1289). This can result in a significant decrease in several chronic diseases related to inflammatory processes, such as cancer, neurodegenerative illnesses, inflammatory bowel diseases, etc. (Wall, et al., 2010, pp.280–289). Thus, a monitored intake of PUFAs, i.e. lower levels of n-6 PUFAs is essential in maintaining overall health. Currently, different organisations have different positions on the recommended intake of n-3 and n-6 PUFA or LA and ALA. However, continuous research and debates are aimed towards finding the optimal levels of their consumption. Based on recent reports, an effective total intake of PUFAs in order to prevent chronic diseases is 6 – 11% of daily energy intake (Ristić-Medić et al., 2013, pp.1269–1289).
Conclusion
A wide range of physiological and pathological processes involve the need of PUFAs. Decreasing the intake of dietary saturated fats and replacing it with PUFAs is the main dietary strategy to prevent cardiovascular diseases, cancer, diabetes, etc. It is also essential to maintain a healthy ratio of n-6/n-3 fatty acids. Hence, dietary modifications should be made based on the health benefits and risks of specific food items. As discussed in this paper, a better diet can help in the prevention of various diseases and can lead to a good overall health. Thus, a healthy diet rich in omega-3 polyunsaturated fatty acids and limited amounts of omega-6 polyunsaturated fatty acids is beneficial for human health.
(Sanders & Roshanai,
1983
(Sanders & Roshanai,
1983
(Sanders & Roshanai,
198
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