Essential Fatty Acids: A Primer
By Elzi Volk http://www.bulknutrition.com/a21_Essential_Fatty_Acids__A_Primer.html Copyright © 2002-2008 BulkNutrition.com Powered by MirageCommerce
Fat. The word ‘fat’ is ubiquitous in our everyday conversations, in the media, on food labels, in advertisements, and in health care. ‘Fat’ also conjures various reactions, depending on individual perspectives and understanding. It is often the scapegoat for our physical maladies, or the source of our foodstuffs’ pleasing taste. Nonetheless, what are fats? Are all fats the same? In this two-part series, I will present a basic introduction to fats with special attention to a class of fats called polyunsaturated fatty acids, or PUFAs for short. The Anatomy of Fat Fatty acids (FAs) are the simplest type of fats. Their chemical compositions determine their biological activity, so the reader should understand some of the chemistry and structure of FAs. They are composed of chains of carbons of varying lengths and with attached hydrogens. FAs are the components of more complex fats and are important as an energy source. The length and the number of bonds determine the structure and conformation (shape) of each FA molecule and thus its biological activity. Accordingly, FAs are categorized based on these attributes. Length Recall that FAs are chains of carbons, varying from 4 to 24 carbon atoms long. With a chain length from 2 to 4 they are called short-chain; from 6 to 10 they are called medium-chain, and from12 up to 24 they are called long-chain FAs. Most FAs have an even number of carbon atoms. However, odd-numbered FAs occur in some food sources such as certain fish (tuna) and plants (olive oil). Bonds The bonds between the carbon atoms influence the structure and properties of the FA. The more carboncarbon double bonds that occur in the chain, the more the chain bends. A FA may be 'saturated' with hydrogen, the degree of saturation depending on how many atoms of hydrogen the FA chain contains. All the carbon atoms along the chain of a ‘saturated’ FA have the maximum possible number of hydrogen atoms attached to them. Conversely, a 'monunsaturated' fatty acid has one double bond between carbon atoms, replacing the hydrogen atoms, and a 'polyunsaturated' fatty acid has multiple double bonds.
In Figure 1, you can see that the double bonds and orientation of the hydrogen atoms on either side of these bonds cause the molecule to bend conferring the structure of the FA. The greater the degree of saturation, the straighter the molecule and the more solid the fat molecule is. Predominantly saturated fats are usually solid at room temperature, whereas polyunsaturated fats are liquid (commonly called ‘oils'). Fats with mostly monounsaturated FAs will generally be semi-solid.
Unsaturated Fatty Acids
Monounsaturated Fatty Acids (MUFAs) - MUFAs are considered ‘non-essential’ because they can be synthesized within our bodies. In some parts of the world, MUFAs comprise one-third of total FA intake, with the principal MUFA being oleic acid. Polyunsaturated Fatty Acids (PUFAs) - PUFAs can have two or more (up to six) carboncarbon double bonds and are classified on the basis of those double bonds: their number and placement. For instance, one class of FA, the omega-6 PUFAs, has the first of its double bonds on the sixth carbon atom from one end of the chain. The first double bond of an omega-3 PUFA occurs on the third carbon atom from the same end. This classification will be explained in more detail. Highly Unsaturated Fatty Acids (HUFAs) - Although a less commonly used classification, HUFAs are appearing more frequently in the scientific literature. HUFAs are products of metabolized PUFAs occurring in mammals and also supplied from food sources. HUFAs will be discussed in more detail in the second part of this series. Trans Fatty Acids (TFAs) - Trans' fatty acids have recently received widespread attention. Technically, these are unsaturated FAs, yet their structures have been altered so that they now act more like saturated fats. The word 'trans' refers to the orientation of the hydrogens attached to the carbon on either side of a double bond. In TFAs, the hydrogens are oriented on opposite sides of the double bond. It is thought that this actually makes the molecule behave like a 'saturated' fat because it is relatively straight in character. TFAs can be found in hydrogenated vegetable oils and margarine to make them soft and spreadable. They are suspected to be associated with cardiovascular disease and cause cancer. Therefore, they may pose a nutritional hazard.
Fatty Acids Notation Several confusing notations often describe pUFAs. The two main notation systems used to describe PUFAs are the delta notation used by chemists, and the omega system used by physiologists and biochemists (seen most often). Common names are also assigned to each FA. In both notation systems, the chain length (number of carbons), and the number and position of double bonds present are used to classify a FA. The omega system will be used throughout this series.
Omega system The double bonds in this system are counted from the omega (oo) end, or the methyl group at the end of the chain (see Figure 2). This system puts the emphasis on the double bond closest to this end group. It is noted by oo-x, where oo being the total number of carbons, and x the position of the last double bond. The other double bonds are inferred from the first one by adding 3. The omega symbol is popularly substituted with the letter ‘n’. In Figure 2, linoleic acid is designated 18:2 (n-6). This compound has 18 carbon atoms, 2 double bonds with the first double bond on carbon number 6 from the end methyl group.
Essential Fatty Acids Two unsaturated FAs cannot be synthesized in animal cells and must be acquired from plant or fish sources. Consequently they are considered ‘essential.’ Vertebrate animals lack the enzymes delta-12 and delta-15 desaturases, which incorporate double bonds at the corresponding delta carbons of the FA chain. Mammals cannot convert n-3 to n-6 FAs, nor vice versa. However, mammals can synthesize certain FAs from the precursor EFAs. There are two essential fatty acids (EFAs): linoleic acid (18:2 n-6, or LA) and alpha-linolenic acid (18:3 n3, or ALA). LA is the precursor for the n-6 series of PUFAs. From LA, gamma-linolenic acid (GLA) and arachidonic acids can be formed in the body. ALA is the precursor for the n-3 series of PUFAs. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are metabolites of ALA that have recently received much acclaim in the public and scientific communities for their health benefits and will be discussed further in Part II. Enzymes metabolize the precursors by incorporating double bonds (desaturation) or adding carbon atoms at the carboxylic end (elongation). Figure 3 depicts the metabolism of the two EFAs and the enzymes involved.
Dietary Sources of PUFAs All edible fats contain some MUFAs, but the quantities of saturated FAs and PUFAs vary depending on the origin of the fat. Animal fats derived from sheep and cattle are largely saturated, whereas pigs and poultry have less saturated FAs and contain some PUFAs. Fats from wild animals tend to be even lower in saturated fatty acids and higher in PUFAs. Fish oil is highly unsaturated. Fish and plant oils are rich sources of n-3 FAs. Fish are the major source of DHA and EPA whereas plant oils are the major source of ALA. Minor sources of PUFAs are nuts, seeds, vegetables, fruit, and egg yolk. Fatty fish such as halibut, mackerel, herring, and salmon, are rich sources of EPA and DHA. The content of n-3 FAs vary among different types of fish. For example, Atlantic, Coho and Sockeye salmon contain higher amounts of EPA and DHA than Chinook salmon. Lean varieties of fish provide fewer n-3 FAs. The most commonly consumed oils in the US derive from plants and provide dietary ALA. Primary sources of ALA are soybean and canola, while flaxseed oil is rich in n-3 FAs (ALA). Human intake of n-3 FAs varies due to the wide disparity in food content. Variation of the n-3 FA composition in fish is due to differences in the diet, location, stage of maturity, sex and size of the fish. The season, water temperature, and preparation methods also influence their PUFA content. Farm-raised fish have lower EPA and DHA content than wild-caught because of differences in the nutrient composition of their diet. Similar factors also affect the ALA content of soybean and canola oil: cultivar, growing region, season, and climatic conditions. Recent breeding of soybean cultivars tries to reduce the ALA content of these plants because of increased oxidative stability (for use in deep-frying and non-hydrogenated liquid salad oil). Not All Fatty Acids are Equal! By now the reader should know what fatty acids are and the terms that are used when discussing them.
Fatty acids are smaller units of larger fats and they are classified by their structure and size. Some fatty acids can be synthesized in our bodies and others cannot. Thus, it is essential that we derive these fatty acids for our food. Most of our food groups provide the fatty acids that we need. Accordingly, our diet should include foods that supply a balanced proportion of these fats. However, as we shall discover in Part II of this series, our modern diets may not be adequate. In fact, imbalances in these important fats may be highly associated with the increased incidence of several diseases.
Part II will examine the biological activities of essential fatty acids, the importance of their ratios, their health benefits and possible associations with disease risk. In this context, several specific HUFAs will be discussed, as well as ways to ensure adequate intake. From reading this series on fatty acids readers will learn how they can tailor foods and supplementation with fatty acids to meet their needs.