Wednesday, February 24, 2010
Essential Fatty Acids (Omega-3 and Omega-6)
Certain dietary deficiency diseases are quite straightforward. For instance, if we don't consume enough vitamin C for a long period of time, we will develop scurvy. Children who don't get enough vitamin D and/or calcium, will suffer from rickets. Adults with a persistent deficiency of vitamin D and/or calcium will eventually experience osteopenia and perhaps osteoporosis.
Other dietary deficiencies produce less obvious symptoms. As early as the 1920's, it was noted that a complete dietary deficiency of fatty acids produced impaired growth in animals. When this was investigated farther, it was found that the omega-3 and omega-6 fatty acids were particularly important for growth and development.
Now a detour to explain some nomenclature. Fatty acids are the carbon chains that are attached to glycerol backbones to form triglycerides. Illustrated above are three such fatty acids, alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The zig zag lines are a form of shorthand that circumvents the necessity of writing out all the carbons and hydrogens found in these molecules. Each inflection of the zig zag (plus the right end of the zig zag) represents a carbon atom. Count these points on ALA, and you will find 18 carbon atoms. On DHA there are 22. The straight lines between the points represent covalent bonds. A single straight line between two carbons is a single bond, also called a saturated bond. A double line between two carbons is a double bond, also called an unsaturated bond. You can see that all three of these fatty acids contain several unsaturated bonds, which is why they are called polyunsaturated fatty acids.
On the left of each fatty acid molecule is a carboxyl group, which is used to join the fatty acid to the glycerol backbone. On the extreme right is the omega (or final) carbon. In each of the fatty acids shown above, at carbon #3, there is a double bond. The presence of that bond means that these are omega-3 fatty acids. Fatty acids that have a double bond at carbon #6, but not at carbon #3 are called omega-6 fatty acids.
Back to the dietary deficiency story. In a review article, William Lands describes how further research showed that omega-3 and omega-6 fatty acids are somewhat interchangeable, but that omega-6 fatty acids are particularly important for maintaining skin integrity, renal function and the process of birth. Omega-3 fatty acids may be more important in the support of visual and neural functions.
As Lands describes it, in 1963 it was discovered that one of the omega-6 fatty acids could be converted to a signaling molecule called a prostaglandin. Prostaglandins act on the vascular system, affect platelet aggregation and regulate inflammation. Further research revealed that omega-3 and omega-6 fatty acids could be converted to a large set of short-lived locally-acting signaling molecules called eicosanoids. Along with the prostaglandins, these include thromboxanes, leukotrienes and prostacyclins. Representative examples are illustrated below. (Both figures in this post are modified from figures found at Wikipedia.)
There are dozens of eicosanoid molecules, and each of them has many actions. Because of this, there is no simple relationship between a deficiency of omega-3 and omega-6 fatty acids and a defined profile of symptoms. When taken in optimal amounts, the eicosanoids promote the health of the cardiovascular system, the central nervous system, and the immune system. For scientific citations, please see the extensive Notes and References section at the end of the Wikipedia article on Omega-3 Fatty Acid. Positive effects have been shown for lowering blood pressure, improving blood lipid profiles, decreasing the risk of stroke and preventing psychotic disorders.
In a practical sense, a Westernized diet provides an abundant supply of omega-6 fatty acids and a relatively poor supply of omega-3 fatty acids. During the past few decades, healthy eating recommendations have caused us to transition from animal fats, rich in omega-3 fatty acids, to corn oil, safflower oil, cottonseed oil, peanut oil and soybean oil, which are all rich in omega-6 fatty acids and poor in omega-3 fatty acids. This is important because, when it comes to omega-6 fatty acids, we cannot say that if a little is good, a lot is better. An excess of omega-6 fatty acids causes these molecules to form inflammatory intermediates which are relevant to processes such as asthma, arthritis and atherosclerosis.
A review article by Artemis Simopoulos describes how these inflammatory intermediates can be counteracted by decreasing our intake of omega-6 fatty acids and increasing our intake of omega-3 fatty acids. Unfortunately, in the modern world, it takes some thought and financial resources to balance our intake of omega-6 and omega-3 fatty acids. That will be the subject of the next blog post.