Primer on Fatty Acids and Fish Oil.
March 30, 2010 in Medicine
Omega-3 fatty acids are types of fat (lipid) molecules that have been proven to lower cardiovascular risk.
A fat molecule or fatty acid is a molecule that has both a water soluble end (the carboxyl end which is the weak organic acid that contributes the hydrogen proton [H]+ to the aqueous solution) and a fat or non-aqueous soluble end (the methyl group). Linking each end is a long chain of linked carbon atoms. If each of these carbons is attached to two hydrogen atoms then the fatty acid is said to be “saturated” (with hydrogen). If certain carbon atoms lack bond to a hydrogen atom then this “extra bond” is converted into a double bond with an adjacent carbon and the fatty acid is said to be “UNsaturated”. If there is more than one double bond then the molecule is said to be “POLYunsaturated” (“poly” derived from several Greek words for “many”).
These double bonds can be in one of two configurations. The “cis” configuration is where the two hydrogen atoms are attached on the same side of the double bond and the “trans” configuration is where the hydrogen atoms are on opposite sides. The cis molecule has a curved shape, has a lower melting point, and so tends to be sold as liquids like vegetable oil. The trans molecule is straighter, has a higher melting point, and is sold as a solid like margarine that is more resistant to spoilage.
Saturated fats and trans-fatty acids have been implicated as contributing to higher rates of cardiovascular disease.
Additionally, the carbon atoms are named in order starting at the carboxyl end after their equivalent letter in the Greek alphabet so that the first is “alpha”, the second is “beta”, etc. In fatty acids that have very long chains (such as those with 20 or more carbons) the last carbon is always named “omega”. These long chain fatty acids are also named by the location of the nearest double bond to the omega carbon so that omega-3 fatty acids have a double bond attached to the third carbon atom from the omega end (see diagram to the left).
The human body can create all the different types of fatty acids that it needs with the exception of linolenic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid and the alpha designation in this case is a common name and has no biochemical meaning). Once supplied with these fatty acids, the body can convert them to other fatty acids for use by cells. Linolenic acid and alpha-linolenic acid are both converted by the same enzyme delta-6-desaturase into several intermediaries that are eventually converted into various prostaglandins, leukotrienes, and thromboxanes that control and moderate cardiovascular function, immune function, and clotting. In particular, alpha-linolenic acid is converted into eicosapenaenoic acid (EPA) and docasahexanoic acid (DHA) which are found at high doses in fish.
Alpha-linolenic acid is found at high doses in flax-seed oil and, to a lesser extent, in canola, soy, perilla, and walnut oils and delta-6-desaturase has a enzymatic preference for alpha-linolenic acid that is much greater than for linolenic acid. So why not eat more flax-seed oil instead of fish oil? It turns out that the modern diet is very rich in linolenic acid (omega-6 fatty acids) and this significantly impairs the ability of delta-6-desaturase to convert alpha-linolenic acid (omega-3 fatty acids). So one can either change to a diet that predates agriculture of 10 thousand years ago or eat more fish oil.
Consumption of fish oil rich in both EPA and DHA has been shown to reduce the risk of fatal and non-fatal heart attack and sudden cardiac death in patients with a history of coronary artery disease. When taken within 3 months of a heart attack, fish oil has been shown to reduce the risk of sudden cardiac death. It appears that fish oil does this by reducing the risk of fatal ventricular arrhythmia that can be induced by coronary artery disease and result in sudden death. Fish oil also lowers triglycerides and has been shown to reduce the build up of atherosclerotic plaques in the coronary arteries over a period of 2 years.
However, there is no solid evidence that fish oil helps to prevent the recurrence of coronary artery plaques after angioplasty or that it helps to reduce the risk of stroke or heart failure. And there is far less evidence that fish oil reduces the risk of fatal or non-fatal heart attack or the development of symptomatic coronary artery disease in people without a history of heart disease.
Though these results are modest there are no known short term or long term risks to taking fish oil to supplement the diet with these omega-3 fatty acids and there does not seem to be a down side to taking them in the context of a history of cardiac disease or recent heart attack. However, fish oil should not be used in loo of other medications such as beta blockers, aspirin, and cholesterol lowering statin medications and certainly not as an alternative to smoking cessation, a healthy diet, exercise, and control of diabetes and hypertension. I have literally had patients who smoke a pack of cigarettes per day ask me if they should take fish oil to reduce their risk of heart attack. *facepalm*
Many of the studies are done with varying amounts of fish oil, from 1 to 6 grams total per day but the main cardiovascular protection effect is obtained from the consumption of 250 mg of fish oil (EPA + DHA) per day. Most experts add in a “hedge factor” of 250 additional mg per day since there are no known harmful side effects and recommend 250-500 mg per day. Different over the counter formulations of fish oil contain different amounts of EPA and DHA per gram of weight so it’s important to read the label to see exactly how many mg of EPA and DHA are per gram of the product. Or you can eat 1-2 servings of oily fish (Anchovy, Salmon, Sardine, Albacore Tuna) per week. Enjoy.