2000-05-30

Before we get round to discussing the omega-3 fatty acids, we need to clear up some technical jargon regarding fats in general. Too many health-care professionals tend to lump all of the fats together, which oversimplifies their role in the body.

Fatty acids are straight chains of hydrogen and carbon atoms. There are 24 common fatty acids, differing in chain length as well as in the position and number of double bonds. They are named according to the number of carbon atoms in their chain and the number and location of double bonds within the chain. A fatty acid that is saturated has no double bonds, whereas an unsaturated fatty acid contains either one (mono-unsaturated) or more (polyunsaturated) double bonds. The location of the first double bond is always designated as the "omega" number; hence omega-3 fatty acids have their first double bond at the third carbon atom and omega-6 fatty acids at the sixth carbon atom.

Classification of the essential fatty acids
There are two classes of essential polyunsaturated fatty acids, omega-3 and omega-6. These fatty acids are essential in that they cannot be manufactured by the body and therefore need to be obtained through diet. The omega-3 fatty acids of interest include alpha-linolenic acid (LNA), and its derivatives eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Linoleic acid is the predominant omega-6 fatty acid.

Why do I need to know about omega-3 fatty acids?
About 35 years ago, researchers discovered that Eskimos eating their traditional marine diet were virtually free of heart disease compared to other nationalities. Eskimos also had lower levels of rheumatoid arthritis, and myocardial infarctions (heart attacks) were near absent. It was also interesting to note that Eskimos ate a predominantly high-fat diet, consisting of blubber and seal meat. Researchers eventually came to realise that the high level of polyunsaturated fats in the Eskimo diets, particularly omega-3 fatty acids, were responsible for this cardioprotective effect.

The protective effect of a high fish diet against the incidence of atherosclerosis and heart disease was again demonstrated in Japan. Japanese farmers consume around 90g of fish per day, compared to Japanese fisherman, who consume around 250g per day. Japanese fishermen have lower blood pressures and lower incidences of heart disease than do the farmers, and the Japanese population as a whole has a sixfold lower incidence of atherosclerosis and heart disease compared to Americans, who consume less than 20g of fish per day.

Fish oils are rich in the omega-3 fatty acids EPA and DHA, which originate from the meat of cold water fish, such as herring, mackerel, sardines and trout. Shellfish, although particularly low in fat (0.5 - 2.5% fat) have about half their fat made up of omega-3 fatty acids.

To date, most clinical trials with human subjects consistently show that fish oil supplements cause a decrease in VLDL triglycerides and some even observed a decrease in blood cholesterol. VLDL's are the fat-carrying lipoprotein particles that are predominantly enriched with triglycerides. Metabolically, omega-3 fatty acids decrease liver production of triglycerides. In general, the higher the concentration of fish oils in the diet, the lower the concentration of these VLDL fat carrying packages, and thus the lower is the blood triglyceride levels. Any effect of lowering LDL-cholesterol ("bad" cholesterol) is related to their displacement of animal fat (which is saturated) from the diet.

The fish oils also exert several other beneficial effects by modulating the manufacture of a group of fat-derived compounds known as eicosanoids - these are very important biological molecules with key physiological roles in the body such as controlling pain and inflammation as well as the blood-clotting mechanism.

To understand the role of the fatty acids, it is necessary to get a simple overview of some of the sophisticated biochemistry. Basically, all of the polyunsaturated vegetable oils that we buy in the supermarket (sunflower, safflower, peanut and maize oil, as well as the products made from them, such as margarine, mayonnaise and salad dressing) all contain the essential fatty acid linoleic acid (omega-6). Linoleic acid is also found in the popular supplement Evening Primrose Oil.

It is metabolised to a substance called arachidonic acid, which is then converted into the specific eicosanoids that control clotting in the blood and are also pro-inflammatory substances. Incidentally, arachidonic acid is also obtained directly from animal foods, such as eggs, beef, chicken, veal, etc. EPA is chemically very similar to arachidonic acid and, if present, will be used preferentially in all the biochemical reactions involving arachidonic acid. The EPA will also be converted into eicosanoids, but these specific eicosanoids are less inflammatory and tend to inhibit platelet aggregation or thrombosis (clot formation).

Thus beef fat, high in arachidonic acid, will help induce clotting and inflammation, whereas those fats found in coldwater fish, including EPA and DHA, will prevent clotting and are anti-inflammatory. This is one of the main reasons that substituting fish and fish oil for animal oil and animal products causes a decrease in the clotting potential of the blood and a reduction in inflammatory conditions, such as rheumatoid arthritis.

In fact, the daily use of 1.8g EPA has been shown to provide symptomatic relief in patients with rheumatoid arthritis. EPA is not recommended as the sole basis for management of rheumatoid arthritis, because better symptomatic relief is available from the non-steroidal anti-inflammatory drugs (NSAIDs). However, it does appear worthwhile to increase the consumption of seafood and fish oils that are rich in omega-3 fatty acids.

Although omega-3's prevent blood clotting, on the down side, this translates into an increased bleeding tendency and it has, in fact, been known for centuries that Eskimos have thinner blood and a certain bleeding tendency.

Other effects of omega-3 fatty acids include improvement of oxygen supply to tissues, a moderate reduction in arterial blood pressure, and relaxation of coronary arteries, all of which may contribute to prevention of atherosclerosis.

How much omega-3 fatty acids do we need?
Although initial studies indicated that fish oils needed to be taken in 15 - 30g quantities for them to be effective, we now know that smaller quantities taken over a longer period of time can be sufficient. As little as two fish meals each week may have a significant effect. If you have problems eating fish twice or three times a week, you can obtain a concentrated form of fish oil in a supplement.

Many people believe that if they don't want to eat fish, they can still get omega-3 fatty acids in the form of linseed oil or other similar vegetable based oils that are high in alpha-linolenic acid (LNA). LNA occurs in plant leaves and a few vegetable seed oils, including linseed (57% LNA), flaxseed (57%), walnut (10%), canola oil (8%), and soybean (7%).

It should be understood, however, that even though LNA can theoretically be converted into EPA, in reality this conversion is carried out at a very slow rate and only small amounts of EPA can be manufactured from LNA. This means that the only viable source of EPA is from cold water fish or fish oils. The cold factor is necessary because if fish from warmer tropical waters are eaten, their level of EPA is much lower and arachidonic acid much higher.

Dietary fish oils and vitamin E status
Generally, to obtain the maximum benefit from the two kinds of fatty acids, we need to eat approximately 10g of omega-6 for every 1g of omega-3. Unfortunately, most people are having 20g or 30g of omega-6 for every 1g of omega-3.

As the consumption of fish oil becomes more popular, it is important to understand that fish oils are highly unstable polyunsaturated molecules. With an increased concentration of these oils in our tissues, there will be more free radicals generated in the body. Increasing the daily intake of Vitamin E, which is a powerful antioxidant, solves the answer to this problem. The higher the intake of fish oils in the diet, the higher is the requirement for vitamin E to protect the body tissues from oxidation.

Conclusion
Most recommendations have concluded that the minimum intake of essential fatty acids should be about 3% of total energy intake. Compared to the present very low intake of long chain omega-3 fatty acids in most countries, it would mean a large increase in the intake of fatty fish.

You might be asking "Why should I add fat to my diet when everything I hear tells me to reduce my fat intake?" Well, we don't recommend that you add more fat to your diet if you are already getting 30% or more calories from fat, but you can make a difference in your health if you can replace many of the "bad" fats with these "good" fats.

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