Omega-3: - just another fish story? The pluses and minuses of a highly touted diet supplement.

The pluses and minuses of a highly touted diet supplement.

Population studies comparing the Greenland Eskimo with the Greenland Danish population have demonstrated a lower prevalence of cardiovascular disease in the Eskimos, who include a lot of fish in their diet. Numerous subsequent studies have implicated the high content of the oftenreferred-to omega-3 fatty acids in the fish oil as accounting for the effects of the fish diet.

As a consequence of this diet, Eskimos have a prolonged bleeding time, and their tissues are easily bruised. Their blood contains a decreased number of platelets and a decreased ability to clot, which is consistent with the decreased rate of coronary thrombosis (clotting that contributes to heart attack). However, the Eskimo diet is not perfect, for the prevalence of stroke among the Eskimos is much greater than is observed in the Danish population.

All these observations have led to a large expenditure of research, with the hope of finding a way to reduce cardiovascular disease in the Western civilization. As a result of this interest, commercial enterprises have produced certain products and, together with the news media, have presented bits and pieces of a story extolling the benefits of fish oil.

We want to present a more nearly complete picture of this most fascinating scientific development, and to note some reason for caution in treating fish oils as a panacea for vascular health.

First, we must take note of what are called essential fatty acids. There are two, and they are called essential because the body cannot make them and must therefore rely on obtaining them from the diet. These fatty acids are called linoleic acid (making up most of the polyunsaturated fat of corn oil, for instance) and alpha-linolenic acid.

Linoleic acid has many functions to perform in cell membranes, but the most important and best understood is its role in the production of a whole series of substances, arachidonic acid being the main product. From arachidonic acid the body forms a large array of hormonelike substances (prostaglandins, leukotrienes, thromboxanes, and lipoxins), which are vital in controlling and promoting the activities of every cell in the body. They also are very important in the function of the immune system. This family of compounds derived from linoleic acid is given a class name of "omega-6 fatty acids."

Alpha-linolenic acid has a more specific role to play as an essential fatty acid. It too undergoes a series of changes by the body, very similar to Linoleic acid; it produces two main products of current interest, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These two acids (also referred to as omega-3 fatty acids) account for the essentiality of alpha-linolenic acid. Their contribution is vital to the function of the brain and the nervous system in general, as well as some specialized tissues, such as the retina.

It is important to realize that the socalled omega-3 fatty acids that are in fish oil are made normally in our bodies. To ingest the large quantity of omega-3 acids in the fish oils may result in an excess beyond what our bodies can use. Indeed, it is known that the omega-3 acids interfere with the body's conversion of arachidonic acid to the prostaglandins, thromboxanes, and leukotrienes that are involved in the regulation of so many body functions.

Both of the essential fatty acids are products of the plant kingdom. Linoleic acid is found mainly in the oil of seeds, while alpha-linolenic is primarily in the oils of the leafy part of the plant. Linoleic acid is abundant in vegetable oils. Alphalinolenic acid does occur in some seed oils: linseed (53 percent), rapeseed or low-erucic -acid canola (11 percent), and soybean (7 percent) represent common oils containing larger amounts of this fatty acid.

Alpha-linolenic acid is a product of the chlorophyll-containing part of the plant and relates to the fish-oil story directly. Phytoplankton, which grows in the cold water of the Arctic, serves as food for a multitude of marine creatures. These creatures convert alpha-linolenic acid from the plankton to the omega-3 acids and store them in their tissues. These fats are finally concentrated in the tissues of larger fishes that eat the smaller animals; thus the fatty fishes, e.g., menhaden and salmon, contain large quantities of the omega-3 acids in their fat.

The benefits of the fish oils in lowering blood lipids are compared to polyunsaturated vegetable oils. For the lowering of serum cholesterol, the vegetable oils are superior to the fish oils, and the effects on HDL (high-density lipoprotein, the "good" kind of cholesterol) levels are equivalent between fish and vegetable oils.

However, the omega-3 oils are more effective than the polyunsaturated vegetable oils in reducing the blood triglyceride levels. The omega-3 fatty acids from fish oils accomplish this effect by suppressing the body's ability to make triglycerides; this results in a very marked decrease in the blood triglyceride levels. In contrast to these positive effects, several research groups have reported that in some individuals the omega-3 oils have caused an increased low-density lipoprotein (LDL) in the blood. Since elevated LDL in the blood is a risk factor for coronary heart disease, these observations raise questions regarding the use of fish oil in treatment of elevated blood triglyceride levels.

Besides the effect on blood lipids, the omega-3 fatty acids from fish oils reduce the ability of the blood to clot. Unwanted blood clotting is often the final event in a heart attack. Arachidonic acid, an omega-6 fatty acid derived from Linoleic acid, is used by the artery to produce an anti-blood platelet aggregation agent (prostacyclin, [PGI .sub.2] anti-platelet aggregator from arachidonic acid.

This may sound a bit complicated, but in summary, the presence of omega-3 fatty acids from fish in the blood increases the production of blood platelet antiaggregator agents. Since the blood platelets are involved in the total blood clotting process, the presence of omega-3 fatty acids from fish oils tends to reduce the clotting ability of the blood.

In the normal course of events the blood platelets produce a pro-platelet aggregator (thromboxane A sub 2) from arachidonic acid. The blood platelets are unable to use the omega-3 fish oil fatty acids to produce a pro-platelet aggregator, and furthermore the omega-3 acids suppress the normal blood platelet activity in converting arachidonic acid to thromboxane A sub 2.

Normally, there is a balance between the arterial platelet anti-aggregator agent and the platelet pro-aggregator agent. In a heart attack, or for that matter with any injury, the balance is changed in favor of pro-aggregator (thromboxane A sub 2), which is acting as a normal protective mechanism. For the person at risk of heart attack, this balance has been upset because of poor diet, high cholesterol, high blood pressure, or some other reason. Therefore, a dietary mediated adjustment can be helpful.

High levels of fish oil (for example 40 milliliters, or about 1.5 ounces of cod liver oil per day, or 4 to 5 grams ERA and 5 to 6 grams DHA per day) are able to affect this balance in favor of platelet antiaggregation.

However, again the fish oils are no panacea for good vascular health. While the omega-3 fatty acids from fish oil serve as excellent anti-thrombotic (anti-clotting) agents, high intakes of fish oils are associated with a reduced blood platelet count. This potentially undesirable side effect must be carefully investigated before fish oils can be considered as a safe prophylatic agent for long-term use. It is important to reiterate that the Greenland Eskimo has an increased bleeding time (clotting occurs more slowly) and a decreased blood platelet count, as well as an increased incidence of stroke compared to the Danish population residing in Greenland.

Diabetes mellitus, Type II (onset usually gradual and after age 40), is often associated with abnormal blood lipid (triglyceride and cholesterol) levels. The use of omega-3 fatty acids in the diet appears to be a good solution to reducing the blood triglyceride level. However, there is a word of caution for these individuals. It has been reported that one month's use of omega-3 fatty acids by non-insulin-dependent diabetics produced a deterioration of the glucose tolerance curve and an elevation of fasting glucose levels. Also, the fasting insulin level dropped 30 percent in this study.

Bear in mind that Linoleic acid of the polyunsaturated vegetable oils also has a favorable effect on the balance of platelet pro- and anti-aggregation agents. Although the effects from vegetable oil are not as dramatic as those seen with omega-3 fatty acids from fish oil, the net result of vegetable oil usage would be more desireable for long-term usage. The vegetable polyunsaturated oil effect is most effective when as much saturated fat as possible is eliminated from the diet.

There is one more concern relating to prolonged use of fish oils. A high intake of the highly unsaturated fatty acids may increase the vitamin E requirement and exceed the normal level of vitamin E intake. Thus, those who use fish oils for a prolonged time may need to supplement their diet with vitamin E.

It is not necessary to eat fish in order to get the benefits of omega-3 fatty acids. One important study recommends a near-vegetarian diet for controlling blood lipids and reducing the risk of heart disease. The suggested diet is low in totalfat, saturated fat, and cholesterol, and high in carbohydrate and fiber. Such a diet can help reduce serum cholesterol without the potential undesirable side effects of high fish oil consumption. Those who are already vegetarians but who need to reduce their blood cholesterol levels may need to decrease the visible fat in the diet, such as margarine, spreads, cooking fats, salad dressing, etc.