THE words are big -- eicosapentaenoic acid and docosahexaenoic acid -- but the sales are even bigger, exceeding $45 million a year. These are the scientific names of two prominent fatty acids in fish oils, better known as omega-3's, supplements of which have become hot items among the health conscious.
Despite the popularity of fish oil capsules, most researchers in the field say this pill popping is scientifically unjustifiable and may even be dangerous to some people. Furthermore, some of the early claims for benefits attributed to fish oils have not yet been borne out, despite well-designed studies.
Fish Oil Furor
The enthusiasm for fish oils began with the observation that Greenland Eskimos, whose diet is rich in marine oils, rarely suffer heart attacks or strokes caused by blood clots. Several effects of fish oils have been cited as the probable explanation for this association. First, the oils lower blood levels of artery-damaging fats called triglycerides. Elevated levels of these fats can increase the risk of a heart attack. Some preliminary research also indicates that fish oils may help to prevent a potentially fatal disruption in heart rhythm called ventricular fibrillation.
But most important, fish oils were found to have a potent anti-clotting effect. They both reduce the stickiness of platelets, the blood cells that initiate clots, and inhibit the formation of four coagulation proteins, one of which, fibrinogen, is considered a risk factor for heart disease. Since most heart attacks and strokes are caused by clots trying to squeeze through arteries narrowed by fatty deposits, anything that inhibits clotting would logically lower the risk.
The claims for fish oil then went hog wild, with hints of benefits being turned into established facts long before proper studies had explored them. One was the suggestion that fish oils lower blood pressure. And indeed they do, in relatively large doses in people who have both high blood pressure and high blood cholesterol levels or atherosclerosis. But, according to a recent review of 31 studies involving 1,300 patients, in the majority of people with hypertension, fish oils at practical doses have at best a very modest effect on blood pressure.
More promising has been the association between fish oils and prevention or treatment of autoimmune diseases like lupus, kidney disease and rheumatoid arthritis. The Greenland Eskimos are reported to have a relatively low incidence of certain disorders that involve the immune system, including multiple sclerosis, asthma and psoriasis. Most of the studies to date have been done in laboratory animals that are prone to autoimmune diseases. But studies of patients with rheumatoid arthritis found that fish oil supplements (about six grams a day) can diminish morning stiffness and joint tenderness, a benefit that can persist for four weeks after the supplements are stopped.
Although this may seem unrelated, studies of chronic migraine sufferers who did not respond to other remedies showed significant relief associated with very large (20-gram) doses of fish oil supplements given every day. Researchers believe that fish oils change the balance of substances called prostaglandins and leukotrienes, which are potent mediators of inflammation and immune responses.
The anti-inflammatory effects of fish oils are believed to be responsible for the new finding that frequent consumption of fish helps to protect smokers from chronic obstructive pulmonary diseases, like chronic bronchitis and emphysema. According to the study of 8,960 current and former smokers, published last week in The New England Journal of Medicine, those who ate two and a half or more servings of fish each week halved their odds of developing these lung diseases.
There is also some preliminary evidence to suggest that fish oils may inhibit the spread of some cancers, particularly breast cancer. The omega-3 fatty acids are thought to inhibit proliferation of tumors by preventing the formation of tumor-stimulating prostaglandins. In a study in mice published last November in The Journal of the National Cancer Institute, spread of human breast cancer to the lungs was inhibited in animals fed a diet rich in fish oils, but no such benefit was observed in mice fed primarily corn oil, which is rich in the omega-6 fatty acid, linoleic acid.
The Downside
Despite the suggested benefits of fish oils to the heart, no major health organization, including the American Heart Association, recommends taking fish oil supplements outside of a well-designed research study. There are good reasons for their reluctance.
The clot-inhibiting properties of fish oils have an unfortunate downside: they increase bleeding tendencies. In the Greenland Eskimos, the fish-oil rich diet appears to raise their risk of suffering usually fatal hemorrhagic strokes. In a person susceptible to other bleeding problems, like a bleeding ulcer, or for someone undergoing surgery or injured in an accident, fish oils could make matters worse.
There is also concern about the inhibiting effects of fish oils on the immune system. Whereas suppressing excessive immune reactions may be beneficial to those with autoimmune disorders, it may also reduce the ability of certain white blood cells to fight off infections. And unless taken with vitamin E, large doses of fish oil can deplete the body's supply of this critical antioxidant nutrient.
There is some evidence that fish oils can actually raise low-density lipoprotein, or LDL, cholesterol, the so-called bad cholesterol. There is the possibility of toxic effects from contaminants like pesticide residues in fish oil capsules that are produced with less than stringent care. And there are some some distressing side effects, including excessive belching, diarrhea and a fishy odor among those who take high doses.
What to Do
No one questions the health value of eating fish, a food rich in top-quality protein, B vitamins, minerals like iron, magnesium, zinc and potassium and very low in artery-damaging saturated fats. Most of the benefits of fish oils are associated with studies of people who eat fish regularly. Just two fish meals a week have been shown to reduce the risk of coronary death. A person who takes supplements might continue eating a diet high in damaging saturated fats, but those who eat fish are likely to substitute it for fattier meats.
The fish richest in omega-3 fatty acids include salmon, sardines, mackerel, herring, bluefish, whitefish and halibut, with more modest amounts in rainbow trout, striped bass, shark, swordfish and squid.
No comments:
Post a Comment