The theory is that fish oil lowers cholesterol and triglycerides, makes blood less sticky, and perhaps even decreases blood pressure. Beyond the heart benefits, it's claimed that fish can alleviate rheumatoid arthritis, psoriasis, and other auto-immune disorders. Even more health claims are made for fish-oil capsules, which take up lots of shelf space in health-food stores and drugstores. Fish is good food, certainly, but let's not go overboard. Here are the fish facts.
Note: When we use the term fish oil below, we mean the oil you get by eating fish or taking capsules. While population studies look at fish consumption, clinical studies almost always use fish oil capsules. It's generally assumed that the supplements provide the same health benefits as the fish itself. However, there may be other beneficial compounds in the fish not found in the capsules.
What's special about fish oil?
The fat in fish is rich in polyunsaturated fatty acids called omega-3s, the major marine types being eicosapentenoic acid (EPA) and docosahexenoic acid (DHA). Fatty fish are the richest sources. Like aspirin, these omega-3s make platelets in the blood less likely to stick together and may reduce inflammatory processes in blood vessels. Thus they reduce blood clotting, thereby lessening the chance of a heart attack.
Does fish oil lower elevated cholesterol?
Yes, according to some studies; no, according to others. If you substitute fish for meat (or for other sources of saturated fat), you will lower your blood cholesterol. But the same would happen if you replaced the meat with beans or other foods low in saturated fat. Thus, when researchers control for saturated fat intake, the effect of fish on cholesterol often turns out to be minimal, at best.
What about triglycerides?
Large amounts of fish oil are known to reduce triglycerides, the major type of fat that circulates in the blood. While some scientists believe that even moderately elevated triglycerides are an independent risk factor for heart disease, this is still unclear, as is what level should be considered high.
Does fish oil lower elevated blood pressure?
Once again, the studies have had contradictory results. If there is an effect, it would take large doses, and the effect would be small.
So what effect does fish have on the risk of heart disease?
Most population studies find some beneficial effect, especially against heart attacks, but the results have been surprisingly inconsistent. Interestingly, some important studies-notably the Harvard Health Professionals Follow-up Study in 1995-have found that fish consumption doesn't reduce the risk of heart disease, but may reduce the risk of dying from it. A 1997 study that followed 1,800 men from the Chicago area for 30 years found that those who ate at least eight ounces of fish a week had a 40% lower risk of fatal heart attack than those who ate no fish (the study didn't look at nonfatal heart attacks).
Some research has also looked at fish's effect on the risk of sudden cardiac death. In 1998 the Physicians Health Study found that men who ate fish at least once a week had a 50% lower risk of sudden cardiac death, but at least three studies contradict this.
Why this sea of contradictions?
Much of the debate about fish oil is theoretical, not based on data from clinical studies. Perhaps if fish oil does protect the heart, it may not be by obvious means such as lowering blood cholesterol, triglycerides, or blood pressure. For instance, some studies suggest that omega-3s may be beneficial because they modulate electrical activity in the heart, thus making the heart less susceptible to dangerous, sometimes fatal, rhythm abnormalities.
In addition, in population studies that compare fish eaters to those who eat no fish, it's possible that the people who eat no fish have a less healthy life-style. Researchers adjust the data for such confounding factors, but can't control for everything.
Can fish oil help treat diabetes?
Probably not. Some researchers are hopeful about this, but studies thus far have not found consistent benefits. In fact, some have shown that fish oil may actually impair blood sugar control in diabetics and perhaps also raise their blood cholesterol.
What about rheumatoid arthritis and other inflammatory disorders?
This may be where fish holds the most promise, though the research is preliminary. Fish oil may help relieve inflammatory symptoms of these auto-immune diseases by suppressing the immune response. More than a dozen studies have suggested that high doses of fish oil supplements taken long term and with pain medication can reduce joint swelling, ease morning stiffness, and lessen fatigue in people with rheumatoid arthritis. There is also some preliminary evidence that fish oil may help reduce the itching and redness of psoriasis, another auto-immune disease.
Don't some plants contain omega-3s, too?
Yes. But leafy greens and some vegetable oils (such as walnut, flaxseed, or canola) contain only the short-chain omega-3 linolenic acid, not the longer-chain fatty acids found in fish oils. Fish are able to convert the linolenic acid in algae and other sea plants into EPA and DHA, but humans can do so only to a limited degree.
Do high doses of omega-3 capsules have adverse effects?
There are some serious concerns:
The decreased ability of your blood to clot has a negative side, notably an increased risk of hemorrhagic stroke. People with bleeding disorders, those taking anticoagulants, or those with uncontrolled hypertension (who are already at high risk for a stroke) should definitely not take fish oil capsules.
Large doses can cause nausea, diarrhea, belching, and a bad taste in the mouth.
Like all fats, omega-3s are concentrated sources of calories: some recommended doses supply 200 calories a day.
Fish oil in liquid or capsule form may contain pesticides or other contaminants. These are usually removed in the manufacturing process, but there's no guarantee.
Cod liver oil is overly rich in vitamins A and D, which can be toxic in high doses. There's no reason to take it.
Large doses of fish oil may suppress certain aspects of the immune system.
Last words: We recommend fish, but not fish oil supplements. Exceptions: if you have rheumatoid arthritis or psoriasis, fish oil capsules may be worth a try, but consult your doctor. Fish itself is one of the best foods around. Besides its oil, it is rich in protein, iron, B vitamins, and other nutrients, and it can take the place of meats that are high in saturated fat. Studies finding that fish enhances cardiovascular health suggest that two servings a week are enough. In fact, a higher intake of fish isn't necessarily better for your health.