The practice of giving cod liver oil began when the smoke-filled skies of nineteenth-century England deprived youngsters of exposure to the sun. Without sun, their bodies couldn't make vitamin D, and they developed rickets. Because cod liver oil contains large amounts of vitamin D, it cured rickets and made a great contribution to public health. Today, however, other constituents of cod liver and other fish oils have become of interest: the omega-3 fatty acids.
Fish oil is obtained from the livers of fish or from the meat.
The most important omega-3 fatty acids found in fish oil are called EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). In order to match the dosage used in several major studies, one should probably take enough fish oil to supply about 1,800mg of EPA and 900mg of DHA daily. Certain conditions require a predominance of EPA while others require DHA. Since they compete with each other, careful attention must be paid to their ratio in those conditions which are discussed elsewhere.
Some manufacturers add vitamin E to fish oil capsules to keep the oil from becoming rancid. Another method is to remove all the oxygen from the capsule.
The omega-3 fatty acids found in fish oil can lower blood triglyceride levels, thin the blood, and also decrease inflammation in various parts of the body. These effects, as well as others, may explain many of fish oil's apparent benefits.
There has been a great deal of excitement about the possibility of using fish oil to help prevent heart disease. According to some but not all studies, fish or fish oil appears to lower triglyceride levels, raise HDL ("good") cholesterol, "thin" the blood, reduce levels of homocysteine, slow down atherosclerosis, help prevent death from heart disease, and perhaps also treat hypertension.
Fish oil has also become recognized as an effective treatment for early stages of rheumatoid arthritis. It appears to significantly reduce symptoms without side-effects and may magnify the benefits of standard arthritis drugs. However, there appears to be no evidence that fish oil slows the progress of the disease.
Various essential fatty acids, including fish oil, flaxseed oil, and GLA (gamma-linolenic acid), are widely recommended for dysmenorrhea (menstrual pain), and two studies suggest that fish oil may indeed be effective.
Small studies also suggest that fish oil may be helpful in Raynaud's phenomenon (a condition in which a person's hands and feet show abnormal sensitivity to cold temperatures), psoriasis, sickle-cell anemia, the autoimmune disease lupus, and a form of kidney disease called IgA nephropathy. Preliminary double-blind trials suggest that EPA, but not DHA, might be helpful for schizophrenia.
Evidence is mixed on whether fish oil can help prevent flare-ups of Crohn's disease, a condition in which parts of the digestive tract are highly inflamed. A 1-year double-blind trial, involving 78 participants with Crohn's disease in remission who were at high risk for relapse, found that fish oil supplements helped keep the disease from flaring up. However, a double-blind placebo-controlled trial that followed 120 individuals for 1 year found that fish oil did not reduce the relapse rate as compared to placebo. Negative results were also seen in a smaller double-blind trial.
Small double-blind trials suggest that fish oil might be helpful for reducing symptoms of ulcerative colitis. However, another small double-blind placebo-controlled trial found no such benefit. Larger studies will be necessary to discover for certain whether fish oil helps or not. Regular use of fish oil does not appear to help prevent disease flare-ups.
A large observational study found evidence that higher consumption of fish and other sources of omega-3 essential fatty acids may reduce the risk of stroke. Interesting, but highly preliminary, evidence suggests that fish oil, or its constituents, might be helpful for treating depression, preventing premature birth, improving vision in premature babies, treating kidney stones, alleviating the symptoms of chronic fatigue syndrome, and reducing the risk of prostate cancer.
Fish oil has also been proposed as a treatment for many other conditions, including diabetic neuropathy, allergies, gout, and migraine headaches, but there has been little real scientific investigation of these uses.
Preliminary studies suggest fish oil may help symptoms of multiple sclerosis; however, one double-blind study found no difference between people taking fish oil and those taking olive oil (used as a placebo).
Typical dosages of fish oil are 3 to 9gm daily, but this is not the upper limit. In one study, participants ingested 60gm daily.
Often there is a digestive disturbance that prevents some people from using fish oils. There are preparations which are coated to overcome this difficulty.
CAUTION: Do not confuse "fish oil" with "cod liver oil" as they are not the same thing. Cod liver oil contains high concentrations of vitamin A. Taking cod liver oil in the same amounts that are recommended for fish oil can be toxic, and even more so in people who have chronic renal failure (because vitamin A can build up to toxic levels).
Several studies have shown an inverse relationship between blood levels of fish oils (eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]) and the risk of prostate cancer. A study by medical researchers at the Karolinska Institute confirms this association. [The Lancet, Vol. 357, June 2, 2001, pp. 1764-66 (research letter), British Journal of Cancer, Vol. 81, No. 7, December 1999, pp. 1238-42]
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