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If you're old enough, you may remember your mother giving you cod liver oil. This practice actually 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.
Omega-3 fatty acids are one type of essential fatty acids, special fats that the body needs as much as it needs vitamins. (The other type is the omega-6 fatty acids).Much of the research into the potential therapeutic benefits of omega-3 fatty acids began when studies of the Inuit (Eskimo) people found that although their diets contain an enormous amount of fat from fish, seals, and whales, they seldom suffer heart attacks or develop rheumatoid arthritis. This is presumably because those sources of fat are very high in omega-3 fatty acids.
Subsequent investigation found that 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 is no daily requirement for fish oil. However, a healthy diet should provide at least 5 g of essential fatty acids daily.
Many grains, fruits, vegetables, and vegetable oils contain significant amounts of essential omega-6 and/or omega-3 fatty acids. Some authorities believe that it is important to consume several times more omega-3 fatty acids than omega-6 fatty acids. If this theory is true, taking fish oil supplements might help ensure the proper balance.
Cod liver oil is the most common form of fish oil, but it may not be the best for reasons of safety (see Safety Issues). Salmon oil, mackerel oil, halibut oil, and the oils from other coldwater fish might be better choices.
Typical dosages of fish oil are 3 to 9 g daily, but this is not the upper limit. In one study, participants ingested 60 g daily.
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, you should probably take enough fish oil to supply about 1.8 g of EPA (1,800 mg) and 0.9 g of DHA daily (900 mg).
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.
Flaxseed oil also contains omega-3 fatty acids, although of a different kind. It has been suggested as a less smelly substitute for fish oil. However, there is no evidence that it is effective when used for the same therapeutic purposes as fish oil
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, we have no evidence that fish oil slows the progress of the disease. Consult your rheumatologist to determine what treatment is best for you.
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.
A study suggests that fish oil can be very helpful for bipolar disorder, more commonly known as manic-depressive illness. More research is needed, but this appears to be a potential breakthrough for this devastating condition, whose conventional treatment causes a great many side effects.
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, 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.
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.
Small double-blind trials suggest that fish oil might be helpful for reducing symptoms of ulcerative colitisHowever, 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.
Despite widely publicized claims that fish oil helps asthma, most preliminary studies have not found it effective, and one study found that fish oil can actually worsen aspirin-related asthma.
Fish oil is also sometimes recommended for enhancing immunity in HIV infection. However, one 6-month double-blind study found that a combination of the omega-3 fatty acids in fish oil plus the amino acid arginine was no more effective than placebo in improving immune function in people with HIV. Fish oil, however, might help individuals with HIV gain weight.
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).
DHA has been evaluated as a possible treatment for male infertility, but a double-blind trial of 28 men with impaired sperm activity found no benefit. Some, but not all, studies suggest that fish oil and GLA taken along with calcium may be helpful in preventing osteoporosis.
Finally, a preliminary trial found some evidence that a supplement containing fish oil and evening primrose oil might improve ADHD symptoms.
What Is the Scientific Evidence for Fish Oil?
There is some evidence that fish oil can help prevent heart disease, but it is not definitive.
Some, but not all, studies have found that fish oil can lower serum triglycerides. However, the standard drug gemfibrozil appears to be more effective than fish oil. Like cholesterol, triglycerides are a type of fat in the blood that tends to damage the arteries, leading to heart disease. Reducing triglyceride levels should help prevent heart disease to some extent. Fish oil has been studied particularly for reducing triglyceride levels in people with diabetes, and it appears to do so safely and effectively.
Fish oil also appears to modestly raise the levels of HDL ("good") cholesterol. Additionally, it may help the heart by "thinning" the blood and by reducing blood levels of homocysteine. Blood clots play a major role in heart attacks, and homocysteine is an amino acid that appears to raise the risk of heart disease.
Studies contradict one another on whether fish oil can lower blood pressure. A 6-week double-blind placebo-controlled study of 59 overweight men suggests that the DHA in fish oil, but not the EPA, can reduce blood pressure.
Evidence from some (but not all) observational trials suggest that higher dietary intake of omega-3 fatty acids may help prevent heart disease and strokes. Despite this, most intervention trials of fish or fish have generally failed to find evidence of improvements in the progression of heart disease. One open trial of 11,324 individuals followed for 3 to 5 years did find that use of fish oil significantly reduced the risk of death from heart attack, and that it was more effective for this purpose than vitamin E. In addition, there is some evidence that higher consumption of fish oil might help prevent sudden cardiac death, possibly by helping prevent abnormal heart rhythms.
However, a double-blind trial of 300 individuals who had just had a heart attack found no meaningful benefits with 4 g of concentrated omega-3 fatty acids, given daily for 12 to 14 months, as compared to corn oil, even though HDL and triglyceride levels improved in the fish oil group.
The omega-3 fatty acids in fish oil can help reduce the symptoms of rheumatoid arthritis, according to 13 double-blind placebo-controlled studies involving a total of over 500 people. This evidence is so strong that it has impressed many conventional physicians. However, unlike some conventional treatments, fish oil probably does not slow the progression of rheumatoid arthritis.
Regular use of fish oil may reduce the pain of menstrual cramps.
In a 4-month study of 42 young women aged 15 to 18, half the participants received a daily dose of 6 g of fish oil, providing 1,080 mg of EPA and 720 mg of DHA daily. After 2 months, they were switched to placebo for another 2 months. The other group received the same treatments in reverse order. The results showed that these young women experienced significantly less menstrual pain while they were taking fish oil.
Another double-blind study followed 78 women, who received either fish oil, seal oil, fish oil with vitamin B12 (7.5 mcg daily), or placebo for three full menstrual periods. Significant improvements were seen in all treatment groups, but the fish oil plus B12 proved most effective, and its benefits continued for the longest time after treatment was stopped (3 months). The researchers offered no explanation why B12 should be helpful.
A 4-month double-blind placebo-controlled study of 30 individuals suggests that fish oil can produce striking benefits in bipolar disorder, preventing relapse and improving emotional state. Eleven of the 14 individuals who took fish oil improved or remained well during the course of the study, while only 6 out of the 16 given placebo responded similarly.
The study will now be repeated by Baylor University and Harvard Medical School/McLean Hospital, enrolling 120 people for a period of 3 years.
In small double-blind studies, high dosages of fish oil have been found to reduce the severe finger and toe responses to cold temperatures that occur in Raynaud's phenomenon. However, these studies suggest that a very high dosage must be used to get results, perhaps 12 g daily. Gamma-linolenic acid (GLA), an omega-6 fatty acid, may work as well.
There is some evidence that EPA from fish oil may be helpful in psoriasis. One double-blind study followed 28 people with chronic psoriasis for 8 weeks. Half received 1.8 g of EPA daily (supplied by 10 capsules of fish oil), and the other half received placebo. By the end of the study, researchers saw significant improvement in itching, redness, and scaling, but not in the size of the psoriasis patches. However, another double-blind study followed 145 people with moderate to severe psoriasis for 4 months and found no benefit as compared to placebo.
There is some evidence that essential fatty acids may enhance the effectiveness of calcium in osteoporosis. In one study, 65 postmenopausal women were given calcium along with either placebo or a combination of omega-6 fatty acids (from evening primrose oil) and omega-3 fatty acids (from fish oil) for a period of 18 months. At the end of the study period, the group receiving essential fatty acids had higher bone density and fewer fractures than the placebo group.
However, a 12-month double-blind trial of 42 postmenopausal women found no benefit.
The explanation for the discrepancy may lie in the differences between the women studied. The first study involved women living in nursing homes, while the second studied healthier women living on their own. The latter group of women may have been better nourished, and already received enough essential fatty acids in their diet.
Lupus is a serious autoimmune disease that can cause numerous problems, including fatigue, joint pain, and kidney disease. One small 34-week double-blind placebo-controlled crossover study compared placebo against daily doses of EPA (20 g) from fish oil.133 A total of 17 individuals completed the trial. Of these, 14 showed improvement when taking EPA, while only 4 did so when treated with placebo. However, evidence suggests that fish oil is specifically not effective for lupus nephritis (kidney damage caused by lupus).
Attention Deficit and Hyperactivity Disorder (ADHD)
Based on evidence that essential fatty acids are necessary for the proper development of brain function in growing children, EFAs have been tried for the treatment of ADHD and related conditions. A preliminary double-blind placebo-controlled trial found some evidence that a supplement containing fish oil and evening primrose oil might improve ADHD symptoms. However, a high dropout rate makes the results of this trial somewhat unreliable.
Fish oil appears to be safe. The most common problem is fishy burps.
Because fish oil has a mild "blood-thinning" effect, it should not be combined with powerful blood-thinning medications, such as Coumadin (warfarin) or heparin, except on a physician's advice.
However, contrary to some reports, fish oil does not seem to cause bleeding problems when it is taken by itself. or even with aspirin.
Also, fish oil does not appear to raise blood sugar levels in people with diabetes.
Nonetheless, if you have diabetes, you should not take any supplement except on the advice of a physician.
Fish oil may temporarily raise the level of LDL ("bad") cholesterol, but this effect seems to be short-lived, and levels return to normal with continued use.
If you decide to use cod liver oil as your fish oil supplement, make sure you do not exceed the safe maximum intake of vitamin A and vitamin D. These vitamins are fat soluble, which means that excess amounts tend to build up in your body, possibly reaching toxic levels. Pregnant women should not take more than 2,667 IU of vitamin A daily because of the risk of birth defects; 5,000 IU per day is a reasonable upper limit for other individuals.
Look at the bottle label to determine how much vitamin A you are receiving. (It is less likely that you will get enough vitamin D to produce toxic effects.)
Interactions You Should Know About
If you are taking Coumadin (warfarin) or heparin, do not take fish oil except on the advice of a physician.
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