Many double-blind trials have demonstrated that fish oils (also called fish-oil concentrates) containing EPA and DHA (mentioned above) lower TG levels.¹ The amount of fish oil used in much of the research was an amount that provided 3,000 mg per day of omega-3 fatty acids. To calculate how much omega-3 fatty acid is contained in a fish oil supplement, add together the amounts of EPA and DHA. For example, a typical 1,000-mg capsule of fish oil provides 180 mg of EPA and 120 mg of DHA (total omega-3 fatty acids equals 300 mg). Ten of these capsules would contain 3,000 mg of omega-3 fatty acids. Other sources of omega-3 fatty acids, such as flaxseed oil, do not lower TGs. While flaxseed oil has other benefits, it should not be used for the purpose of reducing TGs.

Cod liver oil, another source of omega-3 fatty acids, has also been found to lower TGs.² Cod liver oil is less expensive than the fish oil concentrates discussed previously. However, cod liver oil also contains relatively large amounts of vitamin A and vitamin D; too much of either can cause side effects. In contrast, fish oil concentrates have little or none of these vitamins. Individuals wishing to use cod liver oil as a substitute for a fish-oil concentrate should consult a doctor.

Omega-3 fatty acids from fish oil and cod liver oil have been reported to affect blood in many other ways that might lower the risk of heart disease.³ However, these supplements sometimes increase LDL cholesterol—the bad form of cholesterol. A doctor can check to see if fish oil has this effect on an individual. Research shows that when 900 mg of garlic extract is added to fish oil, the combination still dramatically lowers TG levels but no longer increases LDL cholesterol.⁴ Therefore, it appears that taking garlic supplements may be a way to avoid the increase in LDL cholesterol sometimes associated with taking fish oil. People who take fish oil may also need to take vitamin E to prevent the oil from undergoing potentially damaging oxidation in the body.⁵ It is not known how much vitamin E is needed to prevent such oxidation. The amount required would presumably depend on the amount of fish oil used. In one clinical trial, 300 IU of vitamin E per day prevented oxidation damage in individuals taking 6 grams of fish oil per day.⁶

EPA and DHA, the omega-3 fatty acids found in fish oil, lower blood pressure, according to an analysis of 31 trials.⁷ The effect was dependent on the amount of omega-3 fatty acids used, with the best results occurring in trials using unsustainably high levels: 15 grams per day—the amount often found in 50 grams of fish oil. Although results with lower intakes were not as impressive, trials using over 3 grams per day of omega-3 (as typically found in ten 1,000 mg pills of fish oil) also reported significant reductions in blood pressure. One double-blind trial reported that DHA had greater effects on blood pressure than EPA or mixed fish oil supplements.⁸ DHA is now available as a supplement separate from EPA.

The omega-3 fatty acids in fish oil—eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—decrease inflammation. Supplementation with EPA and DHA has prevented autoimmune lupus in animal research.⁹ In a double-blind trial, 20 grams of fish oil daily combined with a low-fat diet led to improvement in 14 of 17 people with SLE in 12 weeks.¹⁰ Other studies also found that supplementing with 10 to 15 grams of fish oil per day,¹¹ or with the amount of EPA and DHA provided by 10 grams per day of fish oil,¹² is beneficial for people with SLE. People wishing to take such a large amount of fish oil should first consult with a doctor.

Many double-blind trials have proven that omega-3 fatty acids in fish oil, called EPA and DHA, partially relieve symptoms of RA.^{13 , 14 , 15 , 16 , 17 , 18 , 19} The effect results from the anti-inflammatory activity of fish oil.²⁰ Many doctors recommend 3 grams per day of EPA and DHA, an amount commonly found in 10 grams of fish oil. Positive results can take three months to become evident. In contrast, a double-blind trial found flaxseed oil (source of another form of omega-3 fatty acid) not to be effective for RA patients.²¹

Fish oil, which contains the fatty acids known as EPA and DHA, has been studied in the treatment of angina. In some studies, enough fish oil to provide a total of about 3 grams of EPA and 2 grams of DHA has reduced chest pain as well as the need for nitroglycerin;²² other investigators could not confirm these findings.²³ People who take fish oil may also need to take vitamin E to protect the oil from undergoing potentially damaging oxidation in the body.²⁴ It is not known how much vitamin E is needed to prevent such oxidation; the amount required would presumably depend on the amount of fish oil used. In one study, 300 IU of vitamin E per day prevented oxidation damage in individuals taking 6 grams of fish oil per day.²⁵

Double-blind research shows that fish oil partially reduces reactions to allergens that can trigger attacks in some asthmatics.²⁷ Another double-blind study showed that fish oil supplements prevented exercise-induced asthma attacks in people with asthma.²⁸ A few other researchers have reported small but significant improvements when asthmatics supplement with fish oil,^{29 , 30} but reviews of the research concluded that most fish oil studies showed little or no benefit.^{31 , 32} It is possible that some of these trials failed to show an improvement because they did not last long enough to demonstrate an effect. There is evidence that children who eat oily fish may have a much lower risk of getting asthma.³³ Moreover, in a double-blind trial, children who received 300 mg per day of fish oil (providing 84 mg of EPA and 36 mg of DHA) experienced significant improvement of asthma symptoms.³⁴ It should be noted that these benefits were obtained under circumstances in which exposure to food allergens and environmental allergens was strictly controlled. Though the evidence supporting the use of fish oil remains somewhat conflicting, eating more fish and supplementing with fish oil may still be worth considering, especially among children with asthma.

Supplementation with fish oil, rich in omega-3 fatty acids, has been associated with favorable changes in various risk factors for atherosclerosis and heart disease in some,^{35 , 36 , 37 , 38 , 39 , 40} but not all, studies.^{41 , 42 , 43} A double-blind trial showed that people with atherosclerosis who took fish oil (6 grams per day for 3 months and then 3 grams a day for 21 months) had significant slowing of progression of atherosclerotic plaques and a decrease in cardiovascular events (for example, heart attack and stroke) compared with those who did not take fish oil.⁴⁴ These results contradict the findings of an earlier controlled trial in which fish oil supplementation for two years (6 grams per day) did not promote major favorable changes in the diameter of atherosclerotic coronary arteries.⁴⁵

People diagnosed with depression may have lower blood levels of omega-3 fatty acids.^{46 , 47} A double-blind trial found that bipolar patients taking 9.6 grams of omega-3 fatty acids from fish oil per day in addition to their conventional medications had significant improvements compared with those taking placebo.⁴⁸ Similar benefits were reported in a preliminary trial that used 1.5 to 2 grams per day of pure eicosapentaenoic acid, a component of fish oil.⁴⁹ In a preliminary trial, children with bipolar disorder saw benefits from omega-3 fatty acids (1,290 to 4,300 mg per day) from fish oil given for eight weeks.⁵⁰

In a double-blind trial, people with a type of arrhythmia known as ventricular premature complexes were supplemented for 16 weeks with either 15 ml (1 tbsp) per day of fish oil or a similar amount of safflower oil as placebo. Patients taking the fish oil had a significantly reduced frequency of abnormal heartbeats compared with those receiving placebo, and 44% of those receiving fish oil experienced at least a 70% reduction in the frequency of abnormal beats.⁵¹ In a separate study, however, men given 20 ml (4 tsp) of cod liver oil per day for six weeks, beginning one week after a heart attack, had the same frequency of irregular heart beats as did men given no supplemental oil.⁵² In a double-blind study, people who had a history of certain potentially life-threatening arrhythmias—sustained ventricular tachycardia or ventricular fibrillation—had an increase in the recurrence rate of these arrhythmias when they took fish oil.⁵³ A similar study found no adverse effect of fish oil supplements in people with these serious arrhythmias.⁵⁴ Because of these conflicting findings, people with a history of either of these arrhythmias should consult a doctor before taking fish oil.

In a double-blind study of patients with chronic heart failure, supplementation with the fatty acids present in fish oil for an average of four years resulted in a small but statistically significant decrease in the number of patients who died or were hospitalized for cardiovascular reasons. The treatment consisted of 850 to 882 mg per day of a mixture of eicosapentaenoic acid and docosahexaenoic acid (as their ethyl esters).⁵⁵

Inflammation within the gut occurs in people suffering from Crohn’s disease. EPA and DHA, the omega-3 fatty acids found in fish oil, have anti-inflammatory activity. Though research is conflicting, there is some evidence that fish oil improves Crohn's disease symptoms. A two-year trial compared the effects of having people with Crohn’s disease eat 3.5 to 7 ounces of fish high in EPA and DHA per day or having them eat a diet low in fish.⁵⁶ In that trial, the fish-eating group had a 20% relapse rate compared with 58% among those not eating fish. Salmon, herring, mackerel, albacore tuna, and sardines are all high in EPA and DHA.

In a double-blind trial, people with Crohn’s disease who took supplements providing 2.7 g of EPA/DHA per day had a recurrence rate of 26% after one year, compared to a 59% recurrence rate among those taking placebo.⁵⁷

The impaired digestion of fats in people with CF often leads to a deficiency of essential fatty acids. This deficiency may in turn lead to lowered immune function, which makes people with CF more susceptible to respiratory infection.⁵⁸ This deficiency may be reversed by supplementation with corn oil (1 gram per 2.2 pounds body weight per day),⁵⁹safflower oil (1 gram per 2.2 pounds body weight per day),⁶⁰ linoleic acid (7.7 grams per day),⁶¹ and eicosapentaenoic acid (EPA from fish oil) (2.7 grams per day).⁶² EPA supplementation was particularly effective. In a double-blind trial, six weeks of supplementation with 2.7 grams of EPA per day led to a reduction in sputum and improvement in lung function in children with chronic respiratory infection due to CF.⁶³

Omega-3 fatty acids found in fish oil, particularly DHA, are needed for normal nervous system function. Depressed people have been reported to have lower omega-3 fatty acid levels (for example, DHA) than people who are not depressed.^{64 , 65 , 66 , 67} Low levels of the other omega-3 fatty acid from fish, EPA, have correlated with increased severity of depression.⁶⁸ In some double-blind trials, supplementation with various amounts of fish oil was beneficial for the treatment of depression in both children and adults,^{69 , 70} but fish oil was ineffective in other double blind trials.^{71 , 72}

EPA alone has also been reported to be beneficial. There is one case report of a man with a long history of severe depression who showed clear improvement within one month of starting a purified EPA supplement (4 grams per day of the ethyl ester of eicosapentaenoic acid [E-EPA]).⁷³ In a double-blind study, supplementation with E-EPA for 12 weeks was significantly more effective than a placebo at relieving symptoms of depression.⁷⁴ E-EPA was beneficial, even though the participants in the study had failed to respond adequately to conventional antidepressant drugs. The conventional medications were continued during treatment with E-EPA or placebo. An effective level of intake was 1 gram per day, whereas larger amounts of E-EPA resulted in little or no benefit. The authors of the study suggested that taking too much E-EPA might cause an imbalance with other essential fatty acids, which could reduce the effectiveness of the treatment.

Ten grams of fish oil providing 1.8 grams of EPA (eicosapentaenoic acid) per day were given to a group of eczema sufferers in a double-blind trial. After 12 weeks, those using the fish oil experienced significant improvement.^{75 , 76} According to the researchers, fish oil may be effective because it reduces levels of leukotriene B4, a substance that has been linked to eczema.⁷⁷ The eczema-relieving effects of fish oil may require taking ten pills per day for at least 12 weeks. Smaller amounts of fish oil have been shown to lack efficacy.⁷⁸

One trial using vegetable oil as the placebo reported that fish oil was barely more effective than the placebo (30% vs. 24% improvement).⁷⁹ As vegetable oil had previously been reported to have potential therapeutic activity, the apparent negative outcome of this trial should not dissuade people with eczema from considering fish oil.

In a preliminary study, supplementation with 3.25 grams per day of a mixture of omega-3 fatty acids (primarily eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA]) for six months markedly reduced the frequency of seizures in five severely retarded epileptic patients.⁸⁰ Additional research is needed to confirm this report and to identify which people with epilepsy are most likely to benefit.

Research on the effect of the omega-3 fatty acids that are abundant in some fish, fish oils, and flaxseed oil is conflicting. Liquid diets containing omega-3 fatty acids used in hospitals for critically ill people have been shown to improve immune function and reduce infections.^{86 87} However, in one controlled study in healthy people, a low-fat diet improved or maintained immune function, but when fish was added to increase omega-3 fatty acid intake, immune function was significantly inhibited.⁸⁸

Supplementation with DHA (an omega-3 fatty acid found in fish oil) in healthy young men has been shown to decrease the activity of immune cells, such as natural killer (NK) cells, and to inhibit certain measures of inflammation in the test tube.⁸⁹ The anti-inflammatory effects of DHA may be useful in the management of autoimmune disorders; however, such benefits need to be balanced with the potential for increased risk of infections. Other studies suggest that increased oxidative damage might be the reason for the negative effects on the immune system sometimes caused by fish oil, and that increased intake of antioxidants, such as vitamin E, could correct the problem.⁹⁰

Men with intermittent claudication consumed a fortified milk product or regular milk daily for one year. The fortified product provided daily 130 mg of eicosapentaenoic acid and 200 mg of docosahexaenoic acid (EPA and DHA, two fatty acids in fish oil), small amounts of supplemental vitamin E, folic acid, and vitamin B6, and additional amounts of oleic acid and alpha-linolenic acid. Compared with regular milk, the fortified milk product significantly increased the distance the participants could walk before the onset of pain.⁹¹

In a double-blind study, supplementation with a proprietary blend of acetyl-L-carnitine, omega-3 fatty acids from fish oil, and coenzyme Q10 for 12 months resulted in an improvement in both visual function and in objective findings on eye examination (a decrease in the drusen-covered area on the retina).⁹²

Although some doctors recommend fish oil capsules for people with MS, few investigations have explored the effects of this supplement. In one small trial, people with MS were given approximately 20 grams of fish oil in capsules per day.⁹³ After one to four months, 42% of these people received slight but significant benefits, including reduced urinary incontinence and improved eyesight. However, a longer double-blind trial involving over 300 people with MS found that half this amount of fish oil given per day did not help.⁹⁴A preliminary, two-year intervention trial tested the effects of fish oil supplements (5 ml of fish oil per day, providing 400 mg of EPA and 500 mg of DHA) combined with other dietary supplements and dietary changes in people with newly diagnosed, relapsing-remitting MS.⁹⁵ The other supplements included 3,333 IU of vitamin A per day, 400 IU of vitamin D per day, and approximately 5.5 IU of vitamin E per day. The dietary recommendations included reducing intake of sugar, coffee, tea, saturated fat from meat and dairy products, and alcohol, while increasing intake of fish, fruit, vegetables, and whole-grain bread. Sixty-nine percent of those following the regimen improved, 25% remained the same, and 6% (one person) deteriorated. The many interventions used in this trial make it impossible to determine what was responsible for the positive outcomes. Given the lack of other effective treatments for MS, though, this approach is worth trying while awaiting further evidence. In another trial, combining fish oil supplementation (6 grams per day) with a low-fat diet (15% of total calories) appeared to reduce the relapse rate in people with the relapsing-remitting form of MS.⁹⁶

In a small preliminary trial, people with MS were given 20 grams of cod liver oil, as well as approximately 680 mg of magnesium and 1,100 mg of calcium per day in the form of dolomite tablets.⁹⁷ After one year, the average number of MS attacks decreased significantly for each person. Unlike fish oil capsules, the cod liver oil in this trial contained not only eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), but 5,000 IU of vitamin D. Therefore, it is not known whether the vitamin D or fatty acids were responsible for the cod liver oil’s effects. (One preliminary study found that giving vitamin D-like drugs to animals with MS was helpful.)⁹⁸ It is also possible that the magnesium and/or calcium given to these people reduced MS attacks. Magnesium⁹⁹ and calcium¹⁰⁰ levels have been reported to be lower in the nerve tissue of people with MS compared with healthy people.

A preliminary trial found that elderly women with osteoporosis who were given 4 grams of fish oil per day for four months had improved calcium absorption and evidence of new bone formation.¹⁰¹ Fish oil combined with evening primrose oil (EPO) may confer added benefits. In a controlled trial, women received 6 grams of a combination of EPO and fish oil, or a matching placebo, plus 600 mg of calcium per day for three years.¹⁰² The EPO/fish oil group experienced no spinal bone loss in the first 18 months and a significant 3.1% increase in spinal bone mineral density during the last 18 months.

Omega-3 fatty acids have anti-inflammatory properties,¹⁰⁶ and animal studies suggest that supplementation with omega-3 fatty acids may improve recovery and prevent infection after surgery.^{107, 108} A controlled human trial found that intravenous nutritional formulas containing omega-3 fatty acids given post-operatively lowered the production of inflammatory chemicals compared with standard nutritional formulas.¹⁰⁹ Other human studies of omega-3 fatty acid-supplemented nutritional formulas used in surgery patients have included other supplemental nutrients as well and are discussed below.¹¹⁰

Supplementation with fish oil (providing either 2.7 g or 6.1 g per day of the omega-3 fatty acids EPA and DHA) significantly reduced recurrence of premature delivery, according to data culled from six clinical trials involving women with a high risk for such complications.¹¹¹ Fish oil supplementation did not prevent premature delivery of twin pregnancies, nor did it have any preventive effect against intrauterine growth retardation or pregnancy-induced hypertension. Fish oils should be free of contaminants, such as mercury and organochlorine pesticides. Women who eat substantial amounts of certain types of seafood (e.g., swordfish, tuna) may be consuming contaminants that can increase the risk of brain and nervous system abnormalities in their offspring. Exposure to mercury and polychlorinated biphenyls (PCBs) was found to be increased in relation to maternal intake of seafood. Higher exposure to these toxic contaminants has been linked to an increased risk of deficits in the developing brains and nervous systems of the children.¹¹²

In a double-blind trial, fish oil (10 grams per day) was found to improve the skin lesions of psoriasis.¹¹⁴ In another trial, supplementing with 3.6 grams per day of purified eicosapentaenoic acid (EPA, one of the fatty acids found in fish oil) reduced the severity of psoriasis after two to three months.^{115 , 116} That amount of EPA is usually contained in 20 grams of fish oil, a level that generally requires 20 pills to achieve. However, when purified EPA was used in combination with purified docosahexaenoic acid (DHA, another fatty acid contained in fish oil), no improvement was observed.¹¹⁷

Additional research is needed to determine whether fish oil itself or some of its components are more effective for people with psoriasis. One trial showed that applying a preparation containing 10% fish oil directly to psoriatic lesions twice daily resulted in improvement after seven weeks.¹¹⁸ In addition, promising results were reported from a double-blind trial in which people with chronic plaque-type psoriasis received 4.2 g of EPA and 4.2 g of DHA or placebo intravenously each day for two weeks. Thirty-seven percent of those receiving the essential fatty acid infusions experienced greater than 50% reduction in the severity of their symptoms.¹¹⁹

Supplementing with fish oil also may help prevent the increase in blood levels of triglycerides that occurs as a side effect of certain drugs used to treat psoriasis (e.g., etretinate and acitretin).¹²⁰

In a double-blind trial, supplementation with 12 large capsules of fish oil per day (providing 4 grams of eicosapentaenoic acid [EPA] per day) for 6 or 12 weeks reduced the severity of blood-vessel spasm in 5 of 11 people with Raynaud’s phenomenon.¹²¹ Fish oil was effective in people with primary Raynaud’s disease, but not in those whose symptoms were secondary to another disorder.

There are two different classes of essential fatty acids: omega-6 fatty acids and omega-3 fatty acids. There is considerable evidence these fatty acids are deficient, or are not used properly, in people with schizophrenia.^{122 , 123 , 124 , 125 , 126 , 127} Some investigators suggest this altered fatty acid metabolism may be involved in the increased need for niacin seen in some schizophrenic patients.¹²⁸ A case has been reported in which a man with schizophrenia had dramatic and sustained improvement while being supplemented with 2 grams daily of omega-3 fatty acids.¹²⁹ In a preliminary trial, schizophrenic patients receiving omega-3 fatty acids showed improvement in symptoms, as well as a reduction in adverse effects from their anti-psychotic medications.¹³⁰ Another study found that people with schizophrenia had lower blood levels of both omega-3 and omega-6 fatty acids, compared with non-schizophrenic people, even though both groups were consuming similar amounts of these fatty acids.¹³¹ In a separate preliminary study, higher intake of omega-3 fatty acids was associated with less severe disease, and supplementation with 10 grams of concentrated fish oil, a source of omega-3 fatty acids, led to significant improvement in symptoms over a six-week period.¹³² In addition, a double-blind trial found that supplementing with 1.2 g per day of omega-3 fatty acids from fish oil prevented the development of psychosis in adolescents and young adults who were at very high risk of developing a psychotic disorder.¹³³

In a double-blind trial that included 87 patients with chronic schizophrenia or a related illness (schizoaffective disorder), supplementation with 3 grams per day of eicosapentaenoic acid (one of the omega-3 fatty acids found in fish oil) was ineffective.¹³⁴ The patients in this negative study were older and had been ill for longer, compared with patients in earlier studies who improved with omega-3 fatty acid supplementation.

Several clinical trials have examined the effects of supplementation with essential fatty acids in people with schizophrenia, with inconsistent results.^{135 , 136 , 137 , 138} While the results of trials using omega-3 fatty acids are promising, further double-blind trials are needed to establish whether fatty acid supplementation is an effective therapy for schizophrenia. Trials of omega-6 fatty acids (like GLA from borage oil) have yielded predominantly negative results.¹³⁹

In a preliminary trial, 13 patients with sickle cell anemia were given two supplement combinations for seven to eight months each. The first combination included 109 mg zinc, 153 IU vitamin E, 600 mg vitamin C, and 400 ml (about 14 ounces) of soybean oil containing 11 grams of linoleic acid and 1.5 grams of alpha linolenic acid. The second combination included 140 IU vitamin E, 600 mg vitamin C, and 20 grams of fish oil containing 6 grams of omega-3 fatty acids. Reduction in diseased cells was observed only during the administration of the first protocol. The authors concluded that zinc was the important difference between the two combinations and may be a protector of red blood cell membranes.¹⁴⁰

Fish oil alone has also been studied. In a double-blind trial, supplementation with menhaden oil, in the amount of 250 mg per 2.2 pounds of body weight per day for one year, reduced the frequency of severe pain episodes by approximately 45%, compared with placebo.¹⁴¹ This treatment may work by correcting an imbalance between omega-3 and omega-6 fatty acids that occurs in people with sickle cell anemia.¹⁴²

One older controlled trial found that children given 3 teaspoons of cod liver oil per day (containing roughly 800 IU of vitamin D) for an entire school year had over 50% fewer new cavities.¹⁴³ These promising results have not been followed up with modern placebo-controlled trials.

Preliminary¹⁴⁴ and double-blind trials^{145 , 146 , 147} have found that fish oil supplementation reduces inflammation, decreases the need for anti-inflammatory drugs, and promotes normal weight gain in people with UC. However, fish oil has not always been effective in clinical trials for UC.¹⁴⁸ Amounts used in successful clinical trials provided 3.2 grams of EPA and 2.2 grams of DHA per day—the two important fatty acids found in fish oil.

In a preliminary trial, people with UC significantly improved on a sugar-free, low-allergen diet with additional nutritional supplementation that included a multivitamin-mineral supplement (2–6 tablets per day); a fish oil supplement (400 mg per day); borage oil (400 mg per day); flaxseed oil (400 mg per day); and a probiotic formula containing Lactobacillus acidophilus and other species of beneficial bacteria.¹⁴⁹ Some participants received slight variations of this regimen. Since so many different supplements were given and since the trial was not controlled, it is not possible to say which, if any, of the nutrients was responsible for the improvement observed by the researchers.

In a double-blind trial, supplementing with the fatty acids present in fish oil (0.6 grams per day of EPA and 1.7 grams per day of DHA) for six months was not beneficial in people with Alzheimer's disease. However, in the subgroup of people with very mild cognitive impairment, supplementation with these fatty acids slowed the rate of cognitive decline compared with a placebo.¹⁵⁰

In a preliminary study, four patients with chronic fatigue syndrome reported an improvement in their symptoms after taking an essential fatty acid supplement daily for at least 12 weeks.¹⁵¹ The amount used was 10 to 18 capsules per day, and each capsule contained 93 mg of eicosapentaenoic acid (EPA), 29 mg of docosahexaenoic acid (DHA), and 10 mg of gamma-linolenic acid. Because there was no placebo group in this study and, because fatigue often improves after treatment with a placebo, additional research is needed to confirm this report.

A greater intake of the omega-3 fatty acids found in fish oils has been linked to reduced risk of COPD,¹⁵² though research has yet to investigate whether fish oil supplements would help people with COPD. In a double-blind trial, people with COPD received a fatty acid supplement (providing daily 760 mg of gamma-linolenic acid, 1,200 mg of alpha-linolenic acid, 700 mg of eicosapentaenoic acid, and 340 mg of docosahexaenoic acid) or a placebo (80% palm oil and 20% sunflower oil) during an eight-week rehabilitation program. Compared with the placebo, the fatty acid supplement significantly improved exercise capacity.¹⁵³ While two of the fatty acids supplied in this supplement (eicosapentaenoic acid [EPA] and docosahexaenoic [DHA] acid) are found in fish oil, is not known which components of the supplement were most responsible for the improvement. Gamma-linolenic acid is found in evening primrose oil, black currant seed oil, and borage oil; alpha-linolenic acid is found in flaxseed oil and other oils.

Diets low in omega-3 fatty acids (EPA and DHA) have been associated with menstrual pain.¹⁵⁷ In one double-blind trial, supplementation with fish oil, a good source of omega-3 fatty acids, led to a statistically significant 37% drop in menstrual symptoms. In that report, adolescent girls with dysmenorrhea took an unspecified amount of fish oil that provided 1,080 mg of EPA and 720 mg of DHA per day for two months to achieve this result.¹⁵⁸ A double-blind trial found that the same amount of EPA and DHA plus 7.5 mcg per day of vitamin B12 led to a greater than 50% decrease in menstrual symptoms, but a group taking only fish oil did not obtain as much relief.¹⁵⁹ Six grams of fish oil per day provides the approximate levels of EPA and DHA used in these trials.

Animal research suggests that fish oils may reduce the severity of endometriosis,^{160 , 161} and fish oils have been shown to improve symptoms of dysmenorrhea (painful menstruation),¹⁶² which may be caused by endometriosis. Therefore, while no specific research has been done on the effects of fish oils in women with endometriosis, some health practitioners recommend several grams of fish oil per day for this condition.

Surveys have shown that Inuit people, who consume large amounts of omega-3 fatty acids, have a much lower incidence of some types of glaucoma than do Caucasians. Although there have been no studies on the use of omega-3 fatty acids to treat glaucoma, one study found that cod liver oil (a rich source of omega-3 fatty acids) reduced intraocular pressure in animals.¹⁶³

Fish oil containing EPA and DHA has been reported to reduce the symptoms of migraine headache in a double-blind trial using 1 gram of fish oil per 10 pounds of body weight.^{165, 166} Fish oil may help because of its effects in modifying prostaglandins (hormone-like substances made by the body).

The omega-3 fatty acids present in fish oil, EPA and DHA, have anti-inflammatory effects and have been studied primarily for rheumatoid arthritis, which involves significant inflammation. However, OA also includes some inflammation.¹⁶⁷ In a 24-week controlled but preliminary trial studying people with OA, people taking EPA had “strikingly lower” pain scores than people who took placebo.¹⁶⁸ However, in a double-blind trial by the same research group, supplementation with 10 ml of cod liver oil per day was no more effective than a placebo.¹⁶⁹

A preliminary trial found that elderly women with osteoporosis who were given 4 grams of fish oil per day for four months had improved calcium absorption and evidence of new bone formation.¹⁷⁰ Fish oil combined with evening primrose oil (EPO) may confer added benefits. In a controlled trial, women received 6 grams of a combination of EPO and fish oil, or a matching placebo, plus 600 mg of calcium per day for three years.¹⁷¹ The EPO/fish oil group experienced no spinal bone loss in the first 18 months and a significant 3.1% increase in spinal bone mineral density during the last 18 months.

In a small preliminary trial, supplementation with fish oil (10 grams per day for three months) reduced photosensitivity in 90% of people suffering from polymorphous light eruptions.¹⁷²

Fish oil supplementation has been proposed to lower the incidence of preeclampsia.^{173, 174} However, controlled clinical trials suggest that fish oil does not reduce symptoms¹⁷⁵ or protect against preeclampsia.^{176, 177}