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

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).¹¹ In another double-blind trial, supplementation with eicosapentaenoic acid and docosahexaenoic acid improved heart function and decreased the number of hospitalizations in patients with heart failure due to dilated cardiomyopathy.¹²

Many double-blind trials have proven that omega-3 fatty acids in fish oil, called EPA and DHA, partially relieve symptoms of RA.^{13,14,15,16,17,18,19,20,21} The effect results from the anti-inflammatory activity of fish oil.²² 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.²³

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.²⁴ 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.²⁵ Other studies also found that supplementing with 10 to 15 grams of fish oil per day,²⁶ or with the amount of EPA and DHA provided by 10 grams per day of fish oil,²⁷ is beneficial for people with SLE. People wishing to take such a large amount of fish oil should first consult with a doctor.