Fish oil contains omega-3 fatty acids, one of the two main classes of essential fatty acids. ( Omega-6 fatty acids are the other main type.) Essential fatty acids are special fats that the body needs for optimum health.

Interest in 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. 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 have various effects that tend to reduce risk of heart disease and strokes. However, research into whether use of fish oil actually prevents these diseases remains incomplete and somewhat inconsistent. In recognition of this, the FDA has allowed supplements containing fish oil or its constituents to carry a label that states: "Supportive but not conclusive research shows that consumption of EPA and DHA omega-3 fatty acids may reduce the risk of coronary heart disease."

In addition, a slightly modified form of fish oil (ethyl-omega-3 fatty acids) has been approved by the FDA as a treatment for high triglyceride levels.237 This specially processed product, sold under the brand name Omacor (Lovaza), is widely advertised as more effective than ordinary fish oil.

Fish oil has also shown promise as an anti-inflammatory treatment for conditions such as rheumatoid arthritis, menstrual pain, and lupus. In addition, it may be helpful for various psychiatric conditions.

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, sea vegetables, and vegetable oils contain significant amounts of essential omega-6 and/or omega-3 fatty acids, but oil from cold-water fish is the richest natural source of omega-3 fats. It is commonly stated that people require a certain optimum ratio of omega-3 to omega-6 fatty acids in the diet; however, there is no real evidence that this is true, and some evidence that it is false.231