Fish oil: what is it?

Fish oil is a dietary source of omega-3 fatty acids which have been linked to various health effects. Omega-3 fatty acids play a role in many processes in the body and in general it is assumed that a higher intake will be beneficial.

Omega 3 fatty acid sources

The bodies capacity to manufacture omega-3 fatty acids is limited and thus we are quite dependent on our food intake to increase our bodies exposure to these fatty acids. Fish oil contains two omega-3 fatty acids that seem particularly important: docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Dietary sources of DHA and EPA are fatty fish, such as salmon, mackerel and trout, and shellfish, such as mussels, oysters and crabs. Omega 3-fatty acids are unsaturated fatty acids and the double bonds that make it "unsaturated" can easily react with other molecules. When fatty acids are saturated they are less likely to react with other molecules. One of the molecules the omega-3 fatty acid react with is oxygen. When a fish oil supplement has been exposed to oxygen the fatty acids may be oxidised and this changes the function of the fatty acids.


Dietary sources of DHA and EPA are fatty fish, such as salmon, mackerel and trout, and shellfish, such as mussels, oysters and crabs.

Although DHA and EPA receive a lot of attention, they are not the only omega-3 fatty acids. Several nuts, seeds and vegetable oils contain alpha-linolenic acid (ALA) which is another omega-3 fatty acid that that can be turned into EPA and DHA. what is important to realise is that this conversion is not easy and only a small percentage of ALA gets converted to EPA or DHA. Therefore, those individuals who do not eat fish may have more difficulty maintaining high levels of omega-3 fatty acids.

Fish oil can be obtained from foods or from supplements. The ratio between DHA and EPA in foods and in supplements varies, and this will likely affect the physiological effects the fish oil may have.


Several nuts, seeds and vegetable oils contain alpha-linolenic acid (ALA) which is another omega-3 fatty acid that that can be turned into EPA and DHA.

Omega 3 : Omega 6 ratio

It has been argued for many years that it is not just the amount of omega 3 fatty acids that are eaten but the ratio between omega 3 and omega 6 fatty acids. Several sources of information suggest that human beings evolved on a diet with a ratio of omega-6 to omega-3 essential fatty acids (EFA) of approximately 1 whereas in Western diets the ratio is 15/1-16.7/1. Western diets are deficient in omega-3 fatty acids, and have excessive amounts of omega-6 fatty acids compared with ancestral diet. The ratio can be influence in several ways: decreasing omega-6 fatty acid intake, increasing omega-3 fatty acid intake or both.


One simple way to affect the omega 3 fatty acid intake (or the ratios) is by taking a fish oil supplement. Fish oil has been heavily researched for many years in relation to a large number of health benefits. There are many claims, but not all of these claims can be backed up. Here is a summary of the evidence to date:


What do we know about the effectiveness of omega-3 supplements?

  • High doses of omega-3 fatty acids can reduce levels of triglycerides

  • Fish oil can also cause a modest reduction in blood pressure in those individuals with high blood pressure.

  • Despite the reduction in triglycerides and reduction in blood pressure, long-term trials haven't found that omega-3 supplements reduce the risk of heart disease (1). However, people who eat seafood one to four times a week are less likely to die of heart disease.