Fish oil has been demonstrated to have many benefits for human health. The active ingredients include two essential fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). They are called essential because we humans cannot manufacture them from scratch ourselves, so we have to consume them ready-made to incorporate them into our bodies. It is possible to convert them from a precursor, as explained below, but only to a limited degree. EPA and DHA belong to a group of fatty acids known as omega 3 fatty acids. This name comes from the fact that the first double bond of the hydrocarbon portion of the molecules is between the third and fourth carbon, when counting from the last (or omega) carbon. Natural sources of omega 3 fatty acids include fatty fish, some kinds of seaweed, and microscopic sea algae. Some can be found also in leafy green vegetables. The fish get these fatty acids through the diet as well, as animals cannot manufacture them. Land plants generally do not contain EPA or DHA either, but they make a precursor of these fatty acids, alpha linolenic acid (ALA), which animals can convert into EPA, and EPA can then be converted into DHA. Birds tend to perform this conversion more efficiently than mammals, and in humans women tend to do it more efficiently than men.
One of the reported benefits of fish oil is the general lowering of the cholesterol level, while raising the high-density lipoprotein (HDL) fraction. Another is that of lowering triglyceride levels in the blood. Diseases like diabetes and cardiovascular disease usually correlate with high levels of triglycerides. Fish oil supplementation of 2-4 grams a day can lower triglyceride levels up to 20%. In the membranes of nerve cells the most frequently occurring fatty acid is DHA. Neurons have long extensions called axons, with which they connect to and transmit messages to each other. The neuron membrane wraps around the axon many times, forming the myelin sheath that protects the axons. This participation of DHA in the correct functioning of our nervous system may partially explain why the consumption of omega 3 fatty acids is correlated with lowering depression, and helping protect older persons against memory loss.
Fish oils are concentrated sources of EPA and DHA fatty acids. Plain fish oil is extracted from muscle tissue of coldwater fish like salmon, mackerel, herring and sardines. Cod liver oil, as the name says, comes from the livers of codfish. It is not as rich in EPA and DHA as fish oil, but it has vitamins A and D in addition to the fatty acids. One to two teaspoons a day of cod liver oil or one to two grams a day of fish oil is sufficient to maintain healthy levels of both EPA and DHA.