These are oils, which originate from the layers of fat within whole fish, as opposed to just the liver as in cod liver oil supplements. Fish oils are particularly rich in omega 3 fatty acids. These fats are essential for the healthy function of the body. They cannot be synthesised by us, so they have to be ingested regularly (like vitamins) and hence the name essential fatty acids. Fish oils are also a good source of the fat-soluble vitamins A, D, E and K. Cod liver oil is also rich in essential oils but there are some concerns that the vitamin A content is too high. Too high a vitamin A, over long periods, can lead to health problems.
Good quality fish oils also contain phospholipids that improve omega 3 absorption. They also contain an antioxidant called astaxanthin which helps keep the oil protected from oxidation.
Bad quality fish oils can be contaminated heavy metals such as cadmium, mercury and arsenic as they use larger fish such as swordfish, king mackerel, tilefish, and albacore tuna. Fortunately, most fish oils have not been found to contain these heavy metals, especially ours which is made from smaller fish such as salmon, herring, mackerel, anchovies and sardines. Likewise some supplement have been found, by consumerlab.com, to contain dioxins and polychlorinated biphenyls (PCBs) as they use fish from polluted seas or again larger fish which are higher up the food chain.