Fish oil is the fat or oil that’s extracted from fish tissue. It usually comes from oily fish, such as herring, tuna, anchovies, and mackerel. Yet it’s sometimes produced from the livers of other fish, as is the case with cod liver oil. The World Health Organization (WHO) recommends eating 1–2 portions of fish per week. This is because the omega-3 fatty acids in fish provide many health benefits, including protection against a number of diseases.