Fatty fishes like mackerel, salmon or herring contain large amounts of so-called omega-3 fatty acids. These are some of the healthy, unsaturated fatty acids that help to strengthen the immune system and prevent cardiovascular disease. The labels “saturated” and “unsaturated” are the technical terms used in chemistry to denote how many hydrogen atoms occur in the long molecule chains of fatty acids. Unsaturated fatty acids contain little hydrogen. Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are especially beneficial. DHA is important for the development of the brain and the eyes, while one of the uses of EPA is for the treatment of rheumatoid arthritis. These two long-chain highly unsaturated omega-3 fatty acids are found almost exclusively in marine fish and marine algae. Phytoplankton is able to generate these two omega-3 fatty acids on its own, whereas fish cannot synthesize the substances themselves. Instead, the fatty acids are taken in by plankton feeders as they feed, and passed up through the food chain to predator fish. The highest contents of DHA and EPA are found in mackerel. Land plants also contain omega-3 fatty acids – particularly alpha-linolenic acid, large amounts of which are found in rapeseed, soya and walnut oil. But this compound is far less effective in the human body than DHA and EPA.