Fish is a key element of a healthy diet — especially fish rich in omega-3 fatty acids, such as salmon, herring, sardines, anchovies and mackerel. Oily fish is also an excellent source of important anti-cancer vitamins and minerals — vitamin D, selenium and iodine – that aren’t easily found in other foods. Aim to eat fish at least twice a week and go for smaller fish that are less likely to contain toxins like mercury.