Olive oil, beans and vegetables are easy to picture. Fish is the one beginners wonder about: how often, how much, and does it have to be salmon? Mediterranean-style eating puts fish and seafood on the table a couple of times a week, not every day, and the official advice in the UK is similarly modest.
The NHS says adults should eat at least 2 portions of fish a week, and one of them should be oily fish. EFSA, the EU food safety agency, has described a range of roughly 1–2 to 3–4 servings a week as the zone where benefits show up. So the question is not "can I eat more fish?" but "does my normal week already get there?"
Choose what is on your plate each day. One fish meal is one portion (about 140 g cooked).
The NHS puts a portion at about 140 g of cooked fish. That is roughly a salmon fillet, a mackerel fillet and a bit, or a palm-and-a-half of cod. A small tin of sardines is a bit under a portion, so a tin plus something on toast is a fair meal.
Fresh, frozen and tinned all count. Frozen fish keeps for months, so it is a handy weekday option. Check labels on tinned fish: fish in water or olive oil are both fine, and fish in brine is saltier.
Oily fish carry more of the long-chain omega-3 fats (called EPA and DHA) in their flesh. The NHS list is herring (including kippers), pilchards, salmon, sardines, sprats, trout and mackerel. White fish such as cod, haddock, plaice and hake are leaner. They are still useful: they give you protein and iodine without much fat.
Tuna is a common surprise. The NHS says fresh and tinned tuna do not count as oily fish, and it is also one of the species EFSA names as higher in mercury. Tuna still counts as fish for your two portions.
Two portions a week, one of them oily: that is the target. It is not a daily habit.
For most adults the main limit is on oily fish, up to 4 portions a week. If you are pregnant, breastfeeding or might become pregnant, the NHS caps oily fish at 2 portions a week, and says to avoid shark, swordfish and marlin. It also limits tuna in that group to no more than 2 steaks or 4 cans a week. Other adults should have no more than 1 portion of shark a week. Children and teenagers aged 16 and under should avoid shark, swordfish and marlin, and boys can have up to 4 portions of oily fish a week.
The reason is methylmercury, which builds up in large, long-lived predator fish. EFSA names swordfish, pike, tuna and hake as examples of species higher in it, and says that choosing lower-mercury species is the most effective way to keep the benefits while limiting exposure. Smaller oily fish such as sardines and herring are generally lower in mercury than the big predators.
People who eat fish regularly tend to have lower rates of heart disease, and pregnancy research links fish to better development in children. Those findings come largely from studies that watch what people eat, which cannot prove cause and effect. Trials that gave people omega-3 capsules have been less consistent than the studies of fish eaters. That is one reason to treat fish as a food, with its protein, iodine, vitamin D and other nutrients, and not as a pill.
If you don't eat fish, plant foods like walnuts, flaxseed and rapeseed oil provide a different omega-3 (ALA). The body converts only a small share of it into EPA and DHA, so people who avoid fish may want to speak to a dietitian about what suits them.
You do not need a special recipe. Swap one of your usual dinners for baked salmon or trout, and add a second fish meal: tinned sardines or mackerel on toast, a fish pie with white fish, or a prawn and bean stew. Both are easy to cook from frozen or from a tin. Plan the oily one first, since it is the one most weeks leave out.
This guide is general information, not medical advice. If you have a medical condition, are pregnant or breastfeeding, have or have had an eating disorder, take medication, or have a fish or shellfish allergy, talk to a doctor or dietitian before changing how you eat.