How often can I eat tinned tuna and oily fish?

Two different substances drive two different limits, and conflating them causes a lot of confusion. Dioxins and PCBs sit in fat, so the limit applies to oily fish as a category. Methylmercury binds to muscle protein, so it applies to large predatory fish including tuna regardless of how lean they are. Tinned tuna and fresh tuna are not treated the same, because fresh tuna steaks come from larger fish.

Safety

Step by step

1. Separate the two reasons for a limit.Oily fish — salmon, mackerel, herring, sardines, trout, fresh tuna — accumulate fat-soluble pollutants, which is why the cap is on the category. Mercury accumulates in muscle and concentrates up the food chain, which is why the cap is on big predators.
2. Note that a minimum exists as well as a maximum.The advice is to eat at least two portions of fish a week, one of which should be oily. Most people in the UK eat considerably less oily fish than the minimum, so the practical problem for the majority is eating too little rather than too much.
3. Understand why tinned and fresh tuna differ.Tinned tuna is mostly skipjack, a small, short-lived species with lower mercury, and it is not counted as an oily fish in the UK guidance because canning reduces the long-chain omega-3. Fresh tuna steaks come from larger, longer-lived fish, carry more mercury, and do count as oily fish.
4. Apply the tighter figures in pregnancy.Women who are pregnant, breastfeeding or may become pregnant have a lower limit on oily fish and a separate, specific limit on tuna, whether tinned or fresh. Shark, swordfish and marlin are to be avoided completely.
5. Apply them for children too.Girls, and women of childbearing age generally, are given the lower oily fish figure because the concern is accumulation before a future pregnancy rather than immediate harm. Shark, swordfish and marlin are off the list for all children.
6. Spread the species around.The limit is on the category, not on any single meal. Alternating sardines, mackerel, herring and salmon and keeping tuna occasional keeps you comfortably inside the advice while eating more fish, not less.
7. Look up the current figures rather than trusting a number.The FSA has revised this advice more than once. The current portions-per-week figures for adults, pregnancy and children are on food.gov.uk and in NHS guidance, and that is the right place to check before changing anything.

Tips

• A portion of fish is generally taken as around 140g cooked, which is roughly a fillet or a standard tin.
• Sardines, herring, sprats and mackerel are the cheapest oily fish and the lowest in mercury, because they are small and short-lived. They are the easy way to hit the minimum without approaching the maximum.
• Shellfish and white fish are not subject to the oily fish cap, so mussels, prawns, cod and haddock can be eaten more freely.
• If you take a fish oil supplement, that counts towards nothing in this advice — the limits are about the fish, and supplements are regulated separately.
• Tinned tuna as a lunch several times a week is where most adults quietly exceed the tuna advice, particularly women of childbearing age.

Common mistakes

• Cutting out oily fish because of the limit — The advice sets both a floor and a ceiling, and the floor is the one most people miss. Oily fish is one of the strongest single dietary recommendations in UK guidance and avoiding it entirely is the wrong response to a cap.
• Assuming tinned tuna is the same as fresh for these purposes — They are treated differently in the guidance for two reasons at once: different species and size, and different omega-3 content after canning. A rule remembered for one does not apply to the other.

Questions people ask