Protein and Fat
Fish and shellfish are generally a source of complete, easily digested protein, with a lower proportion of connective tissue than most red meat, which is part of why fish flesh cooks and breaks down so much faster. Fat content varies enormously by species, from very lean white fish like hake and sole to markedly fattier fish like sardines, mackerel and bonito. That fat is largely unsaturated, including the long-chain omega-3 fatty acids that oily fish are well known for, and which are harder to obtain in meaningful amounts from most other everyday foods.
This is one reason nutrition guidance in many countries recommends eating oily fish regularly rather than treating all seafood as interchangeable — the nutritional profile of a sardine and a prawn, while both broadly healthy, are genuinely different.
Minerals and Micronutrients
Shellfish in particular tend to be a good source of minerals that are harder to obtain elsewhere in the diet, including zinc, iodine and, in the case of mussels and clams, iron. Iodine is worth noting specifically, since seafood is one of the more reliable dietary sources of it, and iodine intake in land-based diets can otherwise be inconsistent depending on soil and food processing. Small fish eaten whole, bones included, such as some preparations of sardines and anchovies, also provide calcium in a form larger fish fillets generally do not.
Sensible Caveats
Larger, longer-lived predatory fish, such as swordfish and larger tuna, tend to accumulate more mercury over their lifetimes than smaller, shorter-lived species, which is a widely recognised reason to eat them less frequently than smaller fish rather than avoiding fish altogether. Preparation also matters: the nutritional advantages of fish are considerably reduced by heavy battering and deep frying in older or reused oil, compared with grilling, baking or a light pescaíto frito style fry in fresh oil.
Beyond these general points, individual nutritional needs vary, and anyone with specific dietary concerns is better served by guidance suited to their situation than by generic claims about any one food.
Eating a Range of Species
Because nutritional profiles vary so much by species, eating a range of fish and shellfish over time, rather than returning to the same one or two species out of habit, is a reasonable way to get a broader spread of the nutrients seafood as a category offers. A week that includes an oily fish for its fat-soluble nutrients, a lean white fish for a lighter source of protein, and some shellfish for their mineral content covers considerably more nutritional ground than any single species eaten repeatedly.
This variety also happens to align with several of the sustainability principles covered elsewhere in this guide, since favouring a wider range of local species over a narrow handful of familiar names benefits both diet and the fisheries that supply it.
Preserved forms of seafood are also worth including in this variety. Tinned anchovies, sardines and mussels retain much of their original nutritional value, since canning locks in nutrients at the point of processing, and they offer a practical, shelf-stable way to eat oily fish regularly without relying solely on fresh purchases.