Fish, along with seafood, is one of the foods causing the highest number of allergies. Their proteins, the histamine they can contain and the parasite anisakis can provoke allergic reactions. A fish allergy is an adverse reaction to certain proteins in this food. It usually produces symptoms immediately or during the first hour after eating the food. Allergic reactions to fish can be serious and often cause anaphylaxis, with the patient’s life being put in danger.
The main fish allergen is a protein called parvalbumin found in muscle cells. These proteins are heat resistant and are not destroyed when the fish is cooked. These proteins, although similar to each other, are specific to each species. That is why some people may be allergic to just one type of fish, several types or even fish from different families. John Dory is the fish that most frequently causes allergies, followed by hake, sardine and cod. The most readily tolerated are swordfish, dogfish and tuna (blue fish contains fewer allergenic proteins). There is usually a cross-allergy between cod, pollock, salmon, trout, tuna, anchovy and mackerel, so a person allergic to one of these fish is usually also allergic to the rest. Although people with a fish allergy can react to crustaceans and other shellfish, no cross-reactivity between these foods has been demonstrated.
Diagnosis, treatment and advice
The diagnosis is made by an allergist based on the patient’s medical history, skin tests using the prick technique and serological tests (specific IgE). With the results from these tests, the specialist can determine which type of fish you are allergic to and tell you which ones you can eat and which to avoid.
If it is suspected that someone, especially a child, has an allergy to any type of fish, it is recommended to eliminate it from their diet and immediately see a specialist who will perform the relevant diagnostic tests. For adults, in principle there is no need to stop eating other seafood products such as crustaceans (shrimp, prawns, crayfish, etc.), cephalopods (squid, octopus) and molluscs (mussels, clams, sea snails, etc.). However, as a precaution, it is preferable to avoid dishes in which various ingredients may be mixed together (fish or broths made from fish, crustaceans and/or molluscs).
Keep in mind that people with a fish allergy may have reactions after eating other foods such as meat (pork, chicken, etc.) fed with fishmeal. It is essential to check product labels. Fish is part of many foods: soups, pizzas, paella, frozen crab rolls, chicken fed with fishmeal, jellies, products enriched with vitamins or omega 3 (when these are taken from fish oil), fishmeal, wines and alcoholic beverages clarified with fish tails, medicines (cod liver tablets or suspensions), and it is even used for the preparation of glues (adhesive for stamps and bookbinding).
People who have a fish allergy should be careful when eating out. They should avoid fish and seafood restaurants, due to all food being at risk of contamination thanks to possible contact with counters, utensils, cooking oil, deep fryers or grills where fish has been prepared. Therefore, avoid eating foods that have been fried in oil previously used to cook fish, or that have been cooked in containers or handled with utensils previously used in the preparation of fish dishes and that have not first been washed and rinsed thoroughly.
The normal recommendation is for people to eat more fish than meat. This is not because of the protein level, as that is similar in meat, but because of the fat, since there is little fat in white fish and the fat in blue fish is unsaturated. Unsaturated fats are healthier essential fatty oils and can be found in olive oil, nuts, hempseed oil and rapeseed oil.
People with a fish allergy are also recommended to have a diet that includes seed oils (sunflower, soy, corn) and nuts (provided they are tolerated), in moderate amounts, as these contain unsaturated fatty acids, which have characteristics that are comparable to the fat found in fish. Another alternative is to eat foods enriched with Omega 3 provided that this does not come from fish.