Shellfish Free Diet
What is shellfish?
Shellfish bear no real resemblance to fish, other than that they are. Shellfish are actually invertebrate animals with an exoskeleton and are found primarily in marine – but sometimes freshwater – environments. These molluscs, crustaceans and echinoderms are harvested as a food source in many parts of the world. Molluscs include: clams, mussels, oysters, winkles and scallops as well as cephalopods like squid and octopus. Crustaceans, which are closely related to insects in terms of classification, include: shrimp, lobster, crayfish and crab. Echinoderms aren’t as commonly consumed but sea urchin roe and sea cucumbers are quite popular in many parts of the world, especially Southeast Asia. Some land-based species are also eaten by humans; snails are a popular delicacy in French cuisine and the blue land crab is consumed in the Caribbean.
What is a shellfish allergy?
Shellfish allergy is the most common allergy after nuts and tends to be severe. It normally emerges in adulthood and tends to remain with you for the rest of your life. Shellfish allergies can be triggered by any or all crustaceans, molluscs and echinoderms; if you are allergic to one type of crustacean, like prawns, it is likely that you are allergic to all of them, and if you have a crustaceous shellfish allergy it is likely that you are also sensitive to molluscs. The protein in shellfish that triggers an allergic response in your body, causing your immune system to overreact to its presence, is known as tropomyosin. Tropomyosin is also found in land snails and insects like dust mites and cockroaches, something that it’s important to be aware of if you have a severe shellfish allergy.
How do I know if I’m allergic to shellfish?
Symptoms of shellfish allergy usually appear very quickly, within minutes of eating shellfish (or even due to external contact), but can sometimes be delayed for up to two hours. Symptoms may include skin rash such as hives or eczema, itchy, red, weepy eyes, as well as gastrointestinal symptoms such as nausea, abdominal pain, vomiting, or diarrhoea. More severe reactions also include swelling of the face, lips or tongue, and the constriction of airways like the throat, causing wheezing, coughing or a runny nose. A shellfish sensitivity will not normally present such severe reactions but, if you think you are sensitive, it is a good idea to get tested for the allergy by your GP; shellfish allergies can sometimes cause anaphylaxis which may be fatal without immediate medical care. Shellfish allergy is also the most common cause of exercise-induced anaphylaxis, in which the combination of consuming a food allergen and exercising causes a sudden and severe allergic reaction.
Symptoms of shellfish poisoning (also called ‘red tide’ or paralytic shellfish poisoning) normally appear within half an hour of eating tainted shellfish and usually include gastro-intestinal complaints such as nausea, vomiting and diarrhoea, as well as a possible tingling or burning sensation in the mouth or extremities. These symptoms can easily be confused with those of an allergy and shellfish poisoning can similarly be very serious or even fatal requiring immediate medical attention. Shellfish poisoning is caused by algae-like organisms releasing the potent and dangerous saxitoxin. This toxin only affects shellfish such as clams and oysters rather than prawns and lobsters. Therefore, if you have a reaction to eating molluscs, its best to adhere to the ‘better safe than sorry’ rule and go straight to hospital.
What does a shellfish-free diet entail?
As well as avoiding all crustaceans, it is wise to avoid molluscs and echinoderms if you are sensitive or allergic to shellfish. Crustaceans to avoid are: crabs, crawfish (also called crayfish or crawdads), langoustines, lobsters, prawns and shrimp. Molluscs to avoid are: abalone, clams (including quahogs, a popular type of hardshell clam) cockles, limpets, mussels, octopus, oysters, scallops, snails, squid (commonly known across the Mediterranean by the Italian ‘calamari’), surimi (Japanese imitation shellfish) and whelks. You should also avoid their land-based cousins which may contain the allergenic protein, such as land snails (known in French as escargot). Echinoderms like sea urchin and sea cucumber are a feature of Oriental cuisine, so if eating in a Vietnamese restaurant for example, make sure you have a clear understanding of the food that you are ordering.
If you have a severe allergy to shellfish you should also be aware, when handling, compost or fertilizers, as well as fish food and pet food that they can all contain shellfish. Shellfish also has medical applications; it is found in some medical dyes and HemCon bandages are a wound dressing made from shrimp shells. It is often found in over the counter supplements like calcium supplements (often made from oyster shells or coral) or Glucosamine and Omega-3 supplements (the latter usually being made from fish, but sometimes from shellfish).
A note on labelling and eating out
Although it is relatively straightforward to avoid dishes containing whole shellfish like prawns, lobsters and mussels, avoiding shellfish altogether can be tricky when dining out as it is often in dishes, especially sauces, as a hidden ingredient. If you have a shellfish allergy always read ingredients lists carefully. Vietnamese fish sauce (MamTom) and Thai Fish sauce (Nam Prik), which are used frequently in both styles of cooking, both contain shellfish. Generally, it is a good idea to completely avoid these cuisines if possible. Other dishes from around the world that contain shellfish include: Bouillabase, Cioppino , Scampi, Etouffée, Gumbo, Paella, Jambalaya and Clamato (a clam broth and tomato juice mixture sometimes used in Bloody Mary drinks).
Even if you assiduously avoid any dishes containing shellfish, it is still advisable to think about the potential for cross-contamination in the restaurant you choose. In seafood restaurants the potential for cross-contamination is especially high as the same pan, or even the same stirring spoon, may have been used to make multiple dishes, some of which may contain the allergenic protein that you are allergic to. In Chinese, Japanese, Thai, Vietnamese, or Malaysian restaurants the risk is also high as these cuisines often contain a fish sauce made from shrimp and Cajun/ Creole food often contains shrimp or other shellfish. Since restaurants may use shellfish stock as a flavouring or base for sauces or soups, always mention that you have a shellfish allergy if you have the slightest concern.
Furthermore, when shopping at a fishmonger, take care that shellfish has not been displayed near the fish you wish to purchase in case the two have made contact and it has contaminated it. Additionally, cross-contamination can happen at the processing or manufacturing stage, so read ingredients labels carefully on pre-packaged products – in the EU it is a requirement that any potential allergen, including shellfish, be listed on the product label. In the US, however, the Food Allergy Labelling Law defines crustacean shellfish, but not molluscs, as one of the big eight allergens, meaning that manufacturers are not required to list the presence of clams, oysters, mussels, scallops or other molluscs on ingredient lists.
The shellfish allergy and iodine myth
Outdated medical advice suggests a cross reactivity between shellfish allergy and radiocontrast material used in some medical scans. However, although it is possible to be allergic to iodine or radiocontrast material, medical professionals now agree that those allergies are not related to shellfish allergies.
What are the risks and/or advantages of going shellfish-free?
Shellfish is a good source of protein, vitamins and minerals as well as being low in fat and a good source of Omega-3 and Omega-6 fatty acids. However, if you exclude shellfish but still maintain a healthy, balanced diet, you will easily source these essentials elsewhere. Many nuts, for example, are also a great source of protein and are rich in Omega-3 fatty acids.