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All About Shellfish

Maybe you are working to keep your cholesterol level down. If you stopped eating lobster, crab, and the like because you thought shellfish was loaded with cholesterol, then think again. Throw another shrimp on the barbecue and read on, because shellfish, which were once blacklisted by the cholesterol police, have been given a reprieve.

Shellfish and Cholesterol    TOP

Shellfish's cholesterol level is not as health threatening as once believed. There are a few reasons why shellfish may have a bad reputation when it comes to cholesterol. First, shellfish contain a variety of sterols, a group of chemical compounds that includes cholesterol. Previously, scientists could not distinguish among the different sterols and all were labeled as cholesterol. As a result, the amount of cholesterol in shellfish was overestimated. In reality, shellfish contain less cholesterol than meat or poultry.

Not Much Fat, Though    TOP

Another factor that worked against shellfish was the thought that dietary cholesterol raised blood cholesterol levels. Because shellfish does contain cholesterol, it was considered bad for you. Now we know that dietary cholesterol is only a minor contributor to blood cholesterol levels: total calorie intake and the quantity and type of fat, such as trans fat and saturated fat, in the diet are far more important. Fortunately, the fats in shellfish are in the healthy category.

Bad Company    TOP

The company that shellfish keep, however, can be a problem. Shellfish are often served with melted butter or a mayonnaise-based tartar sauce. And shellfish are frequently battered and deep fried. Both actions can turn a low-fat dish into a high-fat bomb by increasing the total fat and the saturated fat. Instead, try steaming shellfish and serving with lemon and spices.

Types of Shellfish    TOP

It is as simple as it sounds—shellfish are sea creatures that have a shell of some kind. There are 2 basic categories:

Crustaceans— They have elongated bodies with a jointed, soft shell. These include crabs, crayfish, lobster, and shrimp.

Mollusks— These have soft bodies covered by a shell of one or more pieces. Mollusks are divided into 3 categories:

  • Univalves—a single shell and a single muscle; includes abalone and snail
  • Bivalves—2 shells hinged together by a strong muscle; includes clam, scallop, mussel, and oyster
  • Cephalopods—tentacles attached to the head and an ink sac; includes octopus and squid

Shellfish Allergies    TOP

Shellfish are one of the most common allergens, and the allergy is rarely outgrown. Reactions usually appear within minutes to a few hours after eating shellfish, inhaling cooking vapors, or handling shellfish, but can be delayed as long as 24 hours. Common symptoms include:

  • Nasal congestion
  • Hives
  • Itching
  • Swelling
  • Swelling around mouth
  • Heartburn
  • Wheezing
  • Shortness of breath
  • Throat closing
  • Nausea
  • Upset stomach
  • Cramps
  • Gas
  • Diarrhea
  • Light-headedness
  • Fainting

The key to living with a shellfish allergy is to avoid all foods or products that contain shellfish. Make sure you read a product's label, because shellfish may be a minor ingredient.

Shellfish Poisoning    TOP

Food poisoning can occur after eating tainted shellfish; clams and mussels are the types most frequently at fault. Symptoms can occur in as little as 30 minutes after eating and begin with a tingling and numbness around the lips. Staggering, giddiness, and muscular incoordination may appear and speech is often difficult to understand. In severe cases, shellfish poisoning may result in seizures, coma, or death. If you suspect shellfish poisoning, seek medical attention right away.

The sickness is most often caused by a toxin that shellfish ingest along with the plankton they eat during certain times of the year. Unlike bacteria that can cause food poisoning, these toxins cannot be destroyed through cooking. To protect yourself, always buy from reputable seafood sellers.

Mercury Content     TOP

Considerable concern has grown about mercury levels in fish. This is a problem with some shellfish too. Although shellfish do not usually approach the mercury levels of the worst fish offenders, such as swordfish and shark, lobster has as much mercury as canned white tuna, and scallops and crab have about a sixth as much. Mussels vary in mercury content depending on their origin. Shrimp and oysters have little to no mercury content.

Guidelines for Cooking Shellfish    TOP

Seafood should be cooked so that the internal temperature is 145°F (63ºC). You will also know when seafood is cooked by doing the following:

  • Fish—Cut through the side of the fish and open it up to expose the flesh. The inside should be opaque and separate easily.
  • Shrimp and lobster—When cooked, the flesh should be pearly-opaque.
  • Scallops—The flesh should look milky white/opaque and firm.
  • Clams, mussels, oysters—When the shells open, they are done. Throw out any ones that do not open.

Nutritional Info    TOP

Shellfish Nutritional Information
Item Serving
size (84g/3 oz)
Clams 12 small
12 med
Scallops 6 large or 14 sm

g = grams; mg = milligrams


Eat Right—Academy of Nutrition and Dietetics


Dietitians of Canada


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Updated 2009. Accessed July 31, 2017.
Food poisoning from marine toxins. Centers for Disease Control and Prevention website. Available at:
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Updated May 31, 2017. Accessed July 31, 2017.
Fresh and frozen seafood: selecting and serving it safely. US Food and Drug Administration website. Available at:
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Updated July 13, 2017. Accessed July 31, 2017.
Mercury levels in commercial fish and shellfish. US Food and Drug Administration website. Available at:
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Updated January 18, 2017. Accessed July 31, 2017.
Safe minimum cooking temperatures. website. Available at:
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Accessed July 31, 2017.
Seafood nutrition facts. US Food and Drug Administration website. Available at:
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Shellfish allergy. Cleveland Clinic website. Available at:
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Updated January 3, 2017. Accessed July 31, 2017.
Last reviewed July 2017 by Michael Woods, MD, FAAP
Last Updated: 10/14/2013

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