Definition of Shellfish :  An aquatic animal, such as a mollusk or crustacean, that has a shell or shell-like exoskeleton. Shellfish include mollusks which include bivalves. For example, a mussel is a shellfish, a mollusk, and a bivalve.

While cholesterol is the most abundant sterol in clams, oysters, and scallops, five different major non cholesterol sterols (a C-26 sterol, 22-dehydrocholesterol, brassicasterol, 24-methylene cholesterol, and a C-29 sterol) comprise some 60% of the total sterols, a situation unique among foods of animal origin that humans consume.

Shellfish Allergy - shellfish allergy symptom
Allergic reaction to shellfish is quite common. Seafood, including fish, shrimp, lobster, crab, crayfish, mussel, and clam are among the most frequent causes of food allergy. Seafood poisoning, including reactions to natural toxins, frequently masquerades as an allergic reaction on presentation. Ingestion of contaminated shellfish results in a wide variety of symptoms, depending on the toxins present, their concentrations in the shellfish, and the amount of contaminated shellfish consumed.

Shellfish poisoning
Five types of shellfish poisoning have been identified including paralytic, neurotoxic, diarrhetic, amnestic, and azaspiracid shellfish poisonings. Based on the presence or absence of the toxin at the time of capture, fish poisoning can be considered conceptually in two categories. In ciguatera and puffer fish poisoning, the toxin is present in live fish, whereas in scombroid poisoning, it is produced only after capture, in the fish flesh, by contaminating bacteria because of improper refrigeration. Most shellfish-associated illness is infectious in nature (bacterial or viral), with the Norwalk virus accounting for most cases of gastroenteritis.
     Paralytic shellfish poisoning is a serious illness caused by eating shellfish contaminated with algae that contains a toxin harmful to humans. When this algae increase to high numbers in marine waters, the condition is sometimes referred to as a "red tide".

Pregnancy and Shellfish
Should women eat shellfish during pregnancy? Nearly all fish and shellfish contain traces of mercury. For most people, the risk from mercury by eating fish and shellfish is not a health concern. Yet, some fish and shellfish contain higher levels of mercury that may harm an unborn baby or young child's developing nervous system. The risks from mercury in fish and shellfish depend on the amount of fish and shellfish eaten and the levels of mercury in the fish and shellfish. A good balance is to reduce intake of fish and shellfish that are high in mercury, including shark, swordfish, king mackerel. A woman during pregnancy may eat fish and shellfish that are low in mercury. These include shrimp, canned light tuna, salmon, and catfish.

Shellfish and Cholesterol
Does shellfish in the diet increase cholesterol levels? Six normal men were given two diets containing different shellfish, each preceded by a low cholesterol baseline diet. Diet I contained 449 mg cholesterol per day from lobster, crab, and shrimp. Diet II contained clam, oyster, and scallop and provided 447 mg of sterols of which cholesterol constituted only 40 percent. The other sterols are uniquely characteristic of these shellfish (i.e. brassicasterol, 24-methylene cholesterol, etc.). In a second study, 2 normal men and 1 type II hypercholesterolemic woman were fed the baseline diet and shellfish diet II to provide 623 mg of sterols per day. The plasma cholesterol of the 6 subjects averaged 184 mg/dl during baseline, 192 mg/dl in shellfish diet I and 182 mg/dl during shellfish diet II. In the second study, the plasma cholesterol of the 2 normal men did not change. The cholesterol of the high cholesterol woman increased from 311 mg/dl (baseline) to 352 during the shellfish diet. Plasma triglyceride levels remained unchanged. These data indicate that large quantities of lobster, crab, and shrimp elevate cholesterol only mildly in normals, but less so than other cholesterol-containing foods. Clams, oysters, and scallops were not hypercholesterolemic in normal subjects but were in a hypercholesterolemic patient. Metabolism. 1982 Oct;31(10):1046-51.
     Dr. Sahelian comments: Small or moderate ingestion of shellfish as part of an overall healthy diet is perfectly acceptable.

Shellfish Composition - shellfish and cholesterol
Proximate composition, minerals, fatty acids, and sterols were determined for eight species of shellfish commonly marketed in the Northwest. Moisture and total lipid content varied with the size of the species, with more variation in mollusca than in crustacea; total lipid content ranged from 0.7% in sea scallops to 3.1% in blue mussels but only from 1.2% in Dungeness crab to 1.3% in pink shrimp. The mineral content was highly variable; the mineral content of Northwest samples tended to be lower than that reported in other studies. Generally, shellfish are good sources of zinc, and Pacific oysters, blue mussels, and Manila clams are also good sources of iron. Five fatty acids (16:0, 16:1, 18:1, 20:5n-3, and 22:6n-3) represented from 60% to 84% of the fatty acid content. Palmitic acid ranged from 13% to 32% of the total fatty acids. Long-chain n-3 polyunsaturated fatty acids were predominant (37.6% to 54.3%), with sea scallops containing more than 50%; n-6 polyunsaturated fatty acids ranged from 1.5% to 6.5%. In crustacea, cholesterol was the primary sterol, and brassicasterol was the only other measurable sterol. In all mollusca except California squid, cholesterol averaged 37 mg/100 gm and ranged from 23% to 39% of the total sterols. In squid, cholesterol, at 231 mg/100 gm, was the only measurable sterol. We conclude that shellfish vary widely in their nutrient content but, in general, are valuable additions to the diet.

Molluscs and Bivalves
Bivalves are two shelled aquatic molluscs, bilaterally symmetrical and laterally compressed. The name bivalve indicates that the main feature of this class of molluscs have two shells. Bivalves include mussels, scallops, cockles, oysters, clams and the unusual shipworms.

Shellfish Diet and Bile
To examine the sterol composition of normal human bile and the effects of dietary components from certain shellfish upon bile composition, we fed 7 subjects diets rich in shellfish for 2 wk following a typical American diet. The total cholesterol, bile acid, and phospholipid, and the individual sterols and bile acids of the bile samples during each dietary period were measured. In the bile of 7 subjects consuming the typical American diet, nine different neutral sterols, in addition to cholesterol, were identified. Similar patterns of these sterols have been found in human gallstones and in plasma after shellfish feeding. The five shellfish sterols (22-dehydrocholesterol, 24-methylene cholesterol, brassicasterol, isofucosterol, and a C-26 sterol) increased from 0.3% to a total of 5.2% (p less than 0.001) of total sterols. A comparison of the ratios of various shellfish sterols to cholesterol in plasma and in bile suggested selectively greater excretion of shellfish sterols relative to cholesterol. Our data demonstrated that human bile contains a mixture of sterols and that its sterol composition can be readily altered by dietary changes. The lithogenicity of the bile based on the ratio of cholesterol, bile acids, and phospholipids, and the bile acid composition was not affected by the presence of shellfish sterols. Gastroenterology. 1984 Apr;86(4):611-7.

Sitosterolemia and xanthomatosis
Sitosterolemia and xanthomatosis together are a disease characterized by premature cardiovascular disease, and by elevated plasma concentrations of total sterols and of plant sterols, especially sitosterol which is hyperabsorbed. In order to determine whether this abnormal metabolism also involved other sterols, a patient with sitosterolemia was fed a diet high in shellfish that contain significant quantities of noncholesterol sterols, some of which are less well absorbed than cholesterol in humans. Compared with control subjects, the sitosterolemic subject had an increased absorption of 22-dehydrocholesterol, C-26 sterol, brassicasterol, and 24-methylene cholesterol. This enhanced absorption was associated with an increased plasma total shellfish sterol level (13.1 mg/dl vs. 1.9 +/- 0.7 mg/dl in normals). In the sitosterolemic subject, as in normals, the shellfish sterols were not preferentially concentrated in any lipoprotein class, and 50-65% of these sterols were in the esterified form in plasma. Bile acids and neutral sterols were quantitated in bile obtained by duodenal aspiration. The bile acid composition did not differ significantly in the sitosterolemic subject compared with the normal controls. The sitosterolemic subject, though, was unable to concentrate normally the neutral shellfish sterols in bile. The normal controls concentrated the shellfish sterols in bile 6.3 +/- 1.7-fold relative to the plasma shellfish sterol concentration whereas the study subject was only able to concentrate them 2.1-fold. We propose that sitosterolemia and xanthomatosis occur from a generalized abnormality in the usual ability of the gut mucosa and other tissues of the body to discriminate among many different sterols. This has important implications for the understanding of the pathophysiology of this disease and for therapeutic recommendations. J Clin Invest. 1986 Jun;77(6):1864-72.

Shellfish questions
Q. Are you familiar with Ciguatera, a toxin that can be found particularly in large warm water fish at the top of the food chain? I got Ciguatera a few years ago and now am afraid of any supplements that come from fish, because, if I come in contact with it again, can have worse consequences than the first time. I would like to take fish oil for the Omega-3 but am afraid to.
   A. Fish oils are extracted from cold water fish who inhabit the North seas. We have never heard of a case of ciguatera poisoning from fish oil capsules.

Q. I got an email newsletter from a nutritionist that said that it was a myth that you get the most benefits by eating a variety of seafood. It explained that "Shellfish, such as shrimp and lobster, and lean fish, such as cod, and catfish are extremely low in omega-3 compared with fatty fish, such as salmon, mackerel, sardines and herring. Recent studies suggest that fatty, not lean fish provided significant benefits. It's ok to eat lean, low-fat seafood, but don't expect much omega-3." Do you agree?
   A. It is true that shellfish has less omega 3 fatty acids than salmon and mackerel, however they may have other beneficial substances. Hence, I think one can eat predominantly the omega-3 rich fish, but it is also a good idea to include shellfish in ones diet.

Q. What are your thoughts on krill oil supplements?
   A. I think they are a healthy addition to one's supplement routine. See the link for more information.