The Fish Foundation
Fish and shellfish are best known as sources of the fat soluble vitamins, A and D , though they also can provide significant amounts of some of the B vitamins. The table below provides details of the vitamin content of selected fish and shellfish. Vitamin A in the form of retinol is found in quite high amounts in huss, and oysters, as well as the oil rich fish such as sprats, herring, and mackerel. 100g portions of these seafoods would provide around 10-15% of the adult Reference Nutrient Intake (RNI) for retinol. The oil-rich, pelagic fish are excellent dietary sources of vitamin D3, (cholecalciferol). Though there is no accepted adult RNI for vitamin D, using a value of 10 mcg (as may be required for certain adults) the oil rich fish provide 50-200% of such a level in a 100g portion.
Table 1 Vitamin Content of Selected Fish and Shellfish (per 100g raw edible portion unless otherwise specified)
tr=trace amounts only; n= data not available
source: Holland, B., Brown, J., & Buss, D.H., 1993. Fish and Fish Products; the third supplement to McCance & Widdowsons The Composition of Foods (5th Edition), HMSO, London.
The fish liver oils have much higher levels of the fat soluble vitamins, and have been used as dietary supplements for these nutrients for over 200 years. The actual levels vary considerably, but halibut liver oil for example can reach levels of 5000 mcg of retinol per gram of oil and up to 120 mcg of cholecalciferol. Such levels would be toxic to humans if ingested over a period of time. Cod liver oil provides generally lower levels, around 100-150 mcg of retinol and 1-2 mcg of cholecalciferol per gram of oil. Oil used for capsule production tends to be somewhat higher in vitamin concentration, so as to supply 100% of the adult RNI for vitamin A in the small amount of oil contained in the capsule (350-500mg). Vitamin E is present in significant amounts in many seafoods, providing around 10-20% of the average daily vitamin E intake of 5-10mg in a 100g portion. Since vitamin E intake is related to the intake of polyunsaturates , and since seafoods supply polyunsaturates, it may be considered that the vitamin E present in seafoods is in general sufficient to provide for the additional vitamin E needs that the seafood polyunsaturate content imposes. It thus makes little or no contribution to the vitamin E needs imposed by other dietary sources of polyunsaturates.
Of the water soluble vitamins, seafoods generally provide little or no vitamin C. The B vitamins are represented to varying extents, with the supply of thiamine, riboflavin and pyridoxine being most significant nutritionally. 100g portions of most seafoods will supply 10% or more of the adult RNIs for these nutrients. Seafoods are especially rich in vitamin B12, supplying 100% or more of the adult RNI in a 100g portion.
Seafoods are better known, nutritionally, for the dietary minerals they supply than for the vitamins. This is because minerals such as iodine and selenium, which are supplied by seafoods, are not readily at the same levels in many other non-marine foods. The table below provides details of the mineral contents of selected seafoods.
Table 2.Mineral Contents of Selected Fish and Shellfish (mg/100g raw edible portion unless otherwise specified)
n= data not available
source: : Holland, B., Brown, J., & Buss, D.H., 1993. Fish and Fish Products; the third supplement to McCance & Widdowsons The Composition of Foods (5th Edition), HMSO, London.
Just why shrimp should have so much sodium is not immediately apparent. Undoubtedly, the shrimp reported in the table were probably cooked in seawater, which would have contributed a substantial amount of the sodium, but on the otherhand, most shrimps are purchased ready cooked, and presumably will also have been so cooked and thus have the same high sodium level. Shrimp should thus be avoided by those with high blood pressure, or who must restrict their sodium intake for other reasons. In general, the balance between sodium and potassium is favourable in fish, with ratios ranging from 1:2 to 1:10. In shellfish, there is more sodium, so the ratio is not so favourable. Shrimp, oysters lobster and crab contain more sodium than potassium.
Calcium levels are not high in most seafoods, though sprats, sardines, oysters and shrimps are clearly exceptions, supplying 10-20% of the adult RNI in a 100g portion. Canned salmon contains softened bones, and when these are eaten with the fish, as they often are, the level of calcium supplied rises to 300mg per 100g, almost half of the adult RNI for calcium. Iron levels are not high in seafoods, but since the iron is easily absorbed, especially from white fish, it is a useful dietary source. The level of iron in molluscs (oysters, mussels) is similar to that in red meat. Zinc is also especially rich in the molluscs, and in particular oysters. The reputed aphrodisiac qualities of oysters is commonly attributed to the high level of zinc present, though this has not been studied in controlled trails! With the adult RNI for zinc at 9.5 mg, most shellfish can make a contribution of around 30-50% to this, though for fish, the contribution is more like 5-10% . Seafood is the richest source of iodine in the normal diet, and one or two seafood meals per week will supply 100-200mcg per day, enough to meet the adult RNI of 140 mcg. Few other commonly eaten foods can match this. The same can be said of selenium, though the variation is greater. Seafoods supply 20-60 mcg per 100g, with the exception of lobster, which contains a massive 130mcg per 100g. The adult RNI for selenium is 75 mcg and 100g of many seafoods will make a large contribution to this. Cereal and meat sources of selenium provide about 10-12 mcg/100g, considerably less than most seafoods.
Other minor components
Though not nutrients in the classic sense of the word, the sterols which seafoods contain are important from the nutritional point of view. Cholesterol is the best known, but the levels are not as significant nutritionally as once thought, partly due to mistaken identity. The is no doubt that the crustaceans as a group contain quite high levels of cholesterol. Prawns in particular contain about 195mg of cholesterol per 100g meat. In the context of a recommended maximum cholesterol intake of 300-600mg per day, this is significant, though it is still less than the cholesterol contained in an egg (250mg). Shrimp meat contains about 130mg/100g, while crab and lobster meat contains 50-100mg per 100g. At one stage it was thought that the molluscs also contained high cholesterol levels, but in fact this was due to an analytical error. What was once identified as cholesterol is now known to include other plant derived sterols, or phytosterols. Cholesterol is still present in molluscs, but a high proportion of the sterols present are now known to be phytosterols. Cholesterol levels in the molluscs range from 40-60 in mussels, scallops and oysters, up to 150-200 mg in cuttlefish and squid. The phytosterols, though not yet widely studied, are considered to be beneficial in the diet, since they interfere with the absorption of cholesterol.