Fish

Nutritionally, the demersel group mainly provide protein, with 80-90% of food energy deriving from this. Lipid levels are 0-2%. Carbohydrate is virtually absent in significant amounts from all bony fish commonly eaten. Levels (in g/100g) of protein in pelagic fish are similar to those in demersel fish (see table below), but lipid levels of 5-15g/100g mean that a smaller proportion of food energy is derived from protein when pelagic fish are corollary of this is that overall food energy intake is higher from pelagic fish than from demersel fish. The higher lipid level in such fish is the reason the are often referred to a fatty fish, oily fish or oil-rich fish.

Protein.

The major proteins in fish are actin and myosin, which combine in muscle to form actomyosin. Albumins are also present. The amino acid composition of fish protein is such that it can provide the sole source of protein for humans, as a consequence of the presence and extent of the essential amino acids it contains. The actual amino acid pattern is comparable to that of other proteins of high Biological Value, such as beef, egg or milk protein. The amount of connective tissue in fish and shellfish muscle is relatively low. It softens and dissolves more readily when heated compared to the connective tissue of land animals, and is readily hydrolysed by digestive enzymes . It is therefore easy to chew and digest when cooked.

Lipid (fats/oils)

The lipid content of seafoods is primarily in the form of   triglycerides, or triacylglycerols, and is the area of most interest nutritionally, since seafoods are the only major source of certain long chain polyunsaturated fatty acids (see Fats/Oils Page). The level of lipid in fish flesh varies widely, not only between different species, but also within the same species depending on season, feeding grounds, water salinity and other factors. Demersel fish in general do not have a great deal of lipid in their flesh. Lipid stores in these fish are to be found in the liver, in the lining of the peritoneum, and/or immediately under the skin. Pelagic fish on the other hand, tend to store their lipid in head and muscle tissue. Some types, such as halibut, store some lipid in the liver, and some in muscle. In general, fish do not feed when they are spawning, in spite of the higher nutritional demands of spawning. Their nutrient supply is thus provided by body stores, and as a result, the level of lipid falls steadily as spawning progresses. Thus as salmon head upstream into freshwater rivers during the early months of the year, to their spawning grounds, flesh lipid content can be as much as 13%. As they journey upriver they do not feed, so that by the time of spawning, around November, the lipid level may be as low as 5%. After spawning, the lipid level continues to fall, and by the time the fish dies, it may be below 1%. Herring show a more seasonal variation in lipid level. From an overwinter low of 3-4% in April, the level rises within a few weeks to as much as 20%. Feeding begins to diminish as the autumn spawning season approaches, so the lipid level begins to drop to around 10-15%. During winter, lower sea temperatures and a scarcity of food causes the level to continue falling. When lipid is drawn from muscle reserves, it is replaced by water, so that the total mass of the fish remains much the same. The muscle water content obviously rises, and this makes the flesh weak and accounts for the poor eating quality of spawning fish.

The high liver lipid content (which can exceed 50% of wet weight) of species in the family Gadidae, such as cod, saithe/coley or haddock, is exploited to produce cod liver oil, a rich source of the omega-3 long chain polyunsaturates as well as vitamins A and D. Fish liver oils are also from time to time produced from ling, shark, huss, halibut and tuna. The livers are removed at the time of evisceration of the fish, and processed to separate the oil. This can be by simple steam cooking to obtain the highest quality medicinal oil, with various other techniques used to extract the residual oil which, being of poorer quality, is used for veterinary or industrial purposes. Though cod liver oil is still a popular product, the level of production now is around 20,000 tonnes /year, down considerably from the 70-80,000 tonnes in the first half of the century.

Effect of Processing

By and large, processing does not have a major impact on the macronutrient content of bony fish. Dehydration potentially can have the biggest effect, by removing water, but this is not practised commercially to any great extent in the UK. Smoking of fish is in effect a partial dehydration exercise, as is the brining which usually accompanies it. Both can result in a partial removal of moisture, with consequent increases in the proportions of the macronutrients. Lack of care (resulting in excessive temperatures) in smoking can cause loss of lipid from herring, mackerel and salmon. The table below shows the impact of processing on the macronutrient content of  of certain fish.  

Canning and freezing do not in and of themselves have much impact on macronutrient content. Tuna is the major exception to this. Since it is a physically large fish, it is customarily cooked before being packed in the can for final processing. During this initial cooking process, some lipid is lost. A more important loss of lipid occurs when the lighter coloured (low lipid) meat is selectively used for canning. This is done for reasons of consumer preference, since the high lipid dark meat has a deep brown-red colour, which is deemed unacceptable to consumers. The parts of the carcass used for producing smoked salmon have a lower inherent lipid level than the carcass as a whole, and so the product has a lower lipid content than whole salmon, in spite of having lost some water in the smoking process. Since salmon are only fairly lightly smoked, the loss of moisture is not great.

Shellfish

The macronutrient content of different types of shellfish is much more variable than in fish, though true comparison is difficult, since analytical data are not available on all species in the same state (i.e., some data is only available on cooked shellfish). Generally speaking, protein levels are a little lower, and lipid levels a little higher than for example in the demersel fish. Small amounts of carbohydrate (primarily glycogen) are found in some shellfish. Shrimp and crab are notable for a relatively high level of lipid (2.5 - 5.5g/100g) , and oysters and scallops for a significant amount of carbohydrate (2.7-3.4g/100g) . The table below shows the macronutrient data on shellfish (g/100g edible food).