Fish has always been seen as a food necessary for good health - since ancient times fish was recognised as being a 'brain food', a reference to its importance in the development of a healthy brain, not to mention the reputed, but as yet unproven, aphrodisiac effects of oysters.

Research over the past few decades has confirmed the importance of the nutritional components of fish in brain development and reproduction and highlighted a role for fish in a variety of other functions in the body. There is strong evidence that fish plays a major role in protecting against heart disease and may also play a role in the prevention of other illnesses. Components of fish are also important in the development and maintenance of the eyes, skin and nervous system.

Much of the importance of fish in health has come from research into long chain polyunsaturated fatty acids (PUFA) of the n-3 family. N-3 Fats are also generally known as omega 3 fats. Fish is a rich source of two important n-3 PUFAs, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Marine algae are the only form of life that can easily make long chain n-3 PUFA. As fish feed on algae they, in turn, become rich sources of these important n-3 fats. Although most animals, including humans, can make ling chain n-3 fats, most have difficulty in doing this and so benefit from eating these fats as part of their normal diet.

As well as being a source of n-3 fats, fish is also a good source of protein, iodine, zinc and selenium and is rich in vitamins A and D and some B vitamins.

Intake of fish has been falling for many years. The North/South Ireland Food Consumption Survey (2000) found that the average intake of seafood in Ireland is about 5 Ѕ oz or 160g per week, with many people never eating fish. It is recommended that we eat fish at least twice a week and include oil-rich fish at least once a week. This level of fish intake has been linked with a lower risk of death from heart disease.

THE 'Nutritional Aspects of Fish' book has been developed as specialised information resource for health professionals and those with an interest in this area. This publication is intended to provide an overview of the nutritional content of fish, reinforcing the proven benefits of fish consumption and outlining other benefits which are currently being researched. We hope it will prove a useful reference for dietitians, doctors, nurses, students all those interested health benefits seafood.

Protein and fats are the major nutrient groups supplied by fish. Fish generally contain very little carbohydrate.

Fish has between 15 and 23% protein, which accounts for 80 to 90% of the energy content of the fish. The protein found in fish is of high biological value, which means that fish can be used as the sole source of protein in the diet. The pattern of amino acids (protein building blocks) in fish is very similar to the amino acids found in other high protein foods such as chicken. The amount of connective tissue in fish and shellfish muscle is quite low and it softens and dissolves more readily when heated compared to the connective tissue of land animals. This makes fish easy to chew. The connective tissue in fish is easily broken down by digestive enzymes making it very easy for the body to digest fish.

Lipid is the term used to describe both fats and oils. Fats are lipids that are solid at room temperature and oils are lipids that are liquid at room temperature. The lipid content of fish varies depending on the type of fish, the time of year and what the fish feeds on. White fish, such as cod, generally have between 0 and 2% lipid whereas oil-rich fish, such as mackerel, can have in excess of 16% lipid. The lipid content of farmed fish can vary widely depending on the feed used. The lipid found in fish is mostly polyunsaturated lipids, also known as PUFA, with small amounts of saturated and monounsaturated lipids also present.

Fish with a high liver lipid content (which can exceed 50% of weight) such as cod or haddock, is used to produce cod liver oil, a rich source of the n-3 long chain polyunsaturates (see the menu item for a more detailed discussion of polyunsaturated fats) as well as vitamins A and D. Although cod liver oil is still popular, the global level of production now is around 20,000 tonnes per year, down considerably from the 70-80,000 tonnes in the first half of the last century.

Effects of Cooking

Fish is very versatile and can be cooked in many ways. Baking, steaming, microwaving, frying and grilling cause little loss of nutrients. Poaching fish does result in almost half of the minerals leaching out into the water, but if the cooking water is used to make a sauce the minerals are not lost. White fish absorbs fat as it is being cooked whereas oil-rich fish does not. Fish that is coated in batter or breadcrumbs will absorb significantly more fat when fried than uncoated fish. While some of the beneficial n-3 fats are destroyed during cooking, fish remains a good source of these healthy fats.

Effects of Processing

In general, processing does not have a major impact on the macronutrient content of fish. The smoking process partly dehydrates fish, as does the brining which usually accompanies it. Smoking and brining both lead to increases in the percentage of nutrients in fish due to the loss of water but also lead to increases in salt levels which may be important for people with high blood pressure. Canning and freezing do not generally 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, including n-3 fat, is lost. A more important loss of lipid occurs as the lighter coloured, lower lipid, meat is selectively used for canning.

The macronutrient content of different types of shellfish is much more variable than in other fish. Generally speaking, protein levels are a little lower and lipid levels a little higher. Small amounts of carbohydrate (mostly glycogen) are found in some shellfish. Crab has a relatively high level of lipid (2.5 - 5.5g/100g), and oysters and scallops have a significant amount of carbohydrate (2.7-3.4g/100g). The table opposite shows the macronutrient data on shellfish.

Fish and shellfish are well known as sources of the fat soluble vitamins A and D.

Fish and shellfish are well known as sources of the fat soluble vitamins A and D. Vitamin A can occur in two different forms - as retinol, which is easily absorbed by the body, or as carotenoids, which are less easily absorbed and have only 50% of the absorption rate of retinol. Carotenoids are converted to vitamin A once absorbed by the body. The easily absorbed retinol is the type of vitamin A found in fish.

Vitamin A Vitamin A is found in quite high amounts in oil-rich fish such as herring and mackerel and in shellfish. 4oz/100g portions of these fish provide around 10 to 15% of the adult recommended daily amount (RDA) for retinol. Oil-rich fish are also excellent dietary sources of vitamin D3, (cholecalciferol) providing 50 to 200% of the RDA in a 4oz/100g portion.

Vitamin D promotes the absorption of calcium and phosphorus from the intestine and regulates blood calcium levels. Without vitamin D the small intestine absorbs no more than 10 to 15% of dietary calcium. Vitamin D is also important in bone metabolism, helping to control bone formation and resorption and it may also play a role in preventing some cancers.

The flesh of oil-rich fish is a better source of vitamin A and D than white fish, but the livers of both white fish and oil-rich fish are very high in both vitamins A and D. This is important to note as pregnant women may be exceeding their vitamin A requirement by taking supplements such as cod liver oil. Too much vitamin A can be dangerous especially during pregnancy when it may be linked with an increased risk of miscarriage.

Vitamin E is present in significant amounts in many seafoods, for example salmon and some shellfish, providing up to 15% of the RDA for vitamin E in a 4oz/ 100g portion. Vitamin E acts as an antioxidant protecting polyunsaturated fats and Low Density Lipoprotein (LDL) cholesterol from oxidation by free radicals and may also have anti-inflammatory effects.

Most fish are a source of some B vitamins, particularly thiamine, riboflavin and pyridoxine. A 4oz/100g portion of most fish will provide approximately 10% of the RDA for these vitamins. Fish are especially rich in vitamin B12, supplying 100% or more of the adult RDA in a 4oz/100g portion. B vitamins are essential for the metabolism of food, particularly carbohydrate. B12 is important for the formation of red blood cells. Fish generally provide little or no vitamin C.

Fish are better known for the dietary minerals they supply than for the vitamins. This is because minerals such as iodine and selenium, which are supplied by fish, are found in much lower amounts in non-marine foods.

Fish contains more iodine than any other food in a normal diet. Eating one or two fish meals every week provides the equivalent of 100 to 200 micrograms (mcg) of iodine a day, enough to meet the RDA of 130mcg for adults. Fish is also an excellent source of selenium although the amount of selenium varies significantly. The RDA for selenium is 55mcg for adults. Fish, on average, supplies 20 to 60mcg of selenium per 4oz/100g, whereas cereal and meat sources of selenium only provide about 10 to 12mcg per 4oz/100g.

Selenium is incorporated into a number of enzymes in the body; the best known is glutathione peroxidase. This enzyme plays an important role in protecting cell membranes from damage by free radicals. Free radicals are substances formed in the body as part of normal metabolism. They can be very damaging to the body and are linked with increased risk of heart disease and cancer. The body has defences in place to help neutralize these free radicals. It uses the antioxidants found in fruit and vegetables and enzymes such as glutathione peroxidase to render free radicals harmless. Selenium also plays key roles in the functioning of the immune system, in thyroid metabolism and in reproduction.

Calcium: Most fish contain only small amounts of calcium although sardines, oysters and shrimps are exceptions, supplying 10 to 20% of the adult RDA in a 4oz/100g portion. The softened bones found in tinned salmon can be eaten, and when they are, the salmon provides up to 300mg of calcium per 4oz/100g portion, almost half of the adult RDA.

Iron levels are not high in white or oil-rich fish, but since the iron present is easily absorbed, especially from white fish, it is a useful dietary source. Shellfish has high levels of iron, similar to that of red meat.

Zinc:

Shellfish, in particular oysters, is a rich source of zinc. The reputed aphrodisiac qualities of oysters are commonly attributed to the high level of zinc present, as zinc is important for reproduction. With the adult RDA for zinc at 9.5mg, most shellfish can make a contribution of up to 50% to this, though for other fish, the contribution is between to 5 and 10%.