Nutritional Value of Fish and Shellfish Introductory Remarks

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Fish and shellfish are excellent sources of protein. A 100 g cooked serving of most types of fish and shellfish provides about 18-20 g of protein, or about a third of the average daily recommended protein intake. The fish protein is of high quality, containing an abundance of essential amino-acids, and is very digestible by people of all ages. Seafood is also loaded with minerals such as iron, zinc, and calcium.

The caloric value of fish is related to the fat content and varies with species, size, diet, and season. Seafood is generally lower in fat and calories than beef, poultry, or pork. Most lean or low-fat species of fish, such as cod, hake, flounder, and sole, contain less than 100kcal (418 kJ) per 100 g portion, and even fatty fish, such as mackerel, herring, and salmon, contain approximately 250 kcal (1045 kJ) or less in a 100 g serving. Most crustaceans contain less than 1% fat in the tail muscle because depot fat is stored in the hepatopancreas, which is in the head region.

Interest in the health benefits of fish and shellfish began decades ago when researchers noted that certain groups of people - including the Inuit and the Japanese, who rely on fish as a dietary staple - have a low rate of ischemic diseases (i.e., heart attack or stroke). Fish, particularly fatty fish, is a good source of the omega-3 fatty acids EPA and DHA. These fats help to lower serum triacylglycerols and cholesterol, help prevent the blood clots that form in heart attacks, and lower the chance of having an irregular heartbeat. In fact, one study found that women who ate fish at least once a week were 30% less likely to die of heart disease than women who ate fish less than once a month. Similar benefits have been found for men. Fish consumption is also related to slower growth of atherosclerotic plaque and lower blood pressure. Especially good sources of omega-3 fats are salmon, tuna, herring, mackerel, and canned tuna and sardines.

When included in the diet of pregnant and breastfeeding women, DHA is thought to be beneficial to infant brain (learning ability) and eye (visual acuity) development. Scientists have found that women who ate fatty fish while pregnant gave birth to children with better visual development. Babies of mothers who had significant levels of DHA in their diet while breastfeeding experienced faster-than-normal eyesight development. Preliminary research also suggests that a diet rich in omega-3 fatty acids - and in DHA in particular - may help to decrease the chance of preterm birth, thus allowing the baby more time for growth and development.

Recent research found that eating just one serving a week of fish decreased the risk of developing dementia by 30%. Eating fatty fish several times a week may also lower the risk of developing prostate cancer by as much as half. A Swedish study of 3500 postmenopausal women eating two servings of fatty fish a week found that they were 40% less likely to develop endometrial cancer than those eating less than one-fourth of a serving a week.

Eating a variety of fish and seafood, rather than concentrating on one species, is highly recommended for both safety and nutrition. It is recommended that pregnant women should avoid certain species of fish and limit their consumption of other fish to an average of 400 g of cooked fish per week. The reason for this recommendation is that, whereas nearly all fish contain trace amounts of methylmercury (an environmental contaminant), large predatory fish, such as swordfish, shark, tile-fish, and king mackerel, contain the most. Excess exposure to methylmercury from these species of fish can harm an unborn child's developing nervous system. It is also suggested that nursing mothers and young children should not eat these particular species of fish.

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