牛奶的成分.doc

牛奶的成分.doc Nutritional Components in Milk This page describes the function of nutritional components in milk: Energy, Water, Carbohydrate, Fat, Protein, Vitamins, Minerals, and Minor Biological Proteins & Enzymes. Links are provided to move the reader to pages that present the content of specific nutrients in milk, important background information on the chemistry of milk carbohydrate (lactose), fat, protein, and enzymes, and other topics that are covered in more depth in other sections of this website. Energy The energy in milk comes from its protein, carbohydrate and fat content, with the exception of skim milk that has virtually no fat. The energy content of some milk varieties is shown in the Nutrient Content Tables. Food provides energy to the body in the form of calories (kcal). There are many components in food that provide nutritional benefits, but only the macronutrients protein, carbohydrate and fat provide energy. The energy value of a food is calculated based on the calories provided by the amount of protein (4 kcal/gram), carbohydrate (4 kcal/gram), and fat (9 kcal/gram) that is present. Water Milk is approximately 87% water, so it is a good source of water in the diet. The water content of some milk varieties is shown in the Nutrient Content Tables. Water does not provide a nutritional benefit in the same manner as proteins or vitamins, for example. However, water is extremely important in human metabolism. Water is a major component in the body. Water maintains blood volume, transports nutrients like glucose and oxygen to the tissues and organs, and transports waste products away from tissues and organs for elimination by the body. Water helps to lubricate joints and cushions organs during movement. Water maintains body temperature regulation through sweating. Lack of water (dehydration) results in fatigue, mental impairment, cramping, and decreased athletic performance. Severe dehydration can be life-threatening. Carbohydrate Milk is approximately 4.9% carbohydrate in the form of lactose. The lactose content of some milk varieties is shown in the Nutrient Content Tables. Carbohydrates are the primary source of energy for activity. Glucose is the only form of energy that can be used by the brain. Excess glucose is stored in the form of glycogen in the muscles and liver for later use. Carbohydrates are important in hormonal regulation in the body. Lack of adequate levels of glucose in the blood and carbohydrate stores leads to muscle fatigue and lack of concentration. Lactose is a disaccharide made up of glucose and galactose bonded together. Before it can be used by the body, the bond must be broken by the enzyme lactase in the small intestine. People that have decreased activity of lactase in the small intestine may have problems digesting lactose and this is referred to as lactose intolerance or malabsorption. Fat Milk is approximately 3.4% fat. The fat content of some milk varieties is shown in the Nutrient Content Tables. Fats are a structural component of cell membranes and hormones. Fats are a concentrated energy source and are the main energy source used by the body during low intensity activities and prolonged exercise over 90 minutes. Fat is the main storage form of excess energy in the body. Fats cushion organs during movement. There are 2 fatty acids that are considered “essential” that cannot be made by the body and must come from the diet, and these are linoleic (18:2) and linolenic (18:3) acids. These fatty acids are used to synthesize the longer chain fatty acids arachidonic acid (AA, 20:4o-6), docopentaenoic acid (DPA, 22:4o-6), eicosapentaenoic acid (EPA, 20:5o-3) and docohexaenoic acid (DHA, 22:6o-3). These fatty acids are essential for the synthesis of hormones such as prostaglandins, thromboxanes, and leukotrienes that are involved in muscle contraction, blood clotting, and immune response. The fatty acids in milk fat are approximately 65% saturated, 29% monounsaturated, and 6% polyunsaturated. The polyunsaturated fatty acids in milk fat include small amounts of the essential fatty acids linoleic and linolenic, and approximately 5% trans fatty acids. An important trans fatty acid in milk fat is conjugated linoleic acid (CLA, 18:2). There are several types (isomers) of CLA in milk that have been shown to inhibit cancer and help maintain lean body mass while promoting the loss of body fat. The health benefits of CLA consumption are discussed in the Milk and Human Health section. The health concerns associated with fats are often linked to the chemical differences in the fatty acids. Saturated and trans unsaturated fats have been associated with high blood cholesterol and heart disease. However, the relationships are not simple. The length of the fatty acid chain and source of the unsaturated bond (naturally-occurring or man-made through processing) can greatly influence the health consequences of a specific fat in the human diet. In addition, the genetics and health status of an individual greatly influences the impact of consuming different types of fats. The subject of fats and health is complex and constantly being updated in the medical literature. Issues relating to milk fat are discussed in the Milk and Human Health section. The content of cholesterol in milk is shown in the Nutrient Content Tables. Cholesterol is an important component of cell membranes and as a starting material for the production of bile salts and steroid hormones. The body manufactures cholesterol to ensure that an adequate level of cholesterol is available for body functions. High levels of blood cholesterol are associated with increased risk for heart disease and are discussed in the Milk and Human Health section. Cholesterol is associated with fat so the content will vary depending on the fat content of the dairy product. An 8 oz serving of 2% milk contains 8% of the Daily Reference Intake (DRI) for cholesterol. Protein Milk is approximately 3.3% protein and contains all of the essential amino acids. The protein content of some milk varieties is shown in the Nutrient Content Tables. Proteins are the fundamental building blocks of muscles, skin, hair, and cellular components. Proteins are needed to help muscles contract and relax, and help repair damaged tissues. They play a critical role in many body functions as enzymes, hormones, and antibodies. Proteins may also be used as an energy source by the body. Nine amino acids must be obtained from the diet and are referred to as the “essential” amino acids: leucine, isoleucine, valine, phenylalanine, tryptophan, histidine, threonine, methionine, and lysine. Proteins that contain all 9 essential amino acids are often called “complete” proteins. Proteins of animal origin and soy are complete proteins, whereas proteins from grains and legumes are missing 1 or more of the essential amino acids, which means that consumers must eat complementary foods in order to get all of the essential amino acids. Milk protein consists of approximately 82% casein and 18% whey (serum) proteins. Both casein and whey proteins are present in milk, yogurt, and ice cream. In most cheeses the casein is coagulated to form the curd, and the whey is drained leaving only a small amount of whey proteins in the cheese. During cheese making, the 6-casein is cleaved between specific amino acids and results in a unique protein fragment that is drained with the whey. This fragment, called milk glycomacropeptide, does not have any phenylalanine and can be used as a source of protein for people with phenylketonuria, the inability to digest proteins that contain phenylalanine. Whey proteins have become popular ingredients in foods as an additional source of protein or for functional benefits. Whey proteins are used as a protein source in high protein beverages and energy bars targeted to athletes. Some examples include the use of whey proteins to bind water in meat and sausage products, provide a brown crust in bakery products, and provide whipping properties that replace a portion of egg whites. Whey proteins contain immunoglobulins which are important in the immune responses of the body. Whey proteins contain branched chain amino acids (leucine, isoleucine, and valine) and have been proposed to have some benefits to athletes for muscle recovery and for preventing mental fatigue. Vitamins Vitamins have many roles in the body including metabolism co-factors, oxygen transport and antioxidants. They help the body use carbohydrates, protein, and fat. The functions of vitamins are described below in alphabetical order. The content of vitamin A in milk is shown in the Nutrient Content Tables. Vitamin A is a fat soluble vitamin involved in vision, gene expression, reproduction, and immune response. The compounds with vitamin A activity are called retinoids and are found in foods in different forms – typically animal foods provide retinol and retinyl esters, and plant foods provide ß-carotene, a starting molecule (precursor) for vitamin A synthesis. Milk contains retinol, retinyl esters, and ß-carotene. Dairy products are a good source of vitamin A, although the vitamin A content will vary with the fat content of the product. An 8 oz serving of 2% milk contains approximately 15% of the daily reference intake (DRI) for vitamin A. The content of thiamin (vitamin B1) in milk is shown in the Nutrient Content Tables. Thiamin is a water soluble vitamin that is an enzyme cofactor involved in the metabolism of carbohydrates and branched chain amino acids. An 8 oz serving of 2% milk contains approximately 8% of the DRI for thiamin. The content of riboflavin (vitamin B2) in milk is shown in the Nutrient Content Tables. Riboflavin is a water soluble vitamin that is an enzyme cofactor involved in electron transport reactions. Milk is a recommended source of riboflavin and an 8 oz serving of 2% milk provides approximately 35% of the DRI for riboflavin. The content of niacin (vitamin B3) in milk is shown in the Nutrient Content Tables. Niacin is a water soluble vitamin that is an enzyme cofactor involved in electron transport reactions required for energy metabolism. There is a small amount of niacin in milk, an 8 oz serving of 2% milk contains less than 2% of the DRI for niacin. The content of pantothenic acid (vitamin B5) in milk is shown in the Nutrient Content Tables. Pantothenic acid is a water soluble vitamin that is an enzyme cofactor in fatty acid metabolism. Milk is a good source of pantothenic acid and an 8 oz serving of 2% milk contains approximately 17% of the DRI for pantothenic acid. The content of vitamin B6 (pyridoxine) in milk is shown in the Nutrient Content Tables. Vitamin B6 is a water soluble vitamin involved in the metabolism of proteins and glycogen (energy stored in the liver and muscles), and in the metabolism of sphingolipids in the nervous system. An 8 oz serving of 2% milk contains approximately 7% of the DRI for vitamin B6. The content of vitamin B12 (cobalamin) in milk is shown in the Nutrient Content Tables. Vitamin B12 is a water soluble vitamin involved in protein metabolism and blood functions. Milk is a recommended source of vitamin B12. An 8 oz serving of 2% milk contains approximately 47% of the DRI for vitamin B12. The content of vitamin C in milk is shown in the Nutrient Content Tables. Vitamin C is a water soluble vitamin that is an important antioxidant. It has a role in collagen formation in connective tissue and helps in iron absorption and healing of wounds and injuries. There is a negligible amount of vitamin C in milk, and a serving of milk contains less than 1% of the DRI for Vitamin C. The content of vitamin D in milk is shown in the Nutrient Content Tables. Vitamin D is a fat soluble vitamin that is important in maintaining blood calcium and phosphorus balance and assists calcium metabolism. Milk is typically fortified with vitamin D. Fortified milk is a good source of vitamin D, and an 8 oz serving of 2% milk contains over 50% of the DRI for vitamin D.The content of vitamin E in milk is shown in the Nutrient Content Tables. Vitamin E is a fat soluble vitamin that has antioxidant activity. The compounds with vitamin E activity are the tocopherols and tocotrienols. Milk contains a small amount of vitamin E, which increases with increasing fat content of dairy products. An 8 oz serving of whole milk contains 1% vitamin E, and an 8 oz serving of 2% milk contains only 0.5% of the DRI for vitamin E. The content of folate in milk is shown in the Nutrient Content Tables. Folate is one of the water soluble B vitamins. Folate is an enzyme cofactor important in the metabolism of proteins and nucleic acids and blood functions. There is a small amount of folate in milk. An 8 oz serving of 2% milk contains 3% of the DRI for folate. The content of vitamin K in milk is shown in the Nutrient Content Tables. Vitamin K is a fat soluble vitamin involved in blood clotting, bone metabolism, and protein synthesis. Milk contains a small amount of vitamin K, which increases with the fat content in dairy products. An 8 oz serving of milk contains less than 1% of the DRI for vitamin K. Minerals Minerals have many roles in the body including enzyme functions, bone formation, water balance maintenance, and oxygen transport. They help the body use carbohydrates, protein, and fat. The functions of minerals are described below in alphabetical order. The content of calcium in milk is shown in the Nutrient Content Tables. Calcium plays an essential role in bone formation and metabolism, muscle contraction, nerve transmission and blood clotting. Dairy products are a significant source of calcium in the diet. Milk is a recommended source of calcium, and an 8 oz serving contains almost 30% of the DRI for calcium.The content of copper in milk is shown in the Nutrient Content Tables. Copper is a component of enzymes used in iron metabolism. Milk contains a small amount of copper. An 8 oz serving of 2% milk contains approximately 3% of the DRI for copper. The content of iron in milk is shown in the Nutrient Content Tables. Iron is a component of blood and many enzymes. It is involved in blood metabolism and oxygen transport. Milk contains a small amount of iron, and an 8 oz serving of milk contains less than 1% of the DRI for iron.The content of magnesium in milk is shown in the Nutrient Content Tables. Magnesium is an enzyme cofactor and is important in bone metabolism. Milk is a recommended source of magnesium, and an 8 oz serving of 2% milk contains approximately 7% of the DRI for magnesium. The content of manganese in milk is shown in the Nutrient Content Tables. Manganese is involved in bone formation, and in enzymes involved in amino acid, cholesterol, and carbohydrate metabolism. There is a small amount of manganese in milk. An 8 oz serving contains less than 1% of the DRI. The content of phosphorus in milk is shown in the Nutrient Content Tables. Phosphorus is involved in maintaining body pH, in storage and transfer of energy, and in nucleotide synthesis. Milk is a recommended source of phosphorus, and an 8 oz serving of milk contains over 30% of the DRI for phosphorus. The content of potassium in milk is shown in the Nutrient Content Tables. Potassium is an electrolyte that is important in the maintenance of water balance, blood volume and blood pressure. Dairy products are a recommended source of potassium, and an 8 oz serving of milk contains approximately 8% of the DRI for potassium. The content of selenium in milk is shown in the Nutrient Content Tables. Selenium is important in oxidative stress response, electron transport, and regulation of thyroid hormone. Milk is a good source of selenium, and an 8 oz serving of 2% milk contains approximately 11% of the DRI for selenium. The content of sodium in milk is shown in the Nutrient Content Tables. Sodium is an electrolyte that is important in the maintenance of water balance and blood volume. An 8 oz serving of milk contains approximately 7% of the DRI for sodium. The content of zinc in milk is shown in the Nutrient Content Tables. Zinc is a component of many enzymes and proteins, and is involved in gene regulation. Milk is a good source of zinc, and an 8 oz serving contains approximately 10% of the DRI for zinc. Minor Biological Proteins & Enzymes Other minor proteins and enzymes in milk that are of nutritional interest include lactoferrin and lactoperoxidase. There are many other enzymes in milk but these do not have a role in human nutrition. Lactoferrin is an iron binding protein that plays a role in iron absorption and immune response. Many other functions of lactoferrin have been proposed, but their confirmation is still under study, including protection against bacterial and viral infections, and it's role in inflammatory response and enzyme activity. The use of lactoferrin as an antimicrobial agent is discussed in the section on Antibacterial Properties of Milk in this website. Lactoperoxidase is an enzyme that, in the presence of hydrogen peroxide and thiocyanate, has antibacterial properties. The use of lactoperoxidase as an antimicrobial agent is discussed in the section on Antibacterial Properties of Milk in this website. Lactoperoxidase does not provide antimicrobial protection to fresh milk because hydrogen peroxide is not normally present in milk – it must be added to activate this system. Lipases, a group of enzymes that break down fats, are present in milk but are inactivated by pasteurization, which increases the shelf life of milk. A popular belief among raw milk consumers is that the native lipase in milk plays an important role in the digestion of fat. Fat digestion begins in the stomach with gastric lipase, and the majority of fat digestion occurs in the small intestine, using enzymes secreted by the pancreas. The relative importance of the native milk lipase in digestion compared to the pancreatic lipases is not clear. Lactase (ß-galactosidase) is the enzyme responsible for the breakdown of lactose into glucose and galactose for digestion. There is no lactase present in fresh milk. Any lactase present in milk products comes from lactic acid bacteria that are either added to milk on purpose, as in the case of yogurt and cheese, or that enter milk from airborne or other contamination. A popular belief is that people with lactose intolerance are able to drink raw milk but not pasteurized milk because the lactase present in raw milk is inactivated during pasteurization. Because there is no lactase present in fresh milk, this concept is a myth. People with lactose intolerance have, themselves, lower levels of lactase which creates problems when it comes to digesting large amounts of lactose in a timely manner. Naturally occurring lactase used to digest milk is normally secreted by the small intestine. Lactase found in any lactic acid bacteria present will minimally help to digest lactose when it is released as the milk is digested in the small intestine. Updated 02/16/07