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