The milk you drink contains a variety of vitamins and minerals, as well as protein and sugar. One of the most abundant proteins in milk is casein. The simple sugar lactose provides most of the natural sugar content of milk. While most people can drink milk without a problem, some people experience abnormal reactions after consuming casein or lactose.
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There are two major proteins in milk: whey and casein. Whey is the protein in the liquid portion of the milk that is left after milk curdles. Casein is the protein in the solid portion of curdled milk, often referred to as the curd. The term "casein" refers to several protein substances. Each of these substances has its own amino acid structure and function. Casein proteins contain phosphate, which allows calcium to bind to the protein. As a result, casein is what gives milk most of its calcium content.
Lactose is a type of simple carbohydrate called a disaccharide, which means it contains two sugars joined together. The simple sugars galactose and glucose make up lactose, often referred to as milk sugar. According to the Virtual Chembook at Elmhurst College, the lactose content of cow’s milk ranges from 4 percent to 6 percent. Your body cannot absorb lactose in its full form. When you consume milk, an enzyme produced in your small intestine, called lactase, breaks the lactose down into its simple sugar components, so they can be properly digested and absorbed by your body.
A milk allergy is characterized by an abnormal immune reaction to the milk proteins. You may be allergy to casein, whey or both of the proteins. When you consume milk, your body identifies the milk protein as a potentially harmful substance and sets off a reaction that sends out antibodies called immunoglobulin E, or IgE, to attack the milk proteins. IgE also triggers your body to release a chemical compound called histamine that causes widespread inflammation. This inflammation causes the symptoms most commonly associated with allergy, which include hives, wheezing, vomiting, runny nose, watery eyes, itchy skin, diarrhea and abdominal cramps.
Lactose intolerance is an inability to properly digest the milk sugar lactose. Lactose intolerance differs from a milk allergy in that it does not involve the immune system. If you are lactose intolerant, your small intestine does not produce enough of the enzyme lactase to properly break down lactose. As a result, lactose moves through your digestive system in its full form. When it reaches the bacteria that live in your large intestine, the bacteria feed on it, which causes the common symptoms of lactose intolerance. These symptoms include abdominal pain, bloating, gas, diarrhea and nausea.