The body does not absorb most nutrient molecules immediately. Most foods, including casein and other proteins, can't be absorbed whole. Instead, they're digested by the stomach and intestine, and absorbed into the bloodstream in the form of their constituent molecules.
Casein is a protein, and like other proteins, it's made up of a long chain of smaller molecules called amino acids. Drs. Reginald Garrett and Charles Grisham, in their book "Biochemistry," explain that amino acids are molecules containing primarily carbon, hydrogen, nitrogen and oxygen -- though some also contain sulfur. There are 20 common amino acids, and when they're linked together in various combinations into chains of various length, they make up all the different proteins in foods and living organisms.
Casein is a specific protein; it's found in dairy products, and is particularly prevalent in cheese. When milk separates into curds and whey, as it does during the process of cheese-making, casein collects in and forms the physical structure of the curds. Cheese makers separate curds from whey, and further age curds, producing cheese. Because casein is a protein when you consume it, whether in combination with whey in milk or relatively isolated in cheese, you digest it just as you would any other protein.
Your body starts digesting protein once it reaches the stomach, explains Dr. Lauralee Sherwood in her book "Human Physiology." The stomach's digestive juices contain a number of protein-digesting chemicals, including hydrochloric acid and enzymes. Enzymes are proteins themselves -- though they're not digested in the stomach like proteins you eat -- and they help speed up the process of digestion. Protein digestion continues in the small intestine.
Between the digestive juices of the stomach and those of the small intestine, which include pancreatic secretions and complete the process of protein digestion, your body breaks down all the proteins you eat into amino acids. As you digest casein and other proteins, the cells of the small intestine wall absorb amino acids from the proteins into the bloodstream, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." Your cells then use the digestion products of casein and other proteins.
Your cells use some nutrient molecules, such as sugars and fat, to provide energy. Cells can use these molecules immediately or store them for later use. Unlike sugars and fats, cells can't store amino acids, explain Drs. Garrett and Grisham. When you digest casein and protein, cells use the products right away for energy, or use them as building blocks to make cellular proteins. Cells can also chemically change the digestion products of casein and other proteins into other molecules, such as neurotransmitters, which send chemical signals in the brain and nervous system.