What Happens to the Proteins You Eat?

The cells in your digestive tract can absorb only single amino acids or very small chains of two or three amino acids called peptides. So proteins from food are broken into their component amino acids by digestive enzymes —which are, of course, specialized proteins. Then other enzymes in your body cells build new proteins by reassembling the amino acids into specific compounds that your body needs to function. This process is called protein synthesis.
During protein synthesis

• Amino acids hook up with fats to form lipoproteins, the molecules that ferry cholesterol around and out of the body. Or amino acids may join up with carbohydrates to form the glycoproteins found in the mucus secreted by the digestive tract.
• Proteins combine with phosphoric acid to produce phosphoproteins, such as casein, a protein in milk.
• Nucleic acids combine with proteins to create nucleoproteins, which are essential components of the cell nucleus and of cytoplasm, the living material inside each cell.

The carbon, hydrogen, and oxygen that are left over after protein synthesis is complete are converted to glucose and used for energy. The nitrogen residue (ammonia) isn’t used for energy. It’s processed by the liver, which converts the ammonia to urea. Most of the urea produced in the liver is excreted through the kidneys in urine; very small amounts are sloughed off in skin, hair, and nails.

Every day, you turn over (reuse) more proteins than you get from the food you eat, so you need a continuous supply to maintain your protein status. If your diet does not contain sufficient amounts of proteins, you start digesting the proteins in your body, including the proteins in your muscles and — in extreme cases — your heart muscle.