What the Body Does with Protein
Of course, protein is an essential nutrient for us all, and what was said above should not be taken to suggest that it is not a critically important part of the diet. Indeed, the word itself, coined in 1838 by Gerardus Johannes Mulder, a Dutch chemist, is taken from the Greek meaning “in first place.” He chose the name because he thought that life could not go on without it. Although we now know that we cannot live on protein alone, we have also discovered during the last century and a half many essential functions for dietary protein.
Every cell in your body contains some protein. Excluding water, 50 percent of the body’s weight is protein-hundreds of different kinds of proteins, each with special properties and functions. Protein is part of muscle, bone, cartilage, skin, blood, and lymph. All enzymes and many hormones are proteins. The only body substances that normally lack protein are bile and urine.
The protein of muscle allows this tissue to contract and hold water.
The protein in hair, skin, and nails is hard and insoluble, providing a protective coating for the body. The protein in blood vessels is elastic, allowing them to expand and contract to maintain normal blood pressure. Protein is also the rigid framework for the minerals of bones and teeth.
With protein, the body forms new tissues, replaces worn-out ones, regulates the balance of water and acids and bases, and transports nutrients in and out of cells. Protein is needed to make antibodies, which help us combat foreign invaders like disease-causing bacteria. Proteins transport oxygen and nutrients in the blood and are essential to the clotting of blood and the formation of scar tissue.
The protein you eat is not used as such to fulfill these roles. Rather, dietary protein must first be broken down into its component parts in the digestive tract and then absorbed into the bloodstream. These components travel in the blood to all parts of the body wherever cells might need them. In tissues throughout the body, new proteins are built up from these components to meet the body’s needs.
The building blocks of proteins, called amino acids, are nitrogen containing chemicals that get strung together in ever-larger units usually 100 to 300 individual amino-acid molecules-until a complete protein is formed. The particular combination of amino acids and the order of the amino-acid necklace determine the properties of the resulting protein. Because there are so many possibilities for constructing proteins, it is our most versatile nutrient.
Living tissue contains 22 different kinds of amino acids. The human body can take apart amino acids derived from the diet and reconstruct new ones from their basic elements, carbohydrates and nitrogen. However, 9 of the amino acids we need we cannot manufacture. These are called essential amino acids; they must be supplied in their final form as part of the protein we eat.
The essential amino acids are methionine, threonine, tryptophan, isoleucine, leucine, lysine, histidine, valine, and phenylalanine. This rather forbidding list is presented merely to help you understand what follows and to appreciate proposals for supplementing certain foods with particular amino acids.