Proteins

Proteins consist of polymers made up of various amino acids joined together by condensation reactions into peptide bonds.

We need protein to supply the building blocks for most of the tissue and organs in the body. The daily requirement of protein for an adult is about 0.8 grams per kilogram of body weight. A 150 pound person weighs about 68 kilograms and would therefore need about 55 grams of protein a day. Infants need about 1.8 grams of protein per kilogram of body weight and children need about 1 gram of protein per kilogram of body weight.

There are ten amino acids that cannot be synthesized in the body. These are called the essential amino acids and include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. We need these amino acids in specific amounts daily in our diet. Arginine is essential for infants but not adults.

Plant proteins are incomplete in that they lack one or more of the essential amino acids. Wheat, rice and corn all lack lysine. Rice also lacks threonine and corn also lacks tryptophan. Beans lack methionine. A vegetarian diet of cereals and legumes (peas, beans, peanuts) can provide all of the amino acids if the  proper combinations of plants are eaten.

Proteins are metabolized into smaller polypeptides in the stomach by an enzyme called pepsin. These polypeptides are metabolized to individual amino acids in the small intestine. The amino acids are absorbed through the intestinal wall into the bloodstream.