Dr. Martin's notes

Monday December 5, 2016

Proteins

Proteins are the building blocks of the body. They are used to repair damaged tissue and for normal cell replacement. Hormones, antibodies, and the enzymes that regulate body functions area all made of protein.

Proteins break down into Amino Acids, which then form DNA and RNA molecules for the body's repair system. If available carbohydrates and fat cannot meet an individual's energy needs, proteins will be broken down and used as a source of emergency energy. Each gram of protein equals 4 calories of energy.

Each protein is a large, complex molecule made up of a string of building blocks called amino acids. The 20 amino acids the body needs can be linked in thousands of different ways to form thousands of different proteins each with a unique function in the body. Both the amino acids manufactured in the liver and those derived from the breakdown of the proteins we eat are absorbed into the bloodstream and taken up by the cells and tissues to build new proteins as needed.

The body cannot use food protein directly, even though the amino acids in food and in the body are the same. After protein is ingested, digestive enzymes break down the protein into shorter amino acid chains (polypeptides and then peptides) and finally into individual amino acids. The amino acids then enter the bloodstream and travel to the cells, where they are incorporated into proteins the body needs.

The quality of a food protein is measured in part by its amino acid content. There are two types: 9 of the 20 amino acids required by human beings are considered essential because they come only from the diet; the other 11 are considered nonessential because the body can make them. A complete protein contains all the essential amino acids in amounts the body needs.

Animal proteins from eggs, meat, fish, poultry, cheese, and milk are generally complete. Plant proteins from fruits, vegetables, grains, and beans are usually low in one or more essential amino acids and are considered incomplete. A well-balanced vegetarian diet, however, can provide the body with all the needed amino acids.

The National Research Council of the National Academy of Sciences considers the average adult's daily requirements to be 0.8 grams (g) of protein for each kilogram (kg) of body weight. To determine this requirement, divide body weight by 2.2 (the number of pounds per kilogram), and then multiply the answer obtained by 0.8 (g of protein for each kg of body weight).