Chemistry

Proteins are composed of many amino acid molecules linked together by peptide bonds. There are 20 different amino acids. And while the basic structure of all these is composed of carbon, hydrogen, oxygen and nitrogen, what makes them unique are the side chains that are attached to the basic structure. Nine of the amino acids are called essential because they cannot be produced in adequate amounts by the body and must be supplied by food. The rest can be manufactured by the body if they are not provided by food. Animal sources of protein are complete because they contain all nine essential amino acids. Most plant sources of protein are incomplete because they lack one or more of the essential amino acids. Complementary proteins are two incomplete proteins that can be combined to provide all the essential amino acids. Some common examples of this are a peanut butter sandwich or beans and rice.

Function

The cells of all our body tissues are constantly dying and needing to be replaced by new cells. Protein is the primary building material for these new cells. So protein is needed to maintain healthy levels of all body tissues such as skin, bone, teeth, skeletal muscle, connective tissue, vital organs and blood. Protein is also necessary for manufacturing hormones, antibodies and enzymes. Other functions of protein are balancing water, electrolytes and the pH of blood and body tissues as well as aiding in the transport of various substances (vitamins, minerals, fatty acids, triglycerides and cholesterol) throughout the body.

Sources

Good sources of protein are meat, poultry, seafood, eggs, legumes, nuts, seeds, soy products and dairy foods. Grains provide small amounts. Foods such as baked goods and chocolate candy also provide small amounts.

Digestions and Metabolism

After food is chewed and swallowed, protein digestion continues in the stomach with the production of hydrochloric acid and the enzyme pepsin. Pancreatic and intestinal enzymes (proteases) complete the breakdown of protein to individual amino acids in the small intestine. Individual amino acids pass through the intestinal wall and travel through the blood stream to the liver. They are then balanced and sent back into the blood stream to be taken up by the cells. In the cells, body proteins are made when synthesizing enzymes create long chains of amino acids directed by the genetic code (DNA).  The sequence of these amino acids differs from person to person and is dependent on the inherited DNA code. Amino acids not needed for body proteins are transformed into glucose and used by the cells for energy.

How Much Is Needed

It is recommended that 10 to 35 percent of our total daily calories come from protein. If inadequate amounts of fat and carbohydrate are eaten, most protein will be used for energy, and there will be inadequate protein available to repair and build body tissue. This can lead to health problems in all body systems and can even eventually lead to multi-organ failure. Excess protein stresses the kidneys and heart and may lead to an increased risk of cancer and osteoporosis as well as decreased athletic performance. Using principles of mindful, intuitive eating, we can determine what amount of protein within the above range results in our feeling the best.
I hope you find the information in this article and the preceding two articles about fat and carbohydrate helpful for understanding our friends, the energy nutrients.  Looking forward to your comments in the section below!

Since 1990 registered dietitian Deborah Kauffmann, RD, LDN has been providing nutrition counseling for disordered eating utilizing an intuitive, Health At Every Size® approach. Deborah works with adults, teens and children/parents. She is one of the pioneers of the HAES (SM) approach in the Baltimore area. Deborah also offers Largely Positive, a free support group for adults of size promoting size acceptance and self care. Her private practice is Nutrition Counseling for Intuitive Eating.