Proteins are the building blocks of life. The body needs protein to repair and maintain itself. The basic structure of protein is a chain of amino acids.
Diet - protein; Complete protein; Incomplete protein
Every cell in the human body contains protein. It is a major part of the skin, muscles, organs, and glands. Protein is also found in all body fluids, except bile and urine.
You need protein in your diet to help your body repair cells and make new ones. Protein is also important for growth and development during childhood, adolescence, and pregnancy.
When proteins are digested, amino acids are left. The human body needs a number of amino acids to break down food. Amino acids need to be eaten in large enough amounts for optimal health.
Amino acids are found in animal sources such as meats, milk, fish, and eggs, as well as in plant sources such as soy, beans, legumes, nut butters, and some grains (such as wheat germ). You do not need to eat animal products to get all the protein you need in your diet.
Amino acids are classified into three groups:
Essential amino acids cannot be made by the body, and must be supplied by food. They do not need to be eaten at one meal. The balance over the whole day is more important. The nine essential amino acids are:
Nonessential amino acids are made by the body from essential amino acids or in the normal breakdown of proteins. They include:
Conditional amino acids are usually not essential, except in times of illness and stress. They include:
Protein foods are no longer described as being "complete proteins" or "incomplete proteins."
A diet high in meat can contribute to high cholesterol levels or other diseases such as gout. A high-protein diet may also put a strain on the kidneys.
A nutritionally balanced diet provides enough protein. Healthy people rarely need protein supplements.
Vegetarians are able to get enough essential amino by eating a variety of plant proteins.
The amount of recommended daily protein depends upon your age and health. Two to three servings of protein-rich food will meet the daily needs of most adults.
The following are the recommended serving sizes for protein:
2 to 3 ounces of cooked lean meat, poultry, or fish (a portion about the size of a deck of playing cards)
1/2 cup of cooked dried beans
1 egg, 2 tablespoons of peanut butter, or 1 ounce of cheese
Escott-Stump S. Nutrition and Diagnosis-Related Care. 6th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2008.
United States Department of Agriculture. Center for Nutrition Policy and Promotion. Dietary Guidelines for Americans. 2010. National Academy Press, Washington, DC, 2010.
National Academy of Sciences. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academy Press. Washington, DC, 2005.
A.D.A.M. Editorial Team: David Zieve, MD, MHA, and David R. Eltz. Previously reviewed by Alison Evert, MS, RD, CDE, Nutritionist, University of Washington Medical Center Diabetes Care Center, Seattle, Washington (5/5/2011).