Proteins are complex structures built out of amino acids. Some proteins are made up of just a couple of amino acids, while others are made of hundreds. In order to properly function, humans need 20 different amino acids. While 11 of these are made in the body the other nine — known as essential amino acids — are accessed through diet.
All foods except for oil and sugar contain protein, though the quality of the protein varies from food to food. Animal protein provides all nine essential amino acids needed by the human body, and is therefore, the highest quality protein. Aside from soy, all plant proteins lack at least one of the essential amino acids. Important sources of animal protein include meat, eggs, and dairy products.
This might make you think that animal protein is absolutely necessary for the human body to function, but this isn't so. If plant proteins are combined correctly, the body can build a complete protein. Vegetarians who combine grain foods with legumes are able to get all the protein they need.
What happens if an essential amino acid is missing from the body? Well, in the beginning the body will break down lean tissue in order to compensate. Eventually, however, the muscles will begin to waste away. While protein deficiencies are rare in the United States, they are more common in countries with poor diets. In addition to muscle wasting, other signs of protein deficiency include mental impairment in children, edema, anemia, decreased immunity, and metabolic abnormalities.
While an ideal diet should contain no more than 12% protein, recently high-protein weight-loss diets have come into vogue. When a high-protein diet is eaten the body eventually conserves protein by burning fat. While most dieters do lose weight in pounds, this is usually water weight, because a high-protein diet causes excessive urination. Weight loss generally returns once carbohydrates are introduced back into the diet. Dieters interested in trying a high-protein diet should think twice, as high-protein diets are difficult on the liver and kidneys; these diets also go hand-in-hand with higher amounts of saturated fat and cholesterol.
You can learn more about the nutritional value of protein by visiting this Protein and Amino Acids resource, published by the United States Department of Agriculture.
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Protein - Guide to Proteins