One of the main reasons americans have trouble maintaining the right balance in their dietary habits is the mistaken notion that if a modest amount of protein is good for you, large amounts must be even better. Because protein is a "body builder," people believe that it makes them strong and that it is better for them than carbohydrates. However, protein consumed in excess of the body's requirement is not converted to muscle; rather, it is converted to energy or stored as fat just as surplus carbohydrates are.

Protein is different from carbohydrates and fats, the other nutrients that supply energy, in that it contains nitrogen, as well as carbon, hydrogen, and oxygen. Because of their unique chemical structures, proteins contain the basic materials for cell growth and repair. They also help the body form antibodies to fight disease and produce substances, such as insulin, enzymes, and hemoglobin.

Protein is made up of chemical structures called amino acids. Both animal and plant proteins are made up of approximately 20 amino acids: can be produced in the body, and must be supplied by the diet. These are called essential amino acids. A complete protein is one that contains all of the essential amino acids. A high quality protein is a complete protein that contains the essential amino acids in amounts proportional to the body's need for them, as well as the nonessential amino acids. Protein sources from animals, including meat, fish, poultry, eggs, milk, and cheese, are examples of high quality, complete proteins.

A low quality protein, or incomplete protein, does rot contain all the essential amino acids in the proportions needed by the body. Plant protein sources are incomplete. Examples include nuts, beans, seeds, wheat, rice, oats, and whole grains. A low quality protein source can be converted into a complete protein source, but it must be matched with another source of low quality protein that is complementary. The body cannot make partial proteins, only complete ones; protein synthesis operates by the "all or none law." If an amino acid is supplied by one source in a smaller amount than is needed, the total amount of protein made from the other amino acids will be limited. Therefore if a diet does not include sources of complete protein, which might occur in a vegetarian diet, the right mix of foods must be included. Some vegetarians (lactoavovegetarians) omit meat, fish, and poultry from their diets but eat eggs and dairy products. Others (lactovegetarians) exclude eggs and consume only dairy products. Both types consume high quality animal proteins and do not need to worry about protein deficiency. Strict vegetarians, people who eat an all plant diet, may need to be discriminating in their food selections because plant proteins are low quality proteins. To obtain all essential amino acids from plant sources, strict vegetarians should eat a variety of foods from the legume group, such as beans, peas, and peanuts, combined with foods from the ceteal and whole grain group, including pasta, wheat, rice, oats, and corn. Combining protein sources from cereal and grains with legumes is called protein complement. Deficiencies in essential amino acids in one group are compensated by the essential amino acid content of the other group.

A plant protein that appears to be unique in comparison with other sources of amino acids is soy protein. Soy protein comes from soybeans and is present in tofu (soybean curd), soy flour, and other dry and moist preparations. Soybeans are high in protein, folate, omega 3 fatty acids, and minerals. People who consume about 3 ounces of tofu (or the equivalent) daily are less likely to develop some forms of cancer, osteoporosis, and kidney disease or to experience symptoms associated with menopause. Its most significant benefit, however, appears to be its ability to lower cholesterol levels. Researchers from the university of kentucky at lexington analyzed data from 38 studies and found that people who ate an average of 47 grams daily had a 12.9% drop in harmful low density lipoprotein and a 9.3% drop in total cholesterol. The beneficial high density lipoprotein stayed the same. The people who benefited the most were those with the highest cholesterol levels. Some experts believe soy protein's ability to clear cholesterol from the circulation is unsurpassed by any other foodstuff. Although scientists cannot document exactly how soy does this, they believe that the photochemical, specifically phys to estrogens (plant hormones) indigenous to soybeans, have an antioxidant effect on LDL cholesterol and inhibit blood platelets from clumping together, two processes that contribute to atherosclerosis.

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