Tuesday, April 1, 2008

High-quality and low-quality proteins

Because an animal’s body is similar to yours, its proteins contain similar combinations of amino acids. That’s why nutritionists call proteins from foods of animal origin — meat, fish, poultry, eggs, and dairy products — high quality proteins. Your body absorbs these proteins more efficiently; they can be used without much waste to synthesize other proteins. The proteins from plants — grains, fruit, vegetables, legumes (beans), nuts, and seeds — often have limited amounts of some amino acids, which means their nutritional content is not as high as animal proteins.
The basic standard against which you measure the value of proteins in food is the egg. Nutrition scientists have arbitrarily given the egg a biological value of 100 percent, meaning that, gram for gram, it’s the food with the best supply of complete proteins. Other foods that have proportionately more protein may not be as valuable as the egg because they lack sufficient amounts of one or more essential amino acids.
For example, eggs are 11 percent protein, and dry beans are 22 percent protein. However, the proteins in beans don’t provide sufficient amounts of all the essential amino acids, so they (the beans) are not as nutritionally complete as proteins from animal foods. The prime exception is the soybean, a legume that’s packed with abundant amounts of all of the amino acids essential for adults. Soybeans are an excellent source of proteins for vegetarians, especially vegans, which are vegetarians who avoid all products of animal origin, including milk and eggs.
The term used to describe the value of the proteins in any one food is amino acid score. Because the egg contains all the essential amino acids, it scores 100.

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