Protein, the most sacred of all nutrients, is a vital component of our bodies and there are hundreds of thousands of different kinds. They function as enzymes, hormones, structural tissue and transport molecules, all of which make life possible. Proteins are constructed as long chains of hundreds or thousands of amino acids, of which there are fifteen to twenty different kinds, depending on how they are counted.
Proteins wear out on a regular basis and must be replaced. This is accomplished by consuming foods that contain protein. When digested, these proteins give us a whole new supply of amino acid building blocks to use in making new protein replacements for those that wore out.
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This process of disassembling and reassembling the amino acids of proteins is like someone giving us a multicolored string of beads to replace an old string of beads that we lost. However, the colored beads on the string given to us are not in the same order as the string we lost. So, we break the string and collect its beads. Then, we reconstruct our new string so that the colored beads are in the same order as our lost string.
But if we are short of blue beads, for example, making our new string is going to be slowed down or stopped until we get more blue beads. This is the same concept as in making new tissue proteins to match our old worn out proteins.
About eight amino acids (“colored beads”) that are needed for making our tissue proteins must be provided by the food we eat. They are called “essential” because our bodies cannot make them. If, like our string of beads, our food protein lacks enough of even one of these eight “essential” amino acids, then the synthesis of the new proteins will be slowed down or stopped.
This is where the idea of protein quality comes into play. Food proteins of the highest quality are, very simply,
those that provide, upon digestion, the right kinds and amounts of amino acids needed to efficiently synthesize our new tissue proteins. This is what that word “quality” really means: it is the ability of food proteins to provide the right kinds and amounts of amino acids to make our new proteins.
Can you guess what food we might eat to most efficiently provide the building blocks for our replacement proteins? The answer is human flesh. Its protein has just the right amount of the needed amino acids. But while our fellow men and women are not for dinner, we do get the next “best” protein by eating other animals. The proteins of other animals are very similar to our proteins because they mostly have the right amounts of each of the needed amino acids. [...] While the “lower quality” plant proteins may be lacking in one or more of the essential amino acids, as a group they do contain all of them.
The concept of quality really means the efficiency with which food proteins are used to promote growth. This would be well and good if the greatest efficiency equaled the greatest health, but it doesn’t, and that’s why the terms efficiency and quality are misleading.
(The China Study – T. Colin Campbell, Ph.D., and his son, Thomas M. Campbell II. Dr. Campbell is a professor of Nutritional Biochemistry)