Amino acids are the building blocks of proteins. Every protein is a long chain of amino acids arranged in a certain sequence and "folded" into a certain shape. Almost everything in your body is composed of proteins, and your body needs amino acids to construct them. Out of the 20 amino acids that make up all of these proteins, your body can build 12 of them "from scratch" and the other 8 need to be absorbed from your food.
The idea of a "complete protein" is one that is highly bioavailable because it contains all 8 amino acids in the proper ratios for your body to absorb. If the ratios are out of whack (too much of one and too little of another) the body will not absorb the amino acids as efficiently. So complete protein foods can be used more efficiently.
The usual cited example of combining food to get a complete protein is rice and beans. Each food on it's own has protein that the body can only absorb inefficiently (much of the protein is eliminated), but combined, the amino acid ratios are such that the total protein can now be absorbed with much higher efficiency.
I can't vouch for accuracy, but I've read that it's less important to have a complete protein at every meal than it is to make sure you ingest the proper amino acids over the length of the day --- apparently the body will not eliminate excess aminos of one type right away, so if you eat more protein at supper that complements the protein you had at lunch, the body will combine them for higher bioavailability. Can anyone comment on whether this is accurate?
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