The word protein comes from the Greek proteios, meaning primary." Your body is composed chiefly of proteins (18 to 20 percent by weight), and consequently needs to be supplied with them in order to build new cells, replacing those that are constantly dying. If the protein intake is so high that some are burned for energy, they are slow to digest. Thus, while they may be likened to hardwoods in our heat-generating analogy, their slow breakdown rate may cause them to remain long in the digestive tract, which is especially dangerous where meat and dairy products are concerned. Every naturally occurring food contains some protein, so the sources are abundant.
Proteins are like words, and the amino acids that comprise them are like letters. When a protein is digested, it is taken apart and the "letters" made available to spell whatever new words the body needs. Proteins (the "words") are used to build and repair the body. In order to perform all the necessary functions, an adequate supply of all the amino acids is imperative. Twenty-two different amino acids are required to build all the proteins needed by humans. Fourteen of these can be manufactured within the body. They are called the nonessential amino acids, not because they aren't necessary but because we don't have to do anything to supply them. The other eight cannot be easily synthesized by the body; they must be supplied with regularity and in certain proportions. They are called the eight essential amino acids. Some foods contain all eight of these essential amino acids; others contain varying amounts. It is important to know that you don't need to consume them all within the same meal.
High amounts of protein are found in meat, fish, eggs, milk, nuts, and soybeans, as well as in lesser-known food sources such as the grains amaranth and quinoa. Even fruits and vegetables contain amino acids that can be combined to form proteins. The idea that you must have superprotein foods like meat or dairy products in order to have adequate protein in the body is a myth, as many vibrantly healthy vegetarians will testify.