Twenty-two different amino acids are known. All but nine of these acids, however, can be made by the cells from fat or sugar combined with the nitrogen freed from the breakdown of used proteins. The nine which the body cannot make are call essential amino acids. This means it is essential that they be present in the diet if health is to be maintained. These nine are tryptophane, lysine, methionine, phenylanlanine, threonine, valine, leucine and isoleucine.
Children also have difficulty making enough arginine, and histidine.
Amino acids are present in greatest abundance in egg yolk, fresh milk, liver, kidneys and cheese. Also, brewers' yeast, some nuts, soybeans, cottonseed and the germ of cereals contain complete proteins. They have also been called smart drugs. Many were originally discovered as a treatment for dementia, or amnesia. It is now known that anyone can benefit from the effects of these substances if taken in proper dosages.