A healthy amount of protein in the diet and/or a quality powdered protein supplement supports general health and prevents deficiency in amino acids from occurring. But if one has an ailment, related to deficiency in a specific amino acid, or one that responds to therapeutic doses of a specific amino acid, then, like with the B vitamins, they must take high amounts of that one amino acid for a specific period of time. A protein supplement, or protein from food, will not work to provide sufficient amounts of single amino acids for therapeutic purposes.
It is important that single amino acids be taken away from protein (either food or supplements). This is because other amino acids found in protein can limit the rate of absorption of specific isolated amino acids. The exemption to this rule is L-glutamine which can be taken with protein powders (most commonly done by people for post-exercise protein shakes designed to repair and build muscle tissue).