Question

If the diet is deficient in one or more of the _____amino acids, the body must break down existing proteins to provide the amino acids.

Answer 1

The answer is essential amino acids. When an essential amino acid is lacking the body break down muscles to make up for it.

Answer 2

Essential amino acids
Amino acids are the molecules that make up proteins when they are linked up. There are certain essential amino acids, namely they are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
All of these must be obtained from one's diet. If they are not, the body will use existing proteins, such as muscle mass, to obtain them.