Excessive intake of any nutrient will produce side effects, but the size of the side effects is different. Cystine you mentioned is an amino acid. There are 20 kinds of amino acids that make up protein, but cystine is not important, because it is not an essential amino acid, but an amino acid that can be synthesized by human body itself. In other words, this amino acid can be produced by the body itself using absorbed nutrients. But those essential amino acids cannot be made by the body itself and must be supplied from food. There are eight essential amino acids. The amino acids you said don't need to be supplemented. Because in general, protein contains this amino acid. Unlike those eight essential amino acids, they only exist in the complete protein.