This is related to the utilization order of methionine and cystine in the body. In the case of extreme shortage of methionine, the organism gives priority to the synthesis of various sulfur-containing bioactive substances by cystine, so as to facilitate more methionine for protein synthesis and methyl metabolism. When methionine is sufficient, the body gives priority to methionine to meet the demand for cystine or cysteine. At this time, intake of high level of cystine may lead to the accumulation of a large amount of cystine or cysteine in the body. The accumulation of excessive cystine, especially cysteine, may induce the automatic oxidation of sulfhydryl groups, form oxygen free radicals, and cause lipid peroxidation damage to the body.