Cysteine is a conditionally essential amino acid–amino acids being the building blocks of proteins. This amino acid is conditionally essential because the body can normally produce it using other essential amino acids, but it may require supplementation when a person is ill. When two cysteines bind together to form cystine, this new amino acid contains a disulfide bond that is very strong and resistant to denaturing, making it important for enzymes functioning at low pH levels and structural proteins like keratin.
Since this amino acid can be produced by modifying existing essential amino acids in the body, consumption of cysteine is not technically required unless one is fighting an illness. Though consumption of this amino acid is still strongly suggested, so as to allow the essential amino acids to fulfill their other purposes in the body.
Sesame seeds, cottonseeds, soy, gelatin, peanuts, pumpkin seeds, sunflower seeds, molluscs, egg whites