Although l - tryptophan can be obtained by eating foods that contain it, such as red meat and turkey, its ability to be converted into 5 - HTP - and ultimately into serotonin - is limited by the availability of an enzyme called tryptophan hydroxylase. (verywellmind.com)
Cerebral tryptophan hydroxylase appears to have a poor affinity for oxygen and to be affected by slight hypoxia. (science.sciencemag.org)
Next there's a study which found that calcium is required for activating tryptophan hydroxylase, which manufactures serotonin. (supernaturalacnetreatment.com)