Development of pellagra like skin lesion in carcinoid syndrome is due to:

One of the main secretory products of carcinoid tumors involved in the carcinoid syndrome is serotonin which is synthesized from tryptophan. Up to 50% of dietary tryptophan can be used in this synthetic pathway by tumor cells, and this can result in inadequate supplies for conversion to niacin; hence, some patients (2.5%) develop pellagra-like lesions. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 3063