Development of pellagra like skin lesion in carcinoid syndrome is due to:
Inadequate conversion of Tryptophan to Niacin
Dietary Niacin deficiency
Serotonin induced skin rash mimicking pellagra
Paraneoplastic manifestation
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