It is also involved in the function of gut microbiota, plasma retinol, CD38 (cluster of differentiation 38), and RORA (Retinoic acid receptor-related orphan receptor alpha4) mRNA.1 Insufficient levels of vitamin A can cause decreased CARS (Childhood Autism Rating Scale) score, increased level of serum 5-hydroxytryptamine (5-HT), and reduced development of the central nervous system.45 Vitamin A is necessary to maintain epithelial integrity and cellular differentiation, production of red blood cell, as well as increase in body resistance against infections. Here, CD38 is linked to autism.