As 5-HT produced in the SI is transported via portal circulation to the liver for subsequent binding with HTR2A, we examined the kinetics of serum 5-HT levels in HFD-fed WT and HFD-fed MC4RKO mice, and in patients with fatty liver disease, to determine whether 5-HT levels reflect hepatic steatosis. This evidence concerns the gene HTR2A and fatty liver disease.