HMDB 4.0: the human metabolome database for 2018. - David S. Wishart; Yannick Djoumbou Feunang; Ana Marcu; An Chi Guo; Kevin Y. H. Liang; Rosa Vázquez-Fresno; Tanvir Sajed; Daniel Johnson; Carin Li; Naama Karu; Zinat Sayeeda; Elvis J. Lo; et al. (Nucleic Acids Res., 2018)