The metabolome is defined as the total set of low molecular weight compounds (generally less than 1.5 kDa) that exist in biological samples. Metabolomics (or metabolome analysis), together with genomics, transcriptomics and proteomics, is one of the main “omics” technologies in systems biology. Therefore, metabolomics has become an important research field for gaining functional insight into cell biology. In particular, metabolites are the final products in biological systems, and their variations are thought to reflect the responses caused by a number of metabolic perturbations. Currently, the human metabolome database lists 74 461 metabolites (as of July 1, 2017),¹  but...