Metabolomics: measuring the molecules that power life A gentle introduction to stable isotope tracing The central dogma of biology goes as follows: genetic information flows from DNA to RNA via transcription, and from RNA to protein via translation. That’s it. This is the fundamental framework for molecular biology. But, this central dogma overlooks an important layer: molecules used in, and resulting from, metabolism, known as metabolites. While genes and proteins provide the blueprint and machinery of life, metabolites represent the chemical activities occurring inside cells at any given moment. They not only provide building blocks for all kinds of molecules and regulate energy balance, but are also required for modifications of proteins and epigenetic regulation at the DNA level. Due to these crucial roles, studying metabolites is essential to understand disease mechanisms, for biomarker discovery, and to identify therapeutic targets. To study metabolites, we rely on mass spectrometry, a technique that converts molecules into charged particles and measures their mass-to-charge ratio with extremely high precision. In essence, mass spectrometry is a highly sophisticated (and expensive) molecular scale. If we know what we are looking for, it allows us to detect metabolites based on their precise molecular weight. You can see what one looks like in the picture below. For scale, I am 1.63 m tall: there is a certain irony in needing an instrument this large to study molecules this small! The abundance paradox: why more isn’t always more For those familiar with transcriptomics or proteomics, the initial analysis is often relatively straightforward: we compare condition A with condition B and generate beautiful volcano plots highlighting up- and downregulated genes or proteins (roughly: more and less active). Since few of us know the function of every gene and protein by heart, we then perform an extra analysis and eventually obtain a list of biological pathways that...