This method allows to analyze if the carbon atoms of lactate are embedded into proteins. Indeed, mammalian cells express the transporter of monocarboxylic acids (called MCT1) that allows the entry of lactate into the cell. To this end, cells are incubated for 24 h with the culture medium containing lactate uniformly labeled with carbon 14 and then, lactate inside the cell is evaluated by counting the radioactivity by a scintillator.