Fig. 1 |. Serum concentration increases linearly with metabolite infusion rate.

a,b, Schematic of the non-perturbative (a) and perturbative (b) infusion experiments. ¹³C-labelled metabolites (green circles) were infused into the circulation at different rates. The circulating concentration and consumption flux were measured to determine their relationship. c, Concentration-infusion rate relationship for fasted perturbative infusions. Mice (fasted from 9:00 to 18:00) were infused for 2.5 h with the indicated [U-¹³C]metabolite at different rates starting at 15:00. Blood was sampled at 17:30 and the total concentration of the infused metabolite (sum of labelled and unlabelled) in serum was measured by LC–MS. Throughout the manuscript, for the BCAA infusions, mice were infused with [U-¹³C]valine with unlabelled leucine and isoleucine at a fixed ratio that was reflective of their abundance in protein (5:6:3). Lines are linear fits to the data.