Fig. 6 |. Mass action-driven oxidation clears amino acid influx from food.

a, Experimental design for measuring tissue-specific protein synthesis rate in fed and fasted mice using ¹³C-valine non-perturbative infusion. For the fasted group, mice were fasted from 9:00 to 18:00 and [U-¹³C]valine infusions were performed from 12:00 to 18:00. For the fed group, mice were fasted from noon to 20:00. At the time of lights off (20:00), food was provided and [U-¹³C] valine infusions were performed from 20:00 to 2:00. During the [U-¹³C]valine infusions, tissues were collected every 2 h and proteins were hydrolysed to measure valine labelling. b, Feeding does not impact protein synthesis rate. For calculations, see Extended Data Fig. 6. n = 6 mice in total per group with n = 2 mice per time point. c, Experimental design for measuring whole-body endogenous protein degradation rate in fed and fasted mice via pulse-chase. Mice were fed ad libitum and received a non-perturbative infusion of [U-¹³C]valine (green circles) for 48 h to label tissue proteins. The infusion was stopped at 16:00 on the third day for 4 h to clear circulating [U-¹³C]valine while mice were fasted. At 20:00, when the lights were turned off, mice received a non-perturbative infusion of the second tracer, [U-²H]valine (red circles) for 2.5 h while one group of mice was kept fasted and another was provided with food. d, Feeding suppresses protein degradation as measured by the serum ratio of [U-¹³C]valine (coming from protein degradation) to [U-²H]valine (infused at a constant rate). n = 2 mice. e, Valine production fluxes calculated from d. Diet flux is calculated by $\frac{\text{food}\text{intake}\times \text{valine}\text{percentage}}{\text{feeding}\text{duration}\times \text{bodyweight}}$. n = 2 mice. Raw data are shown as black dots. f, Valine fluxes during fasting and feeding. The numbers indicate fluxes in units of nmol g body weight min⁻¹.