Fig. 7 |. Mass action explains amino acid homeostasis across high- and low-protein diets.

a, Serum amino acid levels in fasted and fed mice after two weeks on a high-protein, normal-protein or low-protein diet. Heatmaps are normalized to the row mean in the fasted or fed state. b, Consumption flux-concentration relationship across different dietary conditions based on non-perturbative infusions. After two weeks on the different protein diets, mice received a non-perturbative infusion of [U-¹³C]valine, [U-¹³C]lysine or [U-¹³C]methionine. Infusions for fasted groups were carried out exactly as in Fig. 2, except for the infusion rate being minimally perturbative. Infusions for the fed groups were carried out exactly as in Fig. 4a (food provided contained high, normal or low protein as per their diet), except for the infusion rate being minimally perturbative. The lines represent the consumption flux-concentration relationship determined based on fasted perturbative infusions from Fig. 2. c, Tissue TCA labelling-concentration relationship across different dietary conditions based on non-perturbative infusions. Mice were handled as in b, with terminal tissue sampling. The y axis shows the labelling fraction of a representative TCA intermediate (malate) relative to the serum labelling fraction of the infused ¹³C-valine, whose total concentration (sum of labelled and unlabelled) is plotted on the x axis. The lines represent the TCA labelling-concentration relationship determined based on fasted perturbative infusions from Fig. 5 and Extended Data Fig. 5b. d, Protein synthesis rate across dietary conditions based on non-perturbative ¹³C-valine infusions. The whole-body protein synthesis rate was estimated by the sum of the tissue protein synthesis rate multiplied by tissue weight. n = 6 mice in total per group with n = 2 mice per time point.