Natural isotope abundances of carbon and nitrogen in tissue proteins and amino acids as biomarkers of the decreased carbohydrate oxidation and increased amino acid oxidation induced by caloric restriction under a maintained protein intake in obese rats

A growing body of evidence supports a role for tissue-to-diet N-15 and C-13 discrimination factors (N-15 and C-13), as biomarkers of metabolic adaptations to nutritional stress, but the underlying mechanisms remain poorly understood. In obese rats fed ad libitum or subjected to gradual caloric restriction (CR), under a maintained protein intake, we measured N-15 and C-13 levels in tissue proteins and their constitutive amino acids (AA) and the expression of enzymes involved in the AA metabolism. CR was found to lower protein mass in the intestine, liver, heart and, to a lesser extent, some skeletal muscles. This was accompanied by N-15 increases in urine and the protein of the liver and plasma, but N-15 decreases in the proteins of the heart and the skeletal muscles, alongside C-13 decreases in all tissue proteins. In Lys, N-15 levels rose in the plasma, intestine, and some muscles, but fell in the heart, while in Ala, and to a lesser extent Glx and Asx, C-13 levels fell in all these tissues. In the liver, CR was associated with an increase in the expression of genes involved in AA oxidation. During CR, the parallel rises of N-15 in urine, liver, and plasma proteins reflected an increased AA catabolism occurring at the level of the liver metabolic branch point, while N-15 decreases in cardiac and skeletal muscle proteins indicated increased protein and AA catabolism in these tissues. Thus, an increased protein and AA catabolism results in opposite N-15 effects in splanchnic and muscular tissues. In addition, the C-13 decrease in all tissue proteins, reflects a reduction in carbohydrate (CHO) oxidation and routing towards non-indispensable AA, to achieve fuel economy