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.

When the AL and CR rats were considered together, we found negative associations between the liver protein mass and Δ 15 N levels in the urine and plasma proteins (−0.65 ≤ R ≤ −0.35), and on the contrary, positive associations were found between the muscle protein mass and Δ 15 N levels that were highly significant ( p < 0.01) in cardiac and gastrocnemius muscles (0.40 ≤ R ≤ 0.62), but were only marginally significant in the soleus muscle ( R = 0.27, p = 0.077) ( Figure 2 ). δ 15 N was also measured for 11 transaminating (Ala, Asx, Glx, Gly, Pro, Ile, Leu, and Val), non-transaminating, and indispensable (Met, Phe and Lys) AA in tissue proteins from the small intestine, plasma, heart, and the gastrocnemius muscle; Δ 15 N was calculated by subtracting the δ 15 N value of their dietary counterpart.

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