2.4. Fractional synthetic and breakdown rate calculations

The precursor‐product FSR and FBR were directly calculated from the enrichment decay over time after the tracer bolus (Zhang et al., 2002). FSR uses the change in enrichment in tracer bound to muscle proteins divided by the change in intracellular enrichment in muscle over the study period (Equation 1):

where E_F and E_B are the enrichments in the free intracellular and bound amino acid, respectively, pool in muscle, t₁ is the time 0 min, and t₃ is the time 60 min.

Calculations of FBR are also based on the precursor‐product principle; however, the precursor is now the unlabeled protein bound amino acid (phenylalanine), and the product the arterial dilution of L‐[¹⁵N]‐phenylalanine (Equation 2):

where E_m is the enrichment in the muscle intracellular free pool, E_A is the enrichment in the arterial blood, t₂ is the time 30 min, and Q_m/T is the ratio of free to bound Phe in muscle. Muscle net balance was calculated by subtracting FBR from FSR. Therefore, a positive muscle net balance would indicate anabolism, whereas a negative value would indicate catabolism. FSR and FBR results are reported in units of %/h.