Protein Synthesis Rate Calculator

The fractional synthetic rate (FSR) is the proportion of the muscle protein pool synthesized per unit time, expressed as a percentage per hour (%/h) or per day (%/day). It is measured using stable isotope tracer techniques: a labeled amino acid (such as L-[ring-2H5]-phenylalanine) is infused intravenously, and muscle biopsies are taken at multiple time points to measure the enrichment of the tracer in muscle protein (bound form) relative to the free amino acid pool (precursor enrichment).

FSR (%/h) = ((E(p2) - E(p1)) / ((E(precursor1) + E(precursor2)) / 2)) x (1 / t) x 100. Where E(p1) and E(p2) are the tracer enrichments in muscle protein (tracer-to-tracee ratio, TTR) at biopsy 1 and biopsy 2; E(precursor) is the mean precursor pool enrichment (free intracellular or plasma amino acid TTR); and t is the time between biopsies in hours.

At rest and fasted, human skeletal muscle FSR is approximately 0.04-0.07%/h (0.05%/h is a commonly cited mean). After a protein-containing meal, FSR increases to approximately 0.07-0.10%/h. After resistance exercise, FSR can rise to 0.10-0.15%/h for 24-48 hours. Whole-body protein synthesis rates are higher than muscle protein FSR because they include liver, gut, and plasma protein synthesis.

Resistance exercise is the most potent stimulus for increasing muscle protein FSR. Dietary protein intake (particularly leucine-rich sources such as whey and dairy) synergistically increases FSR when consumed post-exercise. Essential amino acids (EAAs), particularly leucine, act as signaling molecules that activate the mTORC1 pathway. Adequate energy intake, insulin signaling, testosterone, growth hormone, and sleep also support protein synthesis.

This calculator computes FSR from tracer enrichment data collected in isotope infusion studies, which are performed in research settings, not in routine clinical practice. The result helps researchers quantify how much new muscle protein is being made under different nutritional and exercise interventions. Clinically, it helps determine the effectiveness of protein supplementation in athletes, elderly patients with sarcopenia, ICU patients, or those recovering from surgery.