Mixed muscle protein synthesis can be measured using stable isotopes like d5 phenylalanine to assess label incorporation into muscle tissue relative to precursor pools.

“you have stable isotopes so everybody knows from high school or most of you will know from high school that in chemistry you have carbon 12 or carbon is is is a weight of 12 but there's in nature there's also carbon 30 and carbon 30 means it has an extra neutrient in its core it's stable doesn't fall apart you also have 14 carbon that's unstable falls apart and you're at radioactivity and that's why it's a radioactive isotope now you can actually purchase carbon 13 labeled metabolites and so you can also buy carbon 13 C carbon 13 labeled filine for example nowadays we often use filine as the Tracer because it's a it's a it's an essential amino acid that is hardly metabolized um in uh tissue in in muscle or not metabolized in muscle and it's not as volatile as for example Lucine and Lucine of course has a lot of other effects so you want to have a nice St amino acid that is a good representation of other essential amino acids so if you actually buy 13 C labeled vanilline you can dilute it in a in a in a in a bag of saline Infuse it in the body”

“then you actually have like 5% of all your Fila alanine floating around in your circulation and then you take a muscle biopsy then you take the muscle out of the biopsy you take the protein out of the biopsy you can even SE separate mitochondial protein or myi protein so that's eosine for example and then hydroly that protein back to its fre amino acids and I use Mass spectrometry to look at how many of those amino acids in the muscle protein are coming from the carbon 13 C lab labeled vanilla now a few hours later you take a new biopsy and you see how much more of that 13 C Filan is built into the muscle protein if you divide that by the availability in the circulation and the time between the two biopsies you have a fractional synthetic rate generally expressed in percent per hour”