Methods for measuring tissue protein breakdown rate in vivo

Purpose of review To describe the latest innovations in measuring protein breakdown in vivo, particularly in muscle. Recent findings The traditional method of using 3-methylhistidine excretion to measure muscle protein breakdown has been updated to include arteriovenous or microdialysis measurements, which address the concern that there are alternative sources of 3-methylhistidine in the body other than muscle. Several variations of a precursor-product method to measure fractional breakdown rate of tissues have been developed that are analogous to fractional synthesis rate of tissues. These methods are more generally applicable than the 3-methylhistidine methods and are less invasive than arteriovenous methods. The various precursor-product methods are distinguished by whether they require an isotopic steady state or multiple tracers and by how many biopsies are required. Summary The new precursor-product methods have enabled assessment in clinical trials of protein breakdown for proteins other than myofibrillar proteins and in circumstances in which arteriovenous sampling is not feasible.