L-?Leucine-?1-?13C is an isotope-labeled analog of L-Leucine, which is an essential amino acid. L-Leucine acts as a nutrient signal to stimulate protein synthesis. It has also activates the mammalian target of rapamycin kinase that regulates cell growth. L-?Leucine-?1-?13C is used in vivo to examine Leucine metabolism in man.