L-Methionine-d3 is intended for use as an internal standard for the quantification of L-methionine by GC- or LC-MS. L-Methionine is an essential amino acid. It is catabolized by methionine adenosyltransferase 2A (MAT2A), yielding the universal methyl donor S-. L-methionine has roles in purine and pyrimidine synthesis, polyamine synthesis, and glutathione production, as well as in various metabolic diseases and aging. Dietary methionine restriction (MR) prevents accelerated aging in a mouse model of progeria and increases the lifespan of yeast, Drosophila, C. elegans, mice, and rats. MR also improves insulin sensitivity and reduces adiposity in mice, as well as inhibits the growth and proliferation of several cancer cell lines.