L-methionine and DL-methionine are two variants of the amino acid methionine, but they have different uses and structures. L-methionine is the form that occurs naturally in food and is used by the body for protein synthesis and other processes. This form is biologically active.
DL-methionine is a synthetic variant that consists of both the L- and D-isomers. The D-isomer has no biological function in the body and is usually not used. With DL-methionine, it is only the L-form that can be converted and used by the body, while the D-form remains inactive.