D-aspartic acid is an essential amino acid and a key ingredient in various testosterone boosting anti-estrogen supplements. D-aspartic acid is not used to build proteins; instead, it plays a role in making and releasing hormones in the body. It is an endogenous NMDA receptor agonist with similar activity to the L-isomer. D-aspartic acid also enhances the release of luteinizing hormone (LH) and testosterone. This action is mediated in the pituitary by cGMP and in the testis by cAMP, which acts as the second messengers in the signal transduction in the pituitary and testes respectively. The pituitary and testis possess a D-Aspartate racemase, which provides the necessary production of this isomer.