This tight regulation is partly why many supplemental
testosterone boosters do not work
over the long term, and if we use D - aspartic acid as a notable example its ingestion is able to increase a rate limiting step (increased activity of the StAR enzyme) and increase
testosterone but is eventually regulated at another step (increased activity of D - amino acid oxidase or DAO, which degrades D - aspartic acid) eventually returning to the levels of
testosterone seen before supplementation.