We consider the phenomenon of natural optical activity, and related chiral magnetic effect in helical metals, broadly defined as the ones with spin-momentum locking. To reveal the correspondence between the two phenomena, we compute the optical conductivity of a noncentrosymmetric semimetal to linear order in the wave vector of the light wave, specializing to the low-frequency regime. We show that it is the orbital magnetic moment of quasiparticles that is responsible for the natural optical activity, and thus the chiral magnetic effect. We further extend the treatment to the case of spatially non-uniform systems, and derive the most general microscopic formulae for the gyrotropic tensor that satisfies general Onsager relations.