There is described a system and method for the in vivo determination of lactate levels in blood using Near-Infrared Spectroscopy (NIRS) and/or Near-infrared Raman Spectroscopy (NIR-RAMAN). The method teaches measuring lactate in vivo comprising: optically coupling a body part with a light source and a light detector the body part having tissues comprising blood vessels; injecting near-infrared (NIR) light at one or a plurality of wavelengths in the body part; detecting, as a function of blood volume variations in the body part, light exiting the body part at at least the plurality of wavelengths to generate an optical signal; and processing the optical signal as a function of the blood volume variations to obtain a lactate level in blood.