This invention is a scheme for monitoring a solute in a biological system comprising the steps of delivering light into a biological system (12) containing a solute, the light having a wavelength selected to be in a range wherein the solute is substantially non-absorbing; detecting at least first and second portions of the delivered light, the first portion having traveled through the biological system along one or more paths characterized by a first average path length, and the second portion having traveled through the biological system along one or more paths characterized by a second average path length that is greater than the first average path length; and comparing the first and second portions of the delivered light to monitor concentration of the solute in the biological system. Also described are schemes for monitoring low molecular weight polyhydroxy solutes, generally sugars (mannitol, fructose, sucrose, glucose, sorbitol), alcohols (methanol, ethanol, propanediol), and electrolytes (sodium, potassium, magnesium, calcium, and chloride ions).