A method of measuring energy expenditure in a living subject comprises: (a) administering a specified dose of doubly-labeled water (2H218O) to a living subject; (b) obtaining samples at three or more times of body water from the living subject; (c) measuring 2H/1H, 17O/16O and 18O/16O ratios in each of the obtained samples using optical spectroscopy; and (d) determining (1) a combined value of flux of body water and exhaled carbon dioxide from a change in measured 18O/16O over time, (2) a value of flux of body water alone from a change in measured 2H/1H over time, and (3) a reference value of isotopic background fluctuation from a change in measured 17O/16O over time. Using 17O measurements to estimate background fluctuations of the 2H and 18O decreases the required isotope dosing of subjects or decreases uncertainty at current dosing levels.