Patent ID: 8823923

Claim:
A method of measuring energy expenditure in a living subject, comprising: administering a specified dose of doubly-labeled water ( 2 H 2 18 O) to a living subject; obtaining samples at three or more times of body water from the living subject; measuring 2 H/ 1 H, 17 O/ 16 O and 18 O/ 16 O ratios in each of the obtained samples using optical spectroscopy; and determining (1) a combined value of flux of body water and exhaled carbon dioxide from a change in measured 18 O/ 16 O over time, (2) a value of flux of body water alone from a change in measured 2 H/ 1 H over time, and (3) a reference value of isotopic background fluctuation from a change in measured 17 O/ 16 O over time, wherein the specified dose is selected to be a low-enrichment dose that produces an initial δ( 18 O) enrichment in the body water of the subject of +20‰ relative to VSMOW.