Summary: Heavy water can be used as a tracer for estimating metabolic rates (in vivo, in vitro) such as total energy expenditure (TEE) and fractional synthesis rates (FSR; eg. protein, lipids, triglycerides). When using both \u00b2H\u00b2O and H\u00b218O (DLW), TEE is estimated from the total production of CO\u00b2 as measured by the differences in decay rates of labeled the 18O and \u00b2H in body water over time following a single bolus of DLW (1). \u00b2H\u00b2O can be used to estimate fractional synthesis rates of metabolic reactions such as those associated with proteins, lipids, triglycerides, and cholesterol (2,4).References: 1. Gas chromatography-mass spectrometry assay of the (18) O enrichment of water as trimethyl phosphate. Brunengraber DZ, McCabe BJ, Katanik J, and Previs SF. Anal Biochem 306: 278\u2013 282 (2002). 2. Increased plasma membrane cholesterol in cystic fibrosis cells correlates with CFTR genotype and depends on de novo cholesterol synthesis. Fang D, West RH, Manson ME, Ruddy J, Jiang D, Previs SF, Sonawane ND, Burgess JD, Kelley TJ.Respir Res.; 11:61 (2010). 3. Triglyceride synthesis in epididymal adipose tissue: contribution of glucose and non-glucose carbon sources. Bederman IR, Foy S, Chandramouli V, Alexander JC, Previs SF. J Biol Chem.; 284(10):6101-8 (2009). 4. Novel application of the "doubly labeled" water method: measuring CO2 production and the tissue-specific dynamics of lipid and protein in vivo. Bederman IR, Dufner DA, Alexander JC, Previs SF. Am J Physiol Endocrinol Metab.; 290(5):E1048-56 (2006).