Patients receive water labeled with deuterium and oxygen O 18 by mouth or by gastrostomy tube on days 1 and 7. Urine samples are collected prior to the first dose, 4-6 hours after the first dose, and then every 24 hours until the second dose. All urine is collected for 4-6 hours after the second dose. Samples are analyzed by dual inlet isotope ratio mass spectrometry.