This is a pilot study to establish a novel methodology using mass-isotopomer flux analysis after infusions of 2-13C-Acetate, 2-15N-Glutamine and D5-phenylalanine to measure the in vivo TCA cycle flux in tissues of human beings. This study will simultaneously determine the validity of measuring the TCA cycle flux in tissue indirectly through dynamic differences in enrichment of labelled TCA cycle intermediates between arterial and venous blood supplies of that particular tissue bed (i.e. arteriovenous model or A-V balance technique). We propose to measure the rates of the different metabolic reactions within the TCA cycle by tracing the position-specific 13C and 15N transfer between the intermediate metabolites in order to characterize the oxidative, anaplerotic, cataplerotic and exchange rates across the TCA cycle. The use of 2-15N-Glutamine will specifically allow us to determine the rate of glutamine entry into the cycle via its conversion to glutamate, thus providing a more accurate quantification of the TCA flux.