Novel application of the 'doubly labeled' water method: measuring C[O.sub.2] production and the tissue-specific dynamics of lipid and protein in vivo

The partitioning of whole body carbon flux between fat and lean compartments affects body composition. We hypothesized that it is possible to simultaneously determine whole body carbon (energy) balance and the dynamics of lipids and proteins in specific tissues in vivo. Growing C57BL/6J mice fed a high-fat low-carbohydrate diet were injected with a bolus of 'doubly labeled' water (i.e., [sup.2][H.sub.2]O and [H.sub.2][sup.18]O). The rate of C[O.sub.2] production was determined from the difference between the elimination rates of [sup.2]H and [sup.18]O from body water. The rates of synthesis and degradation of triglycerides extracted from epididymal fat pads and of proteins extracted from heart muscle were determined by mathematically modeling the [sup.2]H labeling of triglyceride-bound glycerol and proteinbound alanine, respectively. We found that mice were in positive carbon balance (~20% retention per day) and accumulated lipid in epididymal fat pads (~9 [micro]mol triglyceride accumulated per day). This is consistent with the fact that mice were studied during a period of growth. Modeling the [sup.2]H labeling of triglycerides revealed a substantial rate of lipid breakdown during this anabolic state (equivalent to ~25% of the newly synthesized triglyceride). We found equal rates of protein synthesis and breakdown in heart muscle (~10% of the pool per day), consistent with the fact that the heart muscle mass did not change. In total, these findings demonstrate a novel application of the doubly labeled water method. Utilization of this approach, especially in unique rodent models, should facilitate studies aimed at quantifying the efficacy of interventions that modulate whole body carbon balance and lipid flux while in parallel determining their impact on (cardiac) muscle protein turnover. Last, the simplicity of administering doubly labeled water and collecting samples allows this method to be used in virtually any laboratory setting. metabolic regulation; carbon-energy balance; triglyceride turnover; protein turnover; stable isotope tracer kinetics