Substrate cycling in lean and obese Zucker rats

The objective of this study was to compare substrate cycling in lean and obese Zucker rats under conditions where there was (i) a nominal excess or (ii) a deficit in energy intake. Protein turnover was assessed by measuring protein synthesis and breakdown using 15N glycine as the tracer. Glucose/glucose-6-phosphate cycling was calculated from the difference in the glucose appearance rates measured using 2-D- and 6,6-D2-labeled glucose. Information on triglyceride/fatty acid cycling was obtained by measuring free fatty acid kinetics using 1-13C palmitate and glycerol kinetics using 5-D glycerol. Neither protein synthesis or breakdown or glucose cycling rates were significantly different between lean and obese Zucker rats. The relative rate of the triglyceride/fatty acid cycle was higher in the obese rat as a consequence of increased adiposity. Increased intake increased glucose cycling and decreased lipid cycling in both lean and obese rats. The findings indicate that substrate cycling is not different in obese rats and therefore is not a significant factor by which such animals maintain their severely obese state.