SUBSTRATE CYCLING IN LEAN AND OBESE ZUCKER RATS

The objective of this study was to compare substrate cycling in lean a nd obese Zucker rats under conditions where there was (i) a nominal ex cess or (ii) a deficit in energy intake. Protein turnover was assessed by measuring protein synthesis and breakdown using N-15 glycine as th e tracer. Glucose/glucose-6-phosphate cycling was calculated from the difference in the glucose appearance rates measured using 2-D- and 6,6 -D-2-labeled glucose. Information on triglyceride/fatty acid cycling w as obtained by measuring free fatty acid kinetics using 1-C-13 palmita te and glycerol kinetics using 5-D glycerol. Neither protein synthesis or breakdown or glucose cycling rates were significantly different be tween lean and obese Zucker rats. The relative rate of the triglycerid e/fatty acid cycle was higher in the obese rat as a consequence of inc reased adiposity. Increased intake increased glucose cycling and decre ased 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 severel y obese state.