Impact of time and feeding habits on lipid levels in Zucker obese rats.

The influences of time and hyperphagia on cholesterol, triglyceride, glucose and insulin levels were compared in the obese Zucker rat and compared to its lean litter-mates. Following a 28 day acclimation period in a 12 hr light/dark cycle (08-20-08) animal facility, blood samples were obtained every 2 hr in both obese and lean rats over a 24 hr period (N = 48; Dec 1988); serum was measured enzymatically for cholesterol, triglyceride and glucose and by radioimmunoassay for insulin and cortisol levels. Synchronization with other animal studies was established by endogenous serum cortisol measurements (acrophase 18-20 HALO in both groups). Cholesterol, triglyceride, insulin and glucose concentrations were significantly greater per time interval in obese vs. lean rats. No circadian pattern was observed in glucose concentrations in either rat group. Insulin levels peaked in both rat groups during the dark cycle; however, glucose and insulin levels were not correlated. Cholesterol concentrations were unchanged over time in obese as well as lean rats. Although triglyceride levels showed an acrophase at 13 HALO in lean rats, no circadian pattern was found in obese rats. Triglyceride levels remained elevated throughout the 24 hour period in obese rats whereas significant increases were observed in lean rats during the dark cycle. The present results suggest that triglyceride levels, and not insulin and cholesterol levels, are most likely dependent on feeding and activity patterns.