DIETARY-FAT IS SHUNTED AWAY FROM OXIDATION, TOWARD STORAGE IN OBESE ZUCKER RATS

Previous measurements of lipoprotein lipase (LPL) activity in adipose tissue (ATLPL) of lean and obese Zucker rats have consistently documen ted increased activity in obese rats relative to lean. Since LPL is co nsidered to be rate limiting for the delivery of triglyceride fatty ac ids (TGFA) to muscle and adipose tissue, these data have been used to suggest that the metabolic partitioning of TGFA favors storage over ox idation in obese rats. To document the partitioning of TGFA directly, the fate of C-14 labeled oleic acid (42nmols) was fed to lean, obese, and obese Zucker rats fed a hypocaloric diet designed to chronically r educe weight 25% below that of obese controls (reduced-obese). The amo unt of C-14 recovered in CO2 over 6 hours following ingestion was sign ificantly less in obese rats compared to lean (0.45 +/- 0.06 vs. 0.88 +/- 0.09nmols, p=.0004) and less still in the reduced obese group (0.3 4 +/- 0.06nmols p=.00003). Six hours after ingestion, the quantity of label found in adipose tissue was significantly greater in the obese r ats compared to lean (14.51 +/- 1.92 vs. 1.38 +/- 0.29nmols p<.00001), but was intermediate in the reduced-obese group (9.23 +/- 0.98nmols p =.0003). At 2.2 hours there was significantly more label in skeletal m uscle of lean rats compared to either obese or reduced-obese (2.33 +/- 0.24; 1.35 +/- 0.04nmols p=.01; 1.41 +/- 0.27nm p=.02). However, at 6 hours these differences between groups were no longer present. These findings Indicate that dietary fat is shunted away from oxidation towa rd storage in obese Zucker rats. Additionally it appears that there ma y be a relative block in the oxidation of TGFA that is taken up by ske letal muscle in obese rats. Finally the relative normalization of this partitioning defect in reduced-obese rats is at variance with what wa s suggested by previous measurements of tissue specific levels of LPL, and suggests an enhanced recirculation of fatty acids from adipose ti ssue to muscle in reduced-obese rats. This could occur through increas ed delivery of non-esterified fatty acids (NEFA) to muscle as a result of an increase in net lipolysis.