TRAFFICKING OF DIETARY-FAT IN LEAN RATS

Despite increasing interest in the sole that fuel partitioning plays i n determining body composition, the relative importance of oxidative v ersus storage pathways in the clearance of dietary fat remains unclear . A widely held view is that the primary destination of chylomicron tr iglyceride fatty acids (TGFA) is adipose tissue, and the primary sourc e of lipid fuel for skeletal muscle is non-esterified fatty acids (NEF A). An alternate view is that muscle, not adipose tissue, is the prima ry site of TGFA clearance. This view is supported by estimates of the total lipoprotein lipase content of muscle and adipose tissue. To dire ctly study the partitioning of dietary fat between oxidation and stora ge, C-14-labeled oleic acid was fed to Sprague Dawley rats and its met abolic fate followed over 30 days. Two hours after ingestion, more tha n 3.5 times as much label was found in skeletal muscle tissue (2.42 +/ - 0.45 nmols) and CO2 (0.25 +/- 0.01 nmols) than was found in adipose tissue (0.71 +/- 0.14 nmols). Intramuscular triglyceride was the lipid class most extensively labeled. After skeletal muscle, liver was the next most important site of TGFA clearance. Surprisingly a substantial quantity of label remained associated with the GI tract even 24 hours after ingestion. Between 2 and 10 days following ingestion there was a net decline in the C-14 content of muscle, liver and GI tract, assoc iated with a net rise in the C-14 content of adipose tissue. These fin dings demonstrate: 1) the importance of skeletal muscle and liver in w hole organism TGFA clearance, 2) the importance of intramuscular parti tioning of lipid fuels between direct oxidation and storage as TG, 3) the potentially important role of the GI tract in the delivery of diet ary fat to the circulation 10-24 hours following ingestion, and 4) the stability of adipose tissue as a storage site. The complex nature of the tissue-specific clearance of TGFA over time is perhaps better desc ribed by the term ''trafficking' than by the more commonly used term ' 'partitioning.'' Future studies of TGFA clearance combined with sampli ng of relevant tissues over time will provide insight into the specifi c roles that abnormalities in liver, muscle and adipose tissue TGFA me tabolism play in the development of hypertriglyceridemic disorders and states of increased or reduced body weight.