Fatty acid cycling in the fasting rat

Adipose tissue lipolysis and fatty acid reesterification by liver and adipo se tissue were investigated in rats fasted for 15 h under basal and calorig enic conditions. The fatty acid flux initiated by adipose fat lipolysis in the fasted rat is mostly futile and is characterized by reesterification of 57% of lipolyzed free fatty acid (FFA) back into adipose triglycerides (TG ). About two-thirds of FFA reesterification are carried out before FFA rele ase into plasma, whereas the rest consists of plasma FFA extracted by adipo se tissue. Thirty-six percent of the fasting lipolytic flux is accounted fo r by oxidation of plasma FFA, whereas only a minor fraction is channeled in to hepatic very low density lipoprotein-triglycerides (VLDLTG). Total body calorigenesis induced by thyroid hormone treatment and liver-specific calor igenesis induced by treatment with beta,beta'-tetramethylhexadecanedioic ac id (Medica 16) are characterized by a 1.7- and 1.3-fold increase in FFA oxi dation, respectively, maintained by a 1.5-fold increase in adipose fat lipo lysis. Hepatic reesterification of plasma FFA into VLDL-TG is negligible un der both calorigenic conditions. Hence, total body fatty acid metabolism is regulated by adipose tissue as both source and sink. The futile nature of fatty acid cycling allows for its fine tuning in response to metabolic dema nds.