Extending food deprivation reverses the short-term lipolytic response to

摘要

The effects of short-term food deprivation on lipid metabolism are well documented, but little is known about prolonged fasting. This study monitored the kinetics of glycerol (rate of appearance, (R)over dot(a) glycerol) and non-esterified fatty acids ((R)over dot(a) NEFA) in fasting rabbits. Our goals were to determine whether lipolysis is stimulated beyond values seen for short-term fasting, and to characterize the roles of primary (intracellular) and secondary (with transit through the circulation) triacylglycerol/fatty acid cycling (TAG/FA cycling) in regulating fatty acid allocation to oxidation or re-esterification. (R)over dot(a) glycerol (9.62+/-0.72 to 15.29+/-0.96 mu mol kg(-1) min(-1)) and (R)over dot(a) NEFA (18.05+/-2.55 to 31.25+/-1.93 mu mol kg(-1) min(-1)) were stimulated during the first 2 days of fasting, but returned to baseline after 4. days. An initial increase in TAG/FA cycling was followed by a reduction below baseline after 6. days without food, with primary and secondary cycling contributing to these responses. We conclude that the classic activation of lipolysis caused by short-term fasting is abolished when food deprivation is prolonged. High rates of re-esterification may become impossible to sustain, and TAG/FA cycling could decrease to reduce its cost to 3% of total energy expenditure. Throughout prolonged fasting, fatty acid metabolism gradually shifts towards increased oxidation and reduced re-esterification. Survival is achieved by pressing fuel selection towards the fatty acid dominance of energy metabolism and by slowing substrate cycles to assist metabolic suppression. However, TAG/FA cycling remains active even after prolonged fasting, suggesting that re-esterification is a crucial mechanism that cannot be stopped without harmful consequences.