Aging delays the suppression of lipolysis in the postprandial period

Plasma glycerol and free fatty acid concentrations decrease following oral glucose consumption, but changes in the rate of lipolysis during an oral glucose tolerance test (OGTT) have not been documented in conjunction with changes in fatty acid (FA) oxidation or reesterification rates in healthy individuals. After a 12-hr overnight fast, 15 young (21-35 yr; 7 men and 8 women) and 14 older (60-80 yr; 7 men and 7 women) participants had the forearm vein catheterized for primed continuous infusion of 1,1,2,3,3- 2 Hglycerol. A contralateral hand vein was catheterized for arterialized blood sampling. Indirect calorimetry was performed simultaneously to determine total FA and carbohydrate (CHO) oxidation rates (Rox). Total FA reesterification rates (Rs) were estimated from tracer-measured lipolytic and FA oxidation rates. After a 90-min equilibration period, participants underwent a 120-min, 75-g OGTT. Glycerol rate of appearance (Ra), an index of lipolysis, decreased significantly from baseline 5 min post-challenge in young participants and 30 min in older participants. At 60 min, FA Rox decreased in both groups, but was significantly higher in older participants. Between 5-90 min, CHO Rox was significantly lower in older participants. Additionally, FA Rs was significantly lower in older participants at 60 and 90 min. The AUC for FA Rox was greater than that for FA Rs in older, but not young participants. Our results indicate that, in aging, the postprandial suppression of lipolysis and FA oxidation are delayed such that FA oxidation is favored over CHO oxidation and FA reesterification.