Sources of blood glycerol during fasting

1 Endocrine Research Unit, Mayo Clinic, Rochester, Minnesota 55905; 2 Departments of Medicine, Biochemistry and Nutrition, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106; and 3 Division of Clinical Physiology, Karolinska Hospital, S-171 76 Stockholm, Sweden To determine the source(s) of blood and very low density lipoprotein (VLDL)-triglyceride glycerol during fasting, four men ingested 2 H 2 O from 14 to 20 h into a 60-h fast to achieve ~0.5% body water enrichment. At 60 h of fasting, glycerol flux was measured using [2- 14 C]glycerol. Blood was taken for measurement of 2 H enrichment at carbon 6 of glucose and at carbon 3 of free glycerol and VLDL-triglyceride glycerol. 2 H enrichment of the 2 hydrogens bound to carbon 3 of VLDL-triglyceride glycerol was 105 ± 2% of the 2 H enrichment of the 2 hydrogens bound to carbon 6 of glucose, indicating isotopic equilibrium between hepatic glyceraldehyde 3- P and glycerol 3- P . The 2 H enrichment of the 2 hydrogens bound to carbon 3 of free glycerol was 17 ± 3% of VLDL-triglyceride glycerol, indicating that a significant percentage of free glycerol in blood originated from the hydrolysis of circulating VLDL-triglyceride or a pool of glycerol with similar 2 H enrichment. Glycerol flux was 6.3 ± 1.1 µmol · kg 1 · min 1 . Glycerol appearing from nonadipose tissue sources was then ~1.1 µmol · kg 1 · min 1 . Seven other subjects were fasted for 12, 42, and 60 h. A small percentage of glycerol in the circulation after 12 h of fasting was enriched with 2 H. The enrichment of the 2 hydrogens bound to carbon 3 of free glycerol in the longer periods of fasting was ~16% of the enrichment of the 2 hydrogens bound to carbon 6 of glucose. Therefore, as much as 15-20% of systemic glycerol turnover during fasting is not from lipolysis of adipose tissue triglyceride. triglyceride; very low density lipoprotein; lipolysis; deuterated water