Glycerol production and utilization measured using stable isotopes

The rate of appearance of glycerol in the systemic circulation is determine d from the enrichment of arterial blood glycerol when labelled glycerol is infused intravenously. This value provides a good measure of whole-body lip olysis during fasting, except that arterial infusion and venous sampling, i f feasible, would probably give a higher more-accurate value. Lipolysis occ urs primarily in adipose tissue, although other tissues contribute, notably muscle. Measurement is based on the difference in the enrichment of the gl ycerol entering and leaving the tissue. Lipolysis is underestimated by the extent to which glycerol released by lipolysis does not enter the systemic circulation, as occurs when lipolysis takes place in the non-hepatic tissue of the splanchnic bed. Glycerol released into the systemic circulation is utilized mainly by liver, although kidney and muscle are also major users o f glycerol. Measurement of glycerol utilization is based on the amount of l abelled glycerol taken up by the tissues. Other tissues probably utilize gl ycerol to a smaller extent, but in total this represents a significant amou nt. Most glycerol taken up by liver is converted to glucose. Glucose is pro bably the major source of glycerol-3-phosphate used in the esterification o f fatty acids by adipose tissue.