Contributions of gluconeogenesis to glucose production in the fasted state.

Abstract

Healthy subjects ingested 2 H 2 O and after 14, 22, and 42 h of fasting the enrichments of deuterium in the hydrogens bound to carbons 2, 5, and 6 of blood glucose and in body water were determined. The hydrogens bound to the carbons were isolated in formaldehyde which was converted to hexamethylenetetramine for assay. Enrichment of the deuterium bound to carbon 5 of glucose to that in water or to carbon 2 directly equals the fraction of glucose formed by gluconeogenesis. The contribution of gluconeogenesis to glucose production was 47 Ϯ 4% after 14 h, 67 Ϯ 4% after 22 h, and 93 Ϯ 2% after 42 h of fasting. Glycerol's conversion to glucose is included in estimates using the enrichment at carbon 5, but not carbon 6. Equilibrations with water of the hydrogens bound to carbon 3 of pyruvate that become those bound to carbon 6 of glucose and of the hydrogen at carbon 2 of glucose produced via glycogenolysis are estimated from the enrichments to be ‫ف‬ 80% complete. Thus, rates of gluconeogenesis can be determined without corrections required in other tracer methodologies. After an overnight fast gluconeogenesis accounts for ‫ف‬ 50% and after 42 h of fasting for almost all of glucose production in healthy subjects. ( J. Clin. Invest. 1996. 98:378-385.)

Quantifying the contribution of gluconeogenesis to glucose production in fasted human subjects using stable isotopes

“…During a fast, 2 H 2 O is ingested to reach a body water enrichment of 0·25-0·5 % (Landau et al 1995c(Landau et al , 1996Chandramouli et al 1997;Landau, 1997). Ingestion of a single dose to achieve 0·5 % enrichment can cause vertigo, therefore the dose is given over 2-4 h. When the enrichment at C-5 in blood glucose becomes constant, within a few hours after normal subjects ingest 2 H 2 O, i.e.…”

“…Enrichment at C-5 of a portion of blood glucose is determined chemically by converting the glucose to xylose with the removal of C-6 (Landau et al 1996). The xylose is then oxidized with periodate to yield formaldehyde containing C-5 with its H. Condensation of six molecules of the formaldehyde with four molecules of NH 3 yields one molecule of hexamethylenetetramine (HMT).…”

“…Direct release of glucose from hydrolysis of 1,6-glucosyl branches in glycogen would also result in an overestimate of GNG. This overestimation could amount to about 4 % in the subject fasted overnight, but could be reduced by glucose cycling, which labels the H at C-2 of the glucose (Landau et al 1996). The finding that 2 H enrichment at C-2 is approximately equal to 2 H 2 O indicates that the contribution from hydrolysis is very small.…”

“…Using the heavy-water method, the contribution of GNG to glucose production has been estimated at 47 % after 14 h, 67 % after 22 h and 92 % after 42 h of fasting by normal subjects (Landau et al 1996). Between 14 and 22 h, while the contribution of GNG increased, glucose production declined, so that the contribution of GNG remained unchanged (Chandramouli et al 1997).…”

Changes in hepatic glycogen cycling during a glucose load in healthy humans

“…Glycogen cycling has limited the interpretation of estimates of gluconeogenesis [2,8], since its contribution cannot be excluded by current methods, e.g. using 2 H 2 O [9,10] and magnetic resonance spectroscopy (MRS) [11]. Fluxes through the hepatic gluconeogenic pathway could also proceed directly to glycogen rather than into plasma glucose [12,13].…”