Quantifying gluconeogenesis during fasting

Abstract

The use of2H2O in estimating gluconeogenesis’ contribution to glucose production (%GNG) was examined during progressive fasting in three groups of healthy subjects. One group ( n = 3) ingested2H2O to a body water enrichment of ≈0.35% 5 h into the fast. %GNG was determined at 2-h intervals from the ratio of the enrichments of the hydrogens at C-5 and C-2 of blood glucose, assayed in hexamethylenetetramine. %GNG increased from 40 ± 8% at 10 h to 93 ± 6% at 42 h. Another group ingested2H2O over 2.25 h, beginning at 11 h ( n = 7) and 19 h ( n = 7) to achieve ≈0.5% water enrichment. Enrichment in plasma water and at C-2 reached steady state ≈1 h after completion of2H2O ingestion. The C-5-to-C-2 ratio reached steady state by the completion of 2H2O ingestion. %GNG was 54 ± 2% at 14 h and 64 ± 2% at 22 h. A 3-h [6,6-2H2]glucose infusion was also begun to estimate glucose production from enrichments at C-6, again in hexamethylenetetramine. Glucose produced by gluconeogenesis was 0.99 ± 0.06 mg ⋅ kg−1 ⋅ min−1at both 14 and 22 h. In a third group ( n = 3) %GNG reached steady state ≈2 h after2H2O ingestion to only ≈0.25% enrichment. In conclusion, %GNG by 2 h after2H2O ingestion and glucose production using [6,6-2H2]glucose infusion, begun together, can be determined from hydrogen enrichments at blood glucose C-2, C-5, and C-6. %GNG increases gradually from the postabsorptive state to 42 h of fasting, without apparent change in the quantity of glucose produced by gluconeogenesis at 14 and 22 h.

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.…”

“…To measure glucose production, an infusion of [6,6-2 H 2 ]glucose with a primer is given together with the 2 H 2 O (Chandramouli et al 1997). A portion of the blood glucose, when steady-state is reached (usually by 3 h), is oxidized to yield C-6 with its two H in formaldehyde and hence in HMT.…”

“…The major limitation of the method is the long half-life of 2 H 2 O in body water, which prohibits a repeat study in the same subject for many weeks (Chandramouli et al 1997). Since the contribution of GNG increases with fasting by about 1·5 %/h (Chandramouli et al 1997), values based on analysis of a blood glucose sample are underestimated by a few percentage units. The reason for this underestimation is that the glucose in the blood is formed not only at the time of sampling, but also in the preceding period, although in smaller amounts.…”

“…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). There appears to be a small increase above normal in the contribution of GNG in subjects with non-insulindependent diabetes mellitus (Wajmgot et al 1999).…”

“…The evidence is based first on an estimate of glycogen synthesis, assessed using [1-2 H]galactose and glucuronide formation after long-term (60-70 h) fasting. For their estimate they presuppose that the reaction glucose-1-phosphate + uridine triphosphate → uridine diphosphate glucose + pyrophosphate has been established biochemically to be irreversible, when biochemical evidence is to the contrary (Guynn et al 1974;Newsholme & Leech, 1985;Davidson et al 1988;Chandramouli et al 1997). They also conclude that the hepatic glycogen content after 60-70 h fasting is sufficient for such levels of glycogen cycling to occur, despite the extent to which the last glycogen deposited is the first released.…”