Quantifying gluconeogenesis during fasting

The use of (H2O)-H-2 in estimating gluconeogenesis' contribution to glucose production (%GNG) was examined during progressive fasting in three groups of healthy subjects. One group (n = 3) ingested (H2O)-H-2 to a body water enrichment of approximate to 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 ingested (H2O)-H-2 over 2.25 h, beginning at 11 h (n = 7) and 19 h (n = 7) to achieve approximate to 0.5% water enrichment. Enrichment in plasma water and at C-2 reached steady state approximate to 1 h after completion of (H2O)-H-2 ingestion. The C-5-to-C-2 ratio reached steady state by the completion of (H2O)-H-2 ingestion. %GNG was 54 +/- 2% at 14 h and 64 +/- 2% at 22 h. A 3-h [6,6-H-2(2)]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(-1) at both 14 and 22 h. In a third group (n = 3) %GNG reached steady state approximate to 2 h after (H2O)-H-2 ingestion to only approximate to 0.25% enrichment.:[n conclusion, %GNG by 2 h after (H2O)-H-2 ingestion and glucose production using [6,6-H-2(2)]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.