Estimating gluconeogenesis by NMR isotopomer distribution analysis of [13C]bicarbonate and [1-13C]lactate

Effects of visceral adiposity on glycerol pathways in gluconeogenesis

OBJECTIVE

To determine the feasibility of using oral ¹³C labeled glycerol to assess effects of visceral adiposity on gluconeogenic pathways in obese humans.

RESEARCH DESIGN AND METHODS

Obese (BMI ≥30kg/m²) participants without type 2 diabetes underwent visceral adipose tissue (VAT) assessment and stratification by median VAT into high VAT-fasting (n=3), low VAT-fasting (n=4), and high VAT-refed (n=2) groups. Participants ingested [U-¹³C₃] glycerol and blood samples were subsequently analyzed at multiple time points over 3h by NMR spectroscopy. The fractions of plasma glucose (enrichment) derived from [U-¹³C₃] glycerol via hepatic gluconeogenesis, pentose phosphate pathway (PPP), and tricarboxylic acid (TCA) cycle were assessed using ¹³C NMR analysis of glucose. Mixed linear models were used to compare ¹³C enrichment in glucose between groups.

RESULTS

Mean age, BMI, and baseline glucose were 49years, 40.1kg/m², and 98mg/dl, respectively. Up to 20% of glycerol was metabolized in the TCA cycle prior to gluconeogenesis and PPP activity was minor (<1% of total glucose) in all participants. There was a 21% decrease in ¹³C enrichment in plasma glucose in the high VAT-fasting compared with low VAT-fasting group (p=0.03), suggesting dilution by endogenous glycerol. High VAT-refed participants had 37% less ¹³C enrichment in glucose compared with high VAT-fasting (p=0.02). There was a trend toward lower [1,2-¹³C₂] (via PPP) and [5,6-¹³C₂]/[4,5,6-¹³C₃] (via TCA cycle) glucose in high VAT versus low VAT groups.

CONCLUSIONS

We applied a simple method to detect gluconeogenesis from glycerol in obese humans. Our findings provide preliminary evidence that excess visceral fat disrupts multiple pathways in hepatic gluconeogenesis from glycerol.

A simplified mass isotopomer approach to estimate gluconeogenesis rate in vivo using deuterium oxide

We compare a new simplified (2)H enrichment mass isotopomer analysis (MIA) against the laborious hexamethylentetramine (HMT) method to quantify the contribution of gluconeogenesis (GNG) to total glucose production (GP) in calves. Both methods are based on the (2)H labeling of glucose after in vivo administration of deuterium oxide. The (2)H enrichments of plasma glucose at different C-H positions were measured as aldonitrile pentaacetate (AAc) and methyloxime-trimethylsilyl (MoxTMS) derivatives or HMT by gas chromatography/mass spectrometry (GC/MS). Two pre-ruminating fasted Holstein calves (51 kg body mass, BM, age 7 days) received two oral bolus doses of (2)H(2)O (10 g/kg BM, 70 atom% (2)H) at 7:00 h and 11:00 h after overnight food withdrawal. Blood samples for fractional GNG determination were collected at -24 and between 6 and 9 h after the first (2)H(2)O dose. The ratio of (2)H enrichments C5/C2 represents the contribution of GNG to GP. The (2)H enrichment at C2 was calculated based on the ion fragments at m/z 328 (C1-C6) - m/z 187 (C3-C6) of glucose AAc. The (2)H enrichment at C5 was approximated either by averaging the (2)H enrichment at C5-C6 using the ion fragment of glucose MoxTMS at m/z 205 or by conversion of the C5 of glucose into HMT. The fractional GNG calculated by the C5-C6 average (2)H enrichment method (41.4 +/- 6.9%) compared to the HMT method (34.3 +/- 11.4%) was not different (mean +/- SD, n = 6 replicates). In conclusion, GNG can be estimated with less laborious sample preparation by means of our new C5-C6 average (2)H enrichment method using AAc and MoxTMS glucose derivatives.