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.
Keywords: Gluconeogenesis; Glycerol; Pentose phosphate pathway; Tricarboxylic acid cycle; Visceral fat.
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