Visceral Adiposity and Diabetes: Translating Form to Function Using Imaging

Diabetes mellitus type II is the consequence of insulin resistance and pancreatic beta cell failure resulting from a variety of metabolic insults, one of which is excess body adiposity/obesity. In the diabetic individual, hepatic gluconeogenesis may go uninhibited due to failure of the body's normal feedback mechanisms to appropriately incorporate glucose into cells via insulin signaling, leading to excess gluconeogenesis and hyperglycemia. The substrate for this excess glucose derives from multiple sources in the liver including dietary glycerol, adipose-derived glycerol from lipolysis, and substrates from the citric acid cycle. In the normal state, lipolysis is maintained at a steady state in equilibrium between stored dietary triglycerides and free fatty acids. However, in situations of triglyceride excess (e.g. in the obese state), lipolysis may become overactive resulting in increased free fatty acids and adipose-derived glycerol. This excess glycerol drives hepatic gluconeogenesis and is incorporated into glucose and released into the blood, leading to hyperglycemia, and ultimately diabetes and its clinical sequelae.

A popular hypothesis linking visceral fat with excess gluconeogenesis is delivery of glycerol arising from mesenteric triglyceride turnover directly into the portal circulation and to the liver. Glycerol is a primary substrate for gluconeogenesis in the liver. Under normal conditions, hepatic gluconeogenesis begins from glycerol ingested in the diet which is converted to glycerol-3-phosphate and subsequently dihydroxyacetone phosphate (DHAP) in the liver. DHAP is converted to fructose-1,6-bisphosphate which undergoes a series of reactions to become a single 6-carbon glucose molecule. Adipocytes contribute glycerol to hepatic gluconeogenesis through lipolysis of triglyceride stores. Although glycerol-gluconeogenesis has been extensively studied in animals, the traditional reliance on radioactive tracers makes translation to humans difficult for many reasons. We aim to use new techniques to explore the mechanisms behind altered glucose metabolism related to excess visceral adiposity in obese adults by quantifying the relative contributions of varying substrates to liver-derived glucose. One such method uses 13C3 labeled glycerol to trace the incorporation of glycerol from dietary sources to hepatic gluconeogenesis. This technology utilizes nuclear magnetic resonance (NMR) spectroscopy, a technique that does not require ionizing radiation and has been extensively validated, to analyze the NMR spectra of plasma glucose and quantify the "percent enrichment" of the circulating glucose molecules with labeled glycerol. In turn, differences in enrichment reflect variability in hepatic glucose metabolism as it relates to the contribution of glycerol from visceral adipose tissue to gluconeogenesis.

The rationale of this project is to utilize existing technology to investigate the impact of excess visceral adiposity on glycerol metabolism in hepatic gluconeogenesis in obese adults without diabetes and to explore the effects of treatment with EMPA on visceral adiposity related glucose homeostasis.