Determinants and pharmacologic modulation of fasting substrate oxidation in humans

Abstract

Alterations in substrate oxidation are a potential contributor to the pathogenesis of metabolic diseases as they might foster ectopic lipid accumulation. Longitudinal studies identified predominant carbohydrate oxidation to predispose for subsequent weight gain. However, determinants of fasting substrate oxidation remain elusive and evidence for substrate oxidation as a potential pharmacologic target is lacking.

To investigate alterations in energy metabolism that can be involved in the pathogenesis of metabolic diseases, this research project aimed to elucidate determinants of fasting substrate oxidation. This thesis furthermore tested whether the latter can be pharmacologically modulated by the sodium glucose cotransporter 2 inhibitor (SGLT2i) empagliflozin to identify therapeutic approaches for altered fuel use. Moreover, implications of modulated fasting substrate oxidation on intrapancreatic fat and insulin secretion were evaluated with the aim to detect potential approaches for the prevention of type 2 diabetes (T2DM). These research questions were addressed in three publications embedded in this thesis.

In order to elucidate determining factors of substrate oxidation in the fasted state, a cross‑sectional analysis was performed, including 192 individuals with a wide range of BMI as well as different glycemic categories. Following the assessment of fasting respiratory quotient (RQ=VCO2/VO2) by indirect calorimetry as a measure of substrate oxidation, participants underwent a 5-point 75 g oral glucose tolerance test (OGTT). The latter allows to estimate insulin sensitivity and determine the glycemic status beside quantification of several clinical parameters from basal blood. In the fasting state, high free fatty acid (FFA) concentrations were strongly linked to a low RQ, indicative of predominant fat oxidation. Participants with high levels of the ketone body β‑hydroxybutyric acid had significantly lower RQ values, while glucose and insulin levels were not correlated to RQ. Unlike glucagon-like-peptide 1 (GLP-1), fasting levels of glucose‑dependent insulinotropic polypeptide (GIP) and glicentin associated positively with fasting RQ. There was neither a correlation between BMI nor the total amount or the allocation of body fat compartments with fasting RQ. Hyperglycemia, insulin sensitivity or the metabolic syndrome were not related to RQ.

In a double-blind, placebo-controlled randomized trial, 40 participants with prediabetes were treated with empagliflozin or placebo once daily for 8 weeks. Subsequent to overnight fasting, indirect calorimetry as well as a 75 g OGTT were performed before and after treatment. Body fat compartments including lipids in the liver as well as intrapancreatic fat were quantified using magnetic resonance imaging (MRI). A combination of intranasal insulin application with functional MRI was used to determine hypothalamic insulin sensitivity. Empagliflozin reduced fasting RQ, lowered fasting glucose as well as liver fat content and increased hypothalamic insulin sensitivity. Pancreatic fat content as well as insulin secretion remained unaffected upon empagliflozin treatment.

Data of this work support the role of FFA as independent determinants of fuel selection, while metabolic disorders were not linked to substrate preferences. This work gained hints for glicentin and GIP to be involved in fuel choice in the fasting state, representing a promising pharmaceutic target to modulate substrate oxidation with possible implications on whole-body metabolism. Upcoming therapeutic approaches in the preclinical as well as clinical phase targeting GIP receptor highlight the relevance of our results.

Beside the empagliflozin-induced switch towards predominant fat use, intrahepatic lipids were diminished upon SGLT2 inhibition. This demonstrates the potential of pharmacologic modulation of substrate utilization. Since we detected such changes already in prediabetes, substrate oxidation is an interesting target for the development of preventive strategies of metabolic diseases as T2DM. Discussed underlying mechanisms for the empagliflozin-induced change in substrate oxidation are a restoration of diurnal rhythm by caloric restriction, centrally mediated effects and direct cellular effects of SGLT2i interfering with lipid metabolism. Beside SGLT2i, further antidiabetic drugs such as GLP-1 receptor agonists and metformin may have the potential to modulate substrate oxidation. Therefore, approaches to modulate substrate oxidation should be further evaluated and optimized in future studies. Since the observed increase in fat oxidation was neither accompanied by reductions in pancreatic fat nor by enhanced insulin secretion, this work argues for tissue-specific regulation in substrate oxidation and fat mobilization which should be untangled in future studies. It is however possible that prolongation of treatment duration or a population with higher pancreatic fat content might have led to different results. Since lifestyle intervention trials achieved pancreatic fat reduction, approaches to reduce fat in the pancreas remain a promising strategy to improve insulin secretion for the prevention or therapy of T2DM. Future research should put a spotlight on tailored medicine and find out which sub-groups of persons with prediabetes and T2DM could especially profit from such therapies that target substrate oxidation and reduce ectopic fat deposition.