Carnitine Infusion and Insulin Resistance

Overview

Insulin resistant subjects and type 2 diabetic patients are characterized by a decreased metabolic flexibility: a reduced capability to switch from fat oxidation in the basal state to carbohydrate oxidation in the insulin-stimulated state. This metabolic inflexibility is an early hallmark in the development of diabetes. Recent evidence suggests that a low carnitine availability may limit acetylcarnitine formation, thereby reducing metabolic flexibility. Thus, when substrate flux in the muscle is high, acetyl-CoA concentrations increase, leading to inhibition of pyruvate dehydrogenase (PDH) and thereby reducing glucose oxidation. The conversion of acetyl-CoA to acetylcarnitine relieves this acetyl-CoA pressure on PDH. To provide more direct insight into the effect of carnitine in preventing metabolic inflexibility and insulin resistance and to further explore the mechanism of action is the focus of this research. Here, we hypothesize that the capacity to form acetylcarnitine may rescue lipid-induced insulin resistance. To this end, insulin resistance will be induced by lipid infusion in healthy volunteers and it will be tested whether carnitine co-infusion can alleviate insulin resistance.

Rationale: Insulin resistant subjects and type 2 diabetic patients are characterized by a decreased metabolic flexibility: a reduced capability to switch from fat oxidation in the basal state to carbohydrate oxidation in the insulin-stimulated state. This metabolic inflexibility is an early hallmark in the development of diabetes. Recent evidence suggests that a low carnitine availability may limit acetylcarnitine formation, thereby reducing metabolic flexibility. Thus, when substrate flux in the muscle is high, acetyl-CoA concentrations increase, leading to inhibition of pyruvate dehydrogenase (PDH) and thereby reducing glucose oxidation. The conversion of acetyl-CoA to acetylcarnitine relieves this acetyl-CoA pressure on PDH. To provide more direct insight into the effect of carnitine in preventing metabolic inflexibility and insulin resistance and to further explore the mechanism of action is the focus of this research. Here, we hypothesize that the capacity to form acetylcarnitine may rescue lipid-induced insulin resistance. To this end, insulin resistance will be induced by lipid infusion in healthy volunteers and it will be tested whether carnitine co-infusion can alleviate insulin resistance. Objective: The primary objectives are to investigate whether L-carnitine infusion may rescue lipid-induced insulin resistance and whether L-carnitine infusion is improving metabolic flexibility in the state of lipid-induced insulin resistance. Furthermore, a secondary objective is to examine the molecular pathways of carnitine and acetylcarnitine, responsible for muscle insulin sensitivity. Study design: The current study is an interventional randomized crossover trial in which each subject serves as it owns control. Subjects will be blinded for the intervention. Study population: n=10, healthy young (18-40 years) male subjects will be included. Intervention (if applicable): Ten healthy subject will be subjected to the intervention of L-carnitine infusion. To investigate whether L-Carnitine infusion may rescue lipid induced insulin resistance and improve metabolic flexibility three intervention trials are included. The first trial includes lipid infusion combined with L-Carnitine infusion (=LIPID + CAR). In the second trial, L-carnitine infusion will be replaced by placebo infusion in the form of saline (= LIPID + PLAC) in order to investigate the effect of L-Carnitine. During the third trial, lipid infusion will be replaced by infusion of saline and will serve as a control for the lipid infusion (=SALINE + PLAC) and is necessary to investigate to what extend L-carnitine can rescue lipid induced insulin resistance. All three trials will be separated by at least one week. Subjects will be blinded, so no information about the infused substances will be provided to them. The three different trials will be allocated in a random order. Main study parameters/endpoints: The primary study endpoint is whole body insulin sensitivity, measured by the hyperinsulinemic-euglycemic clamp. Secondary endpoints are maximal acetylcarnitine concentrations after exercise, metabolic compounds in the blood and measurements regarding skeletal muscle metabolism in skeletal muscle tissue obtained by needle biopsies.