Impact of L-Carnitine infusion on Lipid induced Insulin resistance

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.

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, secondary objectives are to examine the molecular
pathways of carnitine and acetylcarnitine, responsible for muscle insulin
sensitivity and to investigate the effect of L-carnitine infusion on insulin
signalling pathways in skeletal muscle

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.

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 two weeks. 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.

Subjects will first visit the University once for screening purposes during
which length, weight and blood pressure will be measured. An ECG will be
performed, blood will be drawn and they will fill in 2 questionnaires. If
screening was successfully completed, subjects will visit the university in the
morning (fasted) for a Bod Pod measurement (body composition measurement) and a
maximal cycling test (visit 2: 1 hour). During the third visit subject will
come fasted (for 5 hours, from lunch onwards) to the university in the
afternoon (5PM) for a 30 minute cycling test with a magnetic resonance
spectroscopy (MRS) measurement immediately before and after cycling to
determine exercise-induced acetylcarnitine concentration. Subsequently,
subjects visit the University for the three intervention trials (LIPID+CAR,
LIPID+PLAC, Saline+PLAC). These three days will consist of undergoing a
hyperinsulinemic euglycemic clamp combined with either lipid, carnitine or
saline (11 hours). Furthermore, at the beginning and end of the clamps, a
muscle biopsy will be obtained. For these visits, subjects have to report to
the university in the morning in the fasted state. Three days in advance,
subjects fill out a food diary to monitor their food intake. The evenings prior
to these three test days, subjects have to eat a standardize meal (macaroni
bolognaise). Muscle biopsies lead to mild discomfort and there is a risk of
hematoma. During the hyperinsulinemic euglycemic clamp, a risk of hypoglycaemia
exists. In summary, we will draw approximately 538 ml blood during the entire
study period. During each of the three intervention trial we draw a maximum of
176 ml blood.