The effect of lowered physical activity (induced by unilateral lower limb suspension (ULLS)) on insulin sensitivity and lipid and glucose metabolism

In the Netherlands and worldwide, the number of individuals suffering from
obesity and type 2 diabetes mellitus is rising steadily. It is well established
that obesity predisposes individuals to accumulation of excessive fat in
non-adipose tissues such as the liver, the heart and skeletal muscle (called
steatosis or ectopic fat accumulation). Furthermore, in sedentary humans
ectopic fat accumulation in skeletal muscl is strongly associated with insulin
resistance. However, paradoxically, IntraMyoCellular Lipid (IMCL) content is
also increased in highly insulin sensitive endurance trained subjects (known as
the athlete*s paradox). This is suggesting that IMCL per se is not causative in
skeletal muscle insulin resistance. The increased IMCL storage following
endurance training serves to match training-induced increase in oxidative
capacity and reliance on fat as a substrate during exercise, whereas in
obesogenic/diabetogenic conditions the high fat availability is not matched by
improved oxidative capacity. It is therefore speculated that under the latter
conditions, the lipid intermediates of IMCL metabolism such as diacylglycerol
(DAG), ceramides and fatty acyl-CoAs will accumulate and impede cellular
insulin signalling. The rate of oxidative capacity is regulated by
mitochondria, which are cellular organelles responsible for cellular energy
production and cellular metabolism. Therefore, the overall hypothesis of this
project is that a low muscle mitochondrial oxidative capacity can lead to
muscle fat accumulation and/or accumulation of lipid intermediates when fatty
acid availability is high, and this may result in insulin resistance in
skeletal muscle.

To investigate the effects of lowered physical activity (resulting in decreased
muscle mitochondrial oxidative capacity) together with increased plasma free
fatty acid availability (by infusion of a clinically widely used lipid emulsion
(Intralipid)) on insulin sensitivity and glucose and lipid metabolism. Further,
changes in IMCL and fatty acid intermediates will be determined. To this end,
we will compare skeletal muscle insulin sensitivity and glucose and lipid
metabolism (within one subject) in an immobilized leg (decreased muscle
mitochondrial oxidative capacity) versus a control leg (unchanged muscle
mitochondrial oxidative capacity).
Additionally, the effects of lowered physical activity (resulting in decreased
muscle mitochondrial oxidative capacity) alone (without increased availability
of plasma free fatty acids) on skeletal muscle insulin sensitivity and glucose
and lipid metabolism will be investigated.

15 lean healthy subjects will undergo 12 days of unilateral lower limb
suspension (ULLS). Before the onset of the study subjects* body composition
(DEXA), VO2 max, muscle function and leg volume (by MRI) will be assessed.
During and after the intervention in vivo and ex vivo mitochondrial function,
insulin sensitivity, lipid/glucose metabolism, IMCL and lipid intermediates
will be investigated in an immobilized leg versus a control leg (within one
subject).

This is an intervention study, where each subject will undergo 12 days of
unilateral lower limb suspension. Randomly, the right or left leg of the
subject will be suspended by attachment of a sling to a non-rigid ankle brace
and to a harness on the upper body and unloaded from all weight bearing. The
knee will be slightly flexed at an angle of 130°. Hip, knee and ankle will be
fully mobile. The harness will be used during all locomotory activity, and the
subjects will use crutches for walking.

This study will provide valuable insight on the effects of physical inactivity
on intramyocellular lipid content, mitochondrial function and insulin
sensitivity and the relationship between them. The study carries minor to no
risks for the participants. Taking blood samples and skeletal muscle tissue
biopsies is representing a minor burden for subjects. This burden will be kept
as small as possible, and an experienced medical doctor will perform the
biopsies. During the 12 days of ULLS subjects will have to use crutches for
daily activity and movement of the suspended leg will be limited. Furthermore,
the ULLS carries a minor risk of developing deep venous thrombosis, but this
risk will be minimized by closely monitoring the subjects for signs of
thrombosis during the whole ULLS period. There can be coincidental medical
findings due to the MRS measurements and there is radiation exposure due to
DEXA measurements (0.001 mSv/measurement).