Time course and nature of nutrient sensing during fasting in humans

Supply of fuel is of critical importance for survival. Evolution therefore
provided highly conserved, sensitive cell autonomous and systemic *energy
gauges* to safeguard adequate availability of fuel. Energy deprivation engages
a plethora of endocrine and metabolic mechanisms to reduce energy expenditure
and increase energy production from endogenous sources. Pituitary,
gastrointestinal and adipose hormones are components of the systemic response
to fasting. At the cellular level, AMP activated protein kinase (AMPK) plays a
pivotal role. It is activated by nutrient deprivation via a reduced
intracellular ADP/AMP ratio and a variety of endocrine cues (including insulin
and leptin) and controls energy balance by shutting off energy consuming
processes while activating the machinery to produce ATP (1). The sirtuins are a
family of highly conserved nicotinamide adenosine dinucleotide (NAD)+ dependent
deacetylases that play similar roles by histone modification of genes encoding
proteins involved in energy metabolism (2). Importantly, calorie restriction
extends lifespan in all species examined (including non-human primates) and
prevents chronic disease in mammals, most likely via mechanistic routes that
involve this energy sensing machinery.

Several studies have begun to determine the time course of events sensing
energy availability in rodents. In contrast, there is not a single study that
has examined this in humans to date. A better understanding of this energy
sensing machinery in humans is of utmost importance to give us new insights for
the development of new therapies for common diseases such as obesity, diabetes
mellitus, cardiovascular diseases and cancer. This are diseases in which
disturbances of the energy-sensing machinery might play a role.
To determine the time course of energy sensing events in humans, we will
measure the concentration of various hormones in plasma, and biochemical
changes in skeletal muscle at sequential time points during starvation. We
choose to study the molecular machinery in muscle, since muscle is very
sensitive to fuel deprivation.

1. To determine the nature and time course of the energy sensing machinery
during starvation in humans.
2. To correlate systemic adaptations with changes at the cellular level.

Observational intervention study.

All participants will be screened prior to this intervention study. We will
include 12 healthy young, lean, Caucasian men.
All participants will be admitted to the clinical research unit of the
department of internal medicine for a 24 hour fast. After arrival
anthropometric measurements and a bioelectrical impedance analysis (BIA) will
be performed, after which volunteers will be given a standardized breakfast. We
will take muscle biopsies, preceded by indirect calorimetry and a blood sample,
at different time points: 1 * hours of fasting, 4 hours of fasting, 10 hours of
fasting and 24 hours of fasting.

A muscle biopsy will be taken from the musculus vastus lateralis. Some, but not
all, patients experience some pain at the moment the biopsy is taken. Sometimes
a small haematoma will develop. Furthermore the patient might experience a
heavy feeling in the involved muscle during 24 to 48 hours after the biopsy.
After the muscle biopsy a pressure bandage will be applied to the leg which has
to remain for 24 hours. In the previous years during which we have performed
muscle biopsies, a bleeding or other complication has never occured.