The role of central reward and satiety centers in the etiology of obesity: genetic and environmental influences

It has been suggested that obese individuals are characterized by excessive
eating due to altered central nervous system (CNS) reward and satiety responses
to the consummation of food. This is comparable to the role for CNS reward and
satiety responses in drug addiction. Differences between individuals in CNS
reward and satiety circuits are likely to be influenced by a multitude of
genetic and environmental factors, but whether environmental and genetic risk
factors affect the brain in the same pathways in the brain is unknown. An
additional unresolved question is to what extent the alterations in CNS reward
and satiety circuits are a cause of the development of obesity and to what
extent they are a consequence of obesity.

The overarching goal of this proposal is to investigate the environmental and
genetic influences on the activity of CNS reward and satiety circuits, and to
investigate whether these differences are causal to the development of obesity
in humans.

In order to achieve this aim we will address the following main study
objectives:
1. Do food-stimuli related responses in CNS reward and satiety circuits differ
within obesity-discordant monozygotic twin pairs?
2.a. Do food-stimuli related responses in CNS reward and satiety circuits
differ between individuals at high versus those at low genetic obesity risk?
2 b. Are the genetic effects on CNS reward and satiety circuits independent of
current obesity status?

The above defined objectives will be investigated in 2 cross-sectional studies.

Study 1:
In study 1, we will address the first objective by assessing neuronal activity
in CNS and reward and satiety circuits in response to visual and taste related
food-stimuli, measured as blood oxygen level-dependent (BOLD) response by
functional magnetic resonance imaging (fMRI), according to a special design of
*clonal controls,* i.e. rare monozygotic twins discordant for obesity. Fifteen
monozygotic twin pairs with an intrapair BMI difference of > 3 kg/m2 will be
selected from the Netherlands Twin Registry for analysis. Neuronal activity
will be expressed as signal change from baseline (%) in response to the
food-related stimuli.

Study 2:
In study 2 we will address the second objective (2a and 2b). Neuronal activity
in CNS reward and satiety circuits will be assessed in 60 subjects who are
classified to be at low or high genetic risk based on measured obesity risk
alleles from genome-wide association studies (objective 2a). Neuronal activity
will be measured with BOLD fMRI using the same protocol as in study 1. Changes
in CNS satiety and reward circuit responses may not only be a cause, but also a
consequence of obesity. To investigate the extent to which the impact of
genetic predisposition is independent of current obesity status both obese and
lean individuals with low and high count of obesity risk alleles will be
included (objective 2b).

To study other underlying mechanisms involved in the etiology of obesity, we
will include the role of eating behavior, by assessing dietary habits using the
24 hours recall method during three days and measuring qualitative and
quantitive food intake during a choice-buffet; the role of biomarkers and
hormones in the fasting state, the role of physical activity level, as assessed
by triaxial accelerometry; and basal metabolic rate, measured by indirect
calorimetry. Furthermore, we will assess the role of altered autonomic nervous
system balance by measuring heart rate variability using a small non-invasive
electro- and impedance cardiogram; and the potential role of gut microbiota
species by collecting and analyzing feces samples using phylogenetic
microarrays. Finally, epigenetic changes will be studied by measuring DNA
methylation in a blood sample.

We are aware of the possible demand that may be imposed on the participants in
this observational study. Screening will be performed in part by phone to keep
the number of visits to the research unit by one. After the telephone call
participants will travel one time to the study location. The duration of this
visit is aproximately 4 hours. A maximum amount of 70 mL blood will be drawn
during a single venipuncture. The risks associated with participation in this
study are the risks of venous blood drawing: hematoma and/or flebitis. During
the week after the visit dietary assessment will be done using the 24 hours
recall method during a telephone call on three separate days (2 weekdays and 1
weekendday). The duration of each phone call is about 30 minutes. Also,
participants will be wearing a small accelerometry device placed on their hip
with a belt for 7 days (only during the day and removing it for water-based
activities as swimming and showering). We decided to use these methods so that
participants won't have to keep track of what they eat and how much they
particpate in physical activity themselves (for instance by keeping a diary).
For the collection of the feces sample we will provide participants with a
specially designed feces-collection-container to make this collection as
feasible as possible. Also, we decided to get the feces samples picked up at
home by a research physician/assistant so participants won't need to make a
second visit to the research clinic. The research physician will be daily
available for questions. We will try to make this study as bearable as possible
for our participants. All tests will be done by one researcher.