Investigating Neural Mechanisms Underlying Olfactory Fat Perception

Consumption of dietary fat is exceeding recommended daily intake requirements
in many Western diets. Due to its high energy density and low effect on
satiation (especially in obese individuals) it is considered a major
contributing factor to energy overconsumption and consequential development of
obesity and related comorbidities. To help reduce the public health burden of
excessive fat consumption, the understanding of its perception is crucial.

The alluring flavour of fat arises from a synergy between gustation,
somatosensation, as well as olfaction. Although the role of olfaction in the
perception of dietary fat is relatively unexplored, an increasing body of
evidence underscores the importance of odours in fat perception. It has been
established that humans are capable of detecting and discriminating between
fatty acids using solely olfactory cues. Moreover, research shows that humans
are also capable of detecting minute fat content differences in actual foods.
In our previous two experiments (manuscript submitted for publication; see "K3
Retrofat Study Manuscript* for the submitted manuscript), we replicated
previous findings on olfactory fat perception in real foods and extended them
by showing that humans are also capable of discriminating food fat content
solely based on retronasal olfaction. These findings support the notion that
humans possess a functional olfaction-based system for detecting food fat
content, however, the underlying mechanisms remain to be elucidated.

In contrast to oral fat perception, which has been investigated in numerous
neurobiological studies, the underlying neurobiological mechanisms of olfactory
fat perception remain unexplored. In fact, our literature search did not yield
a single study investigating how olfactory exposure to fat sources is
represented in the human brain. The research proposed in the current protocol
therefore aims at filling this knowledge gap, by uncovering the neural
mechanisms underlying olfactory fat perception and mapping relevant brain
areas. We hypothesise that brain activation in reward-related brain areas will
differ between the fat levels. Specifically, we expect that exposure to higher
fat levels will result in a higher level of activation in the VS (NaC, VP),
VTA, PFC (including OFC), ACC, amygdala, hippocampus, and insula.

See "1. Introduction and Rationale" of the research protocol for more
information.

The primary objective of this study is to determine brain activation in
response to olfactory (orthonasal) exposure to different fat levels contained
in a ecologically-relevant fat source.

The study follows a randomized, counterbalanced, within-subjects design in
which brain responses to olfactory (orthonasal) exposure to three fat
concentrations (low, medium, high) will be measured.

During fMRI, odours from fat sources will be presented using an olfactometer.
Ratings of odour liking, and intensity will be collected using 100-unit Visual
Analogue Scales.

The study is non-therapeutic to the subjects. No immediate benefits for the
subjects are expected from participation in this study, and the risk associated
with participation can be considered negligible. In terms of time, the
subject*s burden is as follows: 30 minutes for the screening/training session;
35 minutes for discrimination testing session; and 1 hour and 10 minutes for
the fMRI task session. In total, the time burden amounts to 2 hours and 15
minutes.

Undergoing an fMRI scan involves: exposure to loud noise (addressed with ear
protection) and a moderate amount of physical restraint (the head is inside an
fMRI coil - this feeling is similar to wearing a motorbike helmet). During the
test session, subjects will smell odours that originate from commercially
available, considered safe, and commonly consumed food products. These odours
will be embedded within a constant stream of odourless air (delivered via an
olfactometer), heated to body temperature and humidified to 80% relative
humidity.