Role of intestinal Acetate Production in human Appetite and Insulin Sensitivity

Obesity is a major public health problem due to its pandemic occurrence and its
association with adverse consequences such as insulin resistance, diabetes and
cardiovascular risk. Comparable to the role of Central Nervous System (CNS)
reward and satiety responses in drug addiction, it has been demonstrated that
obese individuals are characterized by excessive eating due to altered CNS
reward and satiety responses to food. Obesity is an important component of
glucose tolerance and insulin resistance. It has been suggested that intestinal
microbiota might play a role in the regulation of food intake and the
postprandial glucose response and insulin secretion. For example, obesity has
been associated with alterations in composition of intestinal microbiota, and
transfer of *obese* microbiota can induce adiposity and hyperphagia in animals,
suggesting that gut microbiota may influence host feeding behavior. Microbial
metabolites are thought to be key players that mediate the pathways through
which microbiota influence host energy balance. In this regard the short chain
fatty acid (SCFA) acetate is one of the major metabolites produced by
microbiota in the gut from dietary fibers. It has been established in humans
that acetate is absorbed in the blood stream and passes the blood brain barrier
(BBB). Acetate has been shown to reduce food intake and thus to induce satiety
in mice. Previous animal studies have indicated that plasma acetate can
increase the glucose-stimulated insulin secretion (GSIS) by vagal stimulation.
Acetate has been shown to ameliorate insulin resistance by causing a shift from
lipogenesis to fatty acid oxidation in liver and adipose tissue in mice. The
(postprandial) effects of acetate have not been directly studied in humans,
although vinegar suppletion in humans significantly lowers the blood glucose
response to carbohydrates and may inhibit food intake. Another study in humans
showed that vinegar intake reduced body weight and body fat mass in obese
subjects. Since vinegar consists of acetic acid, which is the protonized
variant of the acetate molecule, similar effects are to be expected of acetate.
However, the effects of intestinal acetate on neural responses to food stimuli
and insulin secretion/sensitivity have not been studied in humans.

The main goal of this study is to investigate whether (postprandial) intestinal
acetate production may be related to plasma acetate, insulin
secretion/sensitivity and the CNS regulation of food intake and whether these
findings differ between people with high intestinal acetate production and
people with low acetate production based on their fecal microbiota composition.

Primary objective: Our primary objective is to investigate the difference in
plasma acetate level after the standardized mixed meal test (SMMT) between
individuals with high intestinal acetate production compared to individuals
with low intestinal acetate production.

Secondary objectives: Our secondary objective is to study the difference in CNS
responses to actual and virtual food stimuli between the high acetate producing
group and the low acetate producing group. CNS to food stimuli will be measured
by assessing satiety and appetite functional magnetic resonance imaging (fMRI).
In addition, we aim to compare fasting SCFA, glucose levels and insulin
secretion between high and low acetate producing individuals as well as
postprandial plasma SCFA, glucose and insulin levels in both groups during a
SMMT. Additionally, we will look at individual variation in plasma acetate,
faecal acetate and microbiota composition within individuals by performing a
follow-up measurement on a subset of both the high and the low acetate
producers.

Comparative study

This study consists of 3 test days per subject (and a total of 10 hours per
subject). Participants will visit the AMC 2 times. One week prior to the test
day, subjects will be asked to wear an accelerometer and to register their
dietary intake (https://mijn.voedingscentrum.nl/nl/eetmeter). In total 136 mL
blood will be taken for the regular test (128 mL on the first test day and 8 mL
24h after day 1).
Day 1: the first test day will comprise a visit to the home of the participant
by the investigator. During this visit informed consent will be obtained. In
addition, eating behavior and QOL questionnaires and the materials to collect a
fecal sample will be dropped off. Furthermore, the subject*s medical history
will be determined, as well as the use of medication. At the end of the home
visit, an appointment will be scheduled for their visit to the AMC.
Day 2: subjects will bring morning stool samples, the accelerometer and the
filled out eating behavior and QOL questionnaires on the morning of their
visit. During this visit the following measurements will be performed: body
length, body weight, waist circumference, hip circumference, left upper arm
circumference, left calf circumference and blood pressure. Additionally,
resting energy expenditure, body impedance analysis and standardized mixed meal
test will be performed. Finally, at 7 hours after the MMT, an fMRI will be
performed to measure the individuals* neural responses to virtual food stimuli
and actual food stimuli.
Day 3: Blood samples will be taken 24h after the SMMT, in order to assess the
24h effects of the meal on plasma SCFA, insulin and glucose.
A subset consisting of 20 participants (10 high and 10 low acetate producers)
will be invited back to the AMC for the follow-up blood and feces collection
one week after the main test day in order to assess individual variation of
several of the measures variables.
Day 4 (for the subset of individuals that are invited back for the sub-study):
subjects will bring a morning stool sample on the morning of their visit.
During this visit, a blood sample (60 mL) will be drawn 24h after the SMMT.
Resting energy expenditure and body impedance analysis will be measured. Of
these participants 196 mL blood will be collected in total.
This study will provide valuable insight into the effects of gut microbiota
composition and acetate production on human metabolism, which may help in
developing new strategies to combat obesity and insulin resistance.