The effects of rectal administration of SCFA on human substrate and energy metabolism

Gut microbiota is being increasingly recognized as an important factor in fat
distribution, insulin sensitivity and glucose
and lipid metabolism. Accordingly, the intestinal microbiota could play an
important role in the development of obesity
and type 2 diabetes mellitus. The role of gut-derived short-chain fatty acids
(SCFA), the formation of which is enhanced
by microbial fermentation of fibre, is still controversial. One study found
that an increase in the formation of SCFA
stimulated energy extraction from diet, with subsequent weight gain. In
contrast, supplementation of non-fermentable
carbohydrates, which lead to an increase in SCFA formation, had beneficial
effects on body weight control and insulin
sensitivity. Of note, a study showed that butyrate supplementation in mice
prevented diet-induced obesity and insulin
resistance. At the present time, our understanding of the effects of SCFA on
human metabolism (in gut or systemically)
is still limited. Yet, in light of the health claims of certain dietary fibres
(prebiotics), a detailed picture of the physiology of
human SCFA metabolism and its interaction with the microbiome is of pivotal
importance. We hypothesize that the
differential availability of SCFA impacts human metabolism differently. To
determine whether rectal administration of
SCFA is a good model for studying the metabolic effects of SCFA we first have
performed a pilot study (METC 11-3-079, NL38679.068.11). In the
study we have determined if rectal administration of sodium acetate has the
same effects on substrate and energy
metabolism compared to proximal administration. Our results indicate that the
primary outcome parameter fat oxidation
was significantly changed, when sodium acetate 180mM was administered in the
distal part of the colon (see
objectives). In contrast, no effect on our first outcome parameters were seen,
when sodium acetate was administered
in the proximal colon. Consequently, we will administer the SCFA in this
short-term study rectally
by using enemas.
The total TIFN project will provide more insight into how increased
availability of a beneficial SCFA mixture might serve
as a basis for rational nutritional strategies in the prevention and treatment
of obesity and type 2 diabetes mellitus. To
obtain rational nutritional strategies, a next step in this TIFN project will
be focused on dietary ingredients modulating
intestinal microbiota and subsequent SCFA production.

Based on our hypothesis that differential availability of SCFA will be
beneficial effects on substrate and energy
metabolism, the following objectives have been and will be addressed:
- In our previous study (METC 11-3-079, NL38679.068.11) we have validated
whether rectal administration of SCFA is a good model for studying the acute
metabolic effects of SCFA. For this, we have investigated if site of
administration (in distal or proximal colon) of sodium
acetate differentially affects parameters of substrate and energy metabolism
and we have tested the duration of
short-term effects of sodium acetate administration on markers of substrate and
energy metabolism.
Analyses of our primary outcome parameter fat oxidation showed an 23% increase
in fat oxidation area under curve (AUC) of the first 2h basal period
(significant, p=<0.05), when the high concentration (180mM) sodium acetate
solution was infused distally, compared to placebo.
Proximally, we did not find significant differences in any of our primary
outcome parameters. This data indicated that
rectal infusion is a good model to study metabolic effects of SCFA. Thus, we
will use in this study enemas to
administer the SCFA.
Based on our hypothesis that differential availability of SCFA will have
beneficial effects on substrate and energy
metabolism, the following objectives will be addressed:
- to investigate short-term effects SCFA combinations by rectal administration
on markers
of substrate and energy metabolism during overnight fasted conditions and after
a glucose load.
With the most promising SCFA combinations found in this short-term study , we
will perform a longer
term study (14 days) investigating its effects on insulin sensitivity and
markers of substrate and energy metabolism in
overweight subjects.

After the screening visit, the subject will come to the university 4 times in
21 days. During the test day (each
approximately 6h per test day), rectal infusion of a 200mL solution with a
combination with sodium acetate 24mmol, sodium butyrate
8mmol and sodium propionate 8mmol, a combination with sodium butyrate 14mmol,
sodium acetate 18mmol and sodium propionate 8mmol, a combination with sodium
propionate 14mmol, sodium acetate 18mmol, sodium butyrate 8mmol or a placebo
(saline infusion, 40mmol NaCl) in a randomized will take place. Via a
'ventilated hood' and blood sampling markers of substrate and energy metabolism
will be examined.

During the test day (each approximately 6h per test day), rectal infusion of a
200mL solution with a combination with sodium acetate 24mmol, sodium butyrate
8mmol and sodium propionate 8mmol, a combination with sodium butyrate 14mmol,
sodium acetate 18mmol and sodium propionate 8mmol, a combination with sodium
propionate 14mmol, sodium acetate 18mmol, sodium butyrate 8mmol or a placebo
(saline infusion, 40mmol NaCl) in a randomized will take place. The infusions
will be given twice per test day, in post absorptive conditions and after a
glucose load.

All subjects will be screened before participation and thereby receive
information about their health status. In the future
there can be general health benefits for the public, but the volunteers will
have no personal benefits by participating in
the study. They will, however, receive a financial contribution for
participating in this study. The general interest of this
study is that there have never been human intervention studies with the SCFA
butyrate, propionate and the
combinations we will investigate, and their effects on the human substrate and
energy metabolism. The concentrations
of acetate, propionate and butyrate that will be reached in the study resemble
the concentrations that are reached with
a high dietary fibre diet and are thus within the high physiological range. For
detailed information see paragraph 5.
After initial screening, subjects will have to invest approximately 24 hours in
the study. During the study, blood will be
collected via a venous catheter. Venepunctures can occasionally cause a local
hematoma or bruise to occur. Some
participants report pain during venepuncture.. In the study enemas will be
used. The procedure of the rectal infusion can be a burden for the volunteers
and can be experienced as an embarrassment. During the enema administration,
the coordinating investigator will take care of the privacy of the volunteers.
Enemas are clinically used for several purposes, for example to remove
faeces when an individual is constipated, to cleanse the rectum as preparation
for an endoscopic examination, or to
administer drugs or anaesthetic agents. Enemas differ greatly in volume (3 ml -
1 L) depending on the area of the
colon that has to be reached by the fluid. In the present study 200 ml enemas
will be used. During the enema administration,
the coordinating investigators will take care of the privacy of the subjects.
There is a small risk of rectum perforation when using enemas. In other study
no irritation, consecutive bowel movements or other side effects due to enemas
administered in a higher volume per minute were reported. Also, in the
department of gastroenterology studies were performed where the volunteers
insert enemas twice daily for ten days, and reported no irritations. These
studies were approved by the METC (MEC 06-3-067, MEC 06-3-020, MEC 06-3-071).