Biology Reference
In-Depth Information
the colon because other reactions occur
where the intestinal microbiota are involved.
Several
in vitro
gastrointestinal models have
been designed for different studies. Most of
the
in vitro
models study only gastric diges-
tion and digestion of the small intestine
because, by means of enzymes and different
pH conditions, the compound of interest
may be detected therefore allowing the
determination of the bioaccessibility and/
or bioavailability. Generally, for each
in vitro
incubation, while maintaining the tempera-
ture constant at 37°C, the sample is sub-
jected to digestion in the stomach with a pH
close to 2.0 and incubating with pepsin
(from pork mucosa) in a bath, stirring it for
2 h. Then it is neutralized and pancreatin
(from pork pancreas) and bile salts are
added and incubated for 2 h (Gil-Izquierdo
et al.
, 2001).
Among the models that simulate the
upper gastrointestinal tract is a system using
only a bioreactor to study the passage in the
stomach and intestines; this system can alter
the pH and adapt it to different retention
times. The simulation of the stomach and
small intestine is performed in a flask with
stirring, and combining gastric and acid solu-
tions, specifically with pepsin in acid
medium to simulate stomach and pancreatin
and bile salts in a neutral medium for the
small intestine (Sumeri
et al.
, 2008); this sys-
tem is a more realistic replication of the con-
ditions of the upper gastrointestinal tract.
Of the models that simulate the entire
gastrointestinal tract is the Human Intestinal
Ecosystem Simulator (SHIME), which con-
sists of five or six bioreactors with control-
led pH conditions simulating the stomach,
small intestine, ascending, transverse and
descending colon (Molly
et al.
, 1993; De
Boever
et al.
, 2000; Possemiers
et al.
, 2004).
Another model used is a computer-controlled
dynamic model called TIM (TNO intestinal
model), part of the Netherlands Organization
for Applied Scientific Research (TNO),
which consists of four chambers to simulate
conditions in the stomach and small intes-
tine by kinetic pH, bile salt concentration
and transit of chyme (Minekus
et al.
, 1995).
The TNO intestinal model of the stomach
and small intestine is perhaps the most
elaborate and allows the manipulation of
many parameters, including gastric and
intestinal pH regulation, pancreatic juice
flow including digestive enzymes, peristal-
sis for the mix, transit times and continuous
removal of digested compounds (Carnachan
et al.
, 2012). Both simulators, SHIME and
TIM, because they are very simplified, show
faults in the absorption of metabolites and
fluids, and colonization of microorganisms
in the bowel, among others. There are also
suggestions to incorporate simulated chew-
ing and peristalsis for a complete digestive
process (Yoo and Chen, 2006). To simulate
the digestive process in the colon, the sim-
ulator EnteroMix
®
consists of four reac-
tors that recreate the conditions of the
bowel in all segments - ascending, trans-
verse, descending and sigmoid - using the
same faecal inoculum obtained from one or
more donors, which was developed to
study the effects of carbohydrates fermenta-
tion in the colon microbial composition
(Makivuokko
et al.
, 2006; Makelainen
et al.
, 2007).
To simulate the bioavailability of a bio-
active compound, some authors use a dialy-
sis membrane. Usually, this is a dialysis
semipermeable cellulose membrane, which
is added during the
in vitro
digestion of the
small intestine (Bouayed
et al.
, 2011, 2012).
This
in vitro
model simulates gastrointesti-
nal digestion by subjecting the samples (in
vials) to incubation in a bath with stirring
for 4.5 h at 37°C at different pH values in
the presence of pectic enzyme and by the
fractionation of digestion via dialysis bags,
which are immersed in the samples (Argyri,
et al.
, 2009). The dialysate (fraction inside
the bag) consists of low molecular weight
soluble compounds and the retained solu-
ble and insoluble high molecular weight
compounds. The retained compounds and
dialysate are centrifuged and the super-
natant removed (Kapsokefalou and Miller,
1991). This dialysability method has been
considered a desirable option to calculate
the bioavailability of Fe in a large number of
samples. The molecular weight is presumably
a significant factor that determines the absorp-
tion of Fe; for this reason predicting Fe
bioavailability is based on the measurement
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