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JNM
Journal of Neurogastroenterology and Motility
Original Article
ⓒ2015 The Korean Society of Neurogastroenterology and Motility
J Neurogastroenterol Motil, Vol. 21 No. 4 October, 2015
www.jnmjournal.org
J Neurogastroenterol Motil, Vol. 21 No. 4 October, 2015
pISSN: 2093-0879 eISSN: 2093-0887
http://dx.doi.org/10.5056/jnm15028
511
Received: February 18, 2015 Revised: April 20, 2015 Accepted: April 21, 2015
CC
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.
org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work
is properly cited.
*Correspondence: Rosario Cuomo, MD
Department of Clinical Medicine and Surgery, Federico II University Hospital School of Medicine, Via S. Pansini 5, Building 6, 80131
Napoli, Italy
Tel: +39-081-7463892, E-mail: rcuomo@unina.it
Financial support: None.
Conflicts of interest: None.
Author contributions: Study concept and design: Paolo Andreozzi, Giovanni Sarnelli, and Rosario Cuomo; Subject enrollment and acquisition of data:
Paolo Andreozzi, Giovanni Sarnelli, Marcella Pesce, Francesco P Zito, Alessandra D’Alessandro, Viviana Verlezza; Biochemical
analysis: Ilaria Palumbo and Fabio Turco; Analysis and interpretation of data: Katherine Esposito, Paolo Andreozzi, Giovanni
Sarnelli, and Rosario Cuomo; and Drafting of the manuscript: Paolo Andreozzi, Giovanni Sarnelli, and Rosario Cuomo.
ORCID: Paolo Andreozzi, http://orcid.org/0000-0002-4604-0126; Giovanni Sarnelli, http://orcid.org/0000-0002-1467-1134; Marcella Pesce,
http://orcid.org/0000-0001-5996-4259; Francesco Paolo Zito, http://orcid.org/0000-0002-1084-3373; Viviana Verlezza,
http://orcid.org/0000-0001-9569-2369; Ilaria Palumbo, http://orcid.org/0000-0003-0387-1461; Fabio Turco,
http://orcid.org/0000-0001-9149-3872; Katherine Esposito, http://orcid.org/0000-0002-3652-5154; Rosario Cuomo,
http://orcid.org/0000-0003-0768-8381.
The Bitter Taste Receptor Agonist Quinine
Reduces Calorie Intake and Increases the
Postprandial Release of Cholecystokinin in
Healthy Subjects
Paolo Andreozzi,
1
Giovanni Sarnelli,
1
Marcella Pesce,
1
Francesco P Zito,
1
Alessandra D’Alessandro,
1
Viviana Verlezza,
1
Ilaria Palumbo,
1
Fabio Turco,
1
Katherine Esposito,
2
and Rosario Cuomo
1
*
1
Department of Clinical Medicine and Surgery,
“
Federico II
”
University, Naples, Italy; and
2
Department of Clinical and Experimental Medicine,
Second University of Naples, Italy
Background/Aims
Bitter taste receptors are expressed throughout the digestive tract. Data on animals have suggested these receptors are involved
in the gut hormone release, but no data are available in humans. Our aim is to assess whether bitter agonists influence food
intake and gut hormone release in healthy subjects.
Methods
Twenty healthy volunteers were enrolled in a double-blind cross-over study. On 2 different days, each subject randomly received
an acid-resistant capsule containing either placebo or 18 mg of hydrochloride (HCl) quinine. After 60 minutes, all subjects were
allowed to eat an
ad libitum
meal until satiated. Plasma samples were obtained during the experiment in order to evaluate
cholecystokinin (CCK) and ghrelin levels. Each subject was screened to determine phenylthiocarbamide (PTC) tasting status.
Results
Calorie intake was significantly lower when subjects received HCl quinine than placebo (514 ± 248 vs 596 ± 286 kcal;
P
=
0.007). Significantly higher CCK
Δ
T
90
vs T
0
and
Δ
T
90
vs T
60
were found when subjects received HCl quinine than placebo (0.70
± 0.69 vs 0.10 ± 0.86 ng/mL,
P
= 0.026; 0.92 ± 0.75 vs 0.50 ± 0.55 ng/mL,
P
= 0.033, respectively). PTC tasters ingested
a significantly lower amount of calories when they received HCl quinine compared to placebo (526 ± 275 vs 659 ± 320 kcal;
P
= 0.005), whereas no significant differences were found for PTC non-tasters (499 ± 227 vs 519 ± 231 kcal;
P
= 0.525).
Paolo Andreozzi, et al
Journal of Neurogastroenterology and Motility
512
Conclusions
This study showed that intra-duodenal release of a bitter compound is able to significantly affect calorie intake and CCK release
after a standardized meal. Our results suggest that bitter taste receptor signaling may have a crucial role in the control of food
intake.
(J Neurogastroenterol Motil 2015;21:511-519)
Key Words
Cholecystokinin; Food intake; Ghrelin; Quinine; Taste
Introduction
Taste is a complex sensory modality involved in the detection
of food quality. It is commonly accepted that the so-called “basic”
tastes (ie, bitter, sweet, umami, sour, and salty) have been natu-
rally selected to facilitate nutrient consumption and help avoid the
ingestion of potentially harmful foods.1 For instance, the sweet
taste is perceived as palatable and is classically associated with en-
ergy-dense foods, whereas the bitter taste is innately aversive and
related to toxic compounds.
The mechanisms underlying taste detection in the oral cavity
have been well established; in fact, tastants are recognized by a
number of receptors, including ion channels (salty and sour) and
G protein-coupled receptors (GPCRs) (bitter, sweet, and umami).1
Specifically, the GPCR type 1 taste receptor family (T1R) com-
prises 3 different subunits that heterodimerize to detect sweet
(T1R2 + T1R3) and umami (T1R1 + T1R3) tastants, while
the type 2 taste receptor (T2R) family is responsive to bitter
compounds.2,3 The ability to taste bitter compounds varies greatly
among individuals. The best-known example of this variability is
the genetic ability to taste phenylthiocarbamide (PTC) and 6-N
propylthiouracil (PROP). Based on the ability to recognize
PTC or PROP, individuals can be defined as “tasters” or
“non-tasters.”4
In the last decade, several studies have demonstrated the ex-
pression of taste receptors and their downstream signaling mole-
cules in the gut, leading to the hypothesis that gastrointestinal
taste system may play a role in the gut chemosensitivity.5,6 In par-
ticular, studies in model cell lines7,8 and a histochemical study9
have shown that T2R family members are expressed by en-
tero-endocrine cells (EECs), which constitute a first level of in-
tegration of the information coming from the lumen.10 EECs in
the gastrointestinal (GI) tract play a crucial role in the control of
food intake through several ways. Their activation results in the
release of a large array of GI peptides, such as glucagon-like pep-
tide 1 (GLP-1), cholecystokinin (CCK) and ghrelin,11 all in-
volved in the control of food intake. Ghrelin, released by gastric
EECs, is the only known orexigenic gut peptide and its plasma
levels increase during fasting and rapidly fall after the meal.12,13
Other hormones, such as CCK, GLP 1, and peptide YY (PYY),
exert an opposite effect, thus inhibiting food intake. CCK is the
anorexigenic peptide that has been more thoroughly studied14;
this hormone is released by I-cells into the duodenum in response
to intra-luminal lipids and proteins.15,16 The CCK inhibitory ef-
fects on food intake are thought to be mediated via CCK1 re-
ceptors on vagal afferents.17
A previous study has shown that the stimulation of T2R with
bitter compounds induced the release of the anorexigenic hor-
mone CCK from mouse EEC lines.18 Moreover, the intragastric
gavage of bitter compounds induced the release of ghrelin in
mice.19 However, this evidence derives from experiments carried
out in vitro or on animals, but no study has translated these find-
ings to in vivo human models to define the role of the stimulation
of these receptors on food intake.
In the wake of these results, we hypothesized that bitter taste
receptors in the gut could be involved in the control of food
intake. Our study thus aimed to investigate the effect of in-
tra-luminal release of a bitter compound (HCl quinine) on food
intake and gut hormone release in healthy subjects.
Materials and Methods
Participants
Twenty healthy adult volunteers (12 women, 8 men; median
age 27 years) were recruited from the personnel and students of
the Federico II University Medical School. Their mean ± SD
body mass index (BMI) was 24 ± 4 kg/m2. Exclusion criteria
were: prior abdominal surgery − except for appendectomy, pos-
Bitter Taste and Food Intake
Vol. 21, No. 4 October, 2015 (511-519) 513
Figure 1. Study design. (A) All subjects
randomly received a capsule containing
placebo or 18 mg of hydrochloride
quinine in a randomized, double blind,
cross-over design. (B) The ad libitum test
started 60 minutes after capsule admini-
stration. Blood samples were taken to
assay ghrelin and cholecystokinin at T0,
T60, and T90. Gastrointestinal sensation
assessment was performed at T0, T60,
and at the end of ad libitum test (Tend).
GI, gastrointestinal; HCl, hydrochloride;
PTC, phenylthiocarbamide.
itive history of organic or functional GI diseases, use of medi-
cations able to affect GI motility and binge eating disorders.
Informed written consent was obtained from all the participants
and the study was approved by the Ethics Committee of the
Federico II University of Naples.
Study Protocol
This was a randomized, double blind, placebo-controlled,
cross-over study. Following an overnight fast, approximately at
11:30 AM, all subjects underwent ad libitum test 60 minutes after
they randomly received a capsule containing placebo or 18 mg of
hydrochloride (HCl) quinine (Sigma-Aldrich; Gillingham, UK).
After 1 week, the subjects repeated the same experiment receiv-
ing a capsule containing HCl quinine or placebo, respectively
(Fig. 1A). Randomization was assured by a computer-generated
random list (MS Excel 2007). The experimental dose was chosen
after a preliminary study with different amount of HCl quinine
(18, 36, and 72 mg). Given that no differences were observed in
terms of GI feelings and calorie intake among various doses test-
ed, we decided to use the lower dose (data not shown). The cap-
sule was acid-resistant in order to facilitate the release of bitter
compound or placebo into the duodenum. Considering the avail-
able data on gastric emptying time after an overnight fast,20,21 we
chose a 60-minute interval between capsule administration and
the ad libitum test (see below). During the experiment, a ques-
tionnaire was administered to assess GI sensations and plasma
samples were obtained in order to evaluate GI peptide levels (Fig.
1B). Afterwards, all subjects were screened to determine PTC-
tasting status ("tasters" or "non-tasters").
In addition, 8 out of 20 subjects also underwent 2 sessions of
breath test for gastric emptying study. A standard meal was
served to the subjects 60 minutes after the random administration
of a capsule containing 18 mg of HCl quinine or placebo. Breath
samples were collected during the experiments. After 1- week,
Paolo Andreozzi, et al
Journal of Neurogastroenterology and Motility
514
the same subjects repeated the emptying study receiving a capsule
containing HCl quinine or placebo, respectively. Similarly, ran-
domization was assured by the computer generated random list.
Ad libitum
Test
A standardized test was used to assess calorie intake.22 A
standard buffet meal was served 60 minutes after capsule admini-
stration. The standard meal was composed of white bread, cheese,
and ham spread (Spuntì, Kraft Foods, Italy). The meal was ad-
ministered by single portions, containing 89 kcal each, until max-
imum satiety was reached. The composition of single portions
was the same: 50% carbohydrate, 31% fat, 19% protein. The food
was served in excess to decrease the awareness of food intake. The
quantity of food eaten, the volume of water drunk and the meal
duration were recorded at the end of the test.
Gastrointestinal Sensations Questionnaire
GI sensations were evaluated immediately before the admin-
istration of the capsule (T0), before beginning of the meal (T60)
and at the end of the meal (T end) (Fig. 1B). GI sensations eval-
uated included fullness, nausea, bloating, epigastric pain, heart-
burn, satiety and desire to eat. Measurements were performed by
a visual analogue scale (VAS) calibrated to 100 mm.23,24
Phenylthiocarbamide Status Assessment
To determine PTC tasting status we invited all subjects to
place, consecutively, 2 paper strips on their tongue: the first one
was a control strip, whereas the second one was a strip of filter pa-
per impregnated with 0.3 mg of PTC (Online science mall;
Florida, USA). “Tasters” were defined as subjects who perceived
the bitter taste when PTC-impregnated blotting paper strip was
placed on the tongue. Furthermore, each subject was asked to
rate the intensity of their bitter perception of PTC strip on a VAS
ranging from 0 mm (no taste) to 100 mm (extremely strong
taste).
Gastric Emptying Study
Out of 20 subjects who participated in ad libitum test, 8 (4
women, median age 25 years) underwent a breath test study to
determine gastric emptying in a randomized, double blind, place-
bo-controlled cross-over fashion. Sixty minutes after the admin-
istration of HCl quinine or placebo, they were asked to consume
a test meal consisting of 60 g white bread, 10 g butter, 50 g ham,
an omelet made from 1 egg with egg yolk charged with 74 kBq
13C-octanoic acid (Euriso-top, Saint-Aubin, France) and 100
mL water. The test meal contained 480 kcal (19% protein, 53%
carbohydrate, and 31% fat). Subjects were encouraged to eat the
meal within 10 minutes. Breath samples were collected at
15-minute intervals for 240 minutes postprandially. 13CO2-excre-
tion in breath was subsequently analyzed using isotope-selective
infrared spectroscopy to derive gastric emptying half-time.
Biochemical Analysis
Plasma samples were obtained at 0-60-90 minutes after cap-
sules administration, placed in centrifuge tubes containing apro-
tinin and stored at −80oC immediately after centrifugation.
Total plasma immunoreactive ghrelin and CCK were measured
by enzyme immunoassay. Ghrelin was measured in duplicate us-
ing commercial ELISA kits (Phoenix Pharmaceuticals)25; the in-
ter- and intra-assay coefficients of variance were ＜ 10%. The
lower and upper detection limits for this assay were 0.12 ng/mL
and 100 ng/mL. CCK (26-33 octapeptide non-sulfated form)
was measured in duplicate using a commercial ELISA kit
(Phoenix Pharmaceuticals)26; the inter- and intra-assay co-
efficients of variance were ＜ 10%, with a lower detection limit of
0.04 ng/mL.
Sample Size Calculation
Preliminary data showed a differences of about 100 kcal in
terms of calorie intake between the 2 sessions of the study with a
standard deviation of differences of 120 kcal. Based on this data
we calculated that a sample size of at least 13 p atients would be re-
quired to test our hypothesis with a power of 0.80 and alpha level
of 0.05 (PS Power and Sample Size Calculations; Version 3.0).
Statistical Methods
To evaluate significant differences in terms of GI sensations,
a paired t test was used to compare the VAS scores of these pa-
rameters in the 2 sessions of the study. The same analysis was per-
formed to evaluate differences in calorie intake, volume of water
drunk, meal duration and gastric emptying half-time between the
2 sessions of the experiment, whereas unpaired t test was used to
compare difference of calorie intake in the 2 study sessions (Δ
Kcal = Kcalquinine-Kcalplacebo) between PTC “tasters” and
“non-tasters”. Linear regression was used to evaluate the associa-
tion between bitter PTC intensity and ΔKcal.
Hormone profiles were evaluated both in terms of absolute
values and as difference of T90 values with T0 (before capsule
administration, ΔT90 vs T0) and T60 values (before test meal,
ΔT90 vs T60). Between 2 sessions, ANOVA with repeated meas-
Bitter Taste and Food Intake
Vol. 21, No. 4 October, 2015 (511-519) 515
Figure 2. Gastrointestinal sensations in the 2 sessions of the study. No significant differences were observed between 2 experiments in terms of satiet
y
and desire to eat at T0, T60, and Tend.
Figure 3. Calorie intake and meal duration in the 2 sessions of the
study. Calorie intake was significantly lower when subjects received
hydrochloride quinine than placebo (514 ± 248 vs 596 ± 286 kcal; P =
0.007). Meal duration did not statistically differ in the 2 sessions of the
study. *P = 0.007.
Table 1. Ad libitum Test in 20 Subjects After the Administration of
HCl Quinine and Placebo
After HCl quinine After placebo P-value
Calorie intake (kcal) 514 ± 248 596 ± 286 0.007
Meal duration (min) 13.7 ± 4.7 14.7 ± 5.4 0.403
Water intake (mL) 157 ± 57 165 ± 59 0.186
Data are presented as mean ± SD.
ures was used to evaluate differences of absolute values, whereas a
paired t test was used to compare differences of ΔT90 vs T0 and
ΔT90 vs T60 between 2 sessions of the study.
The statistical analysis was performed by the statistical soft-
ware package SPSS for Windows Version 12.0 (SPSS, Chicago,
IL, USA). The results are reported as mean ± SD. Differences
were considered statistically significant when P was ＜ 0.05.
Results
Effect of Bitter on Gastrointestinal
Sensations
No subject experienced any oral bitter or unpleasant percep-
tion after HCl quinine or placebo administration, nor were there
any adverse reactions after the experiment. No significant differ-
ences were observed between the 2 experiments in terms of satiety
and desire to eat at T0, T60, and T end (Fig. 2). No significant dif-
ferences were found regarding other symptoms or GI sensations
(data not shown).
Effect of Bitter on Food Intake
As shown in Figure 3, subjects ingested a significantly lower
amount of calories when receiving the capsule containing HCl
quinine than placebo (514 ± 248 vs 596 ± 286 kcal; P = 0.007).
Conversely, meal duration was not different between the 2 experi-
ments (13.7 ± 4.7 vs 14.7 ± 5.4 minutes, P = 0.403) (Fig. 3),
nor was the amount of water intake different (157 ± 57 mL vs
165 ± 59 mL; P = 0.186). All data are showed in Table 1.
Effect of Bitter on Gut Hormones Release
There were no significant differences in CCK absolute values
when subjects received HCl quinine or placebo at the different
time points (T0: 1.05 ± 0.57 vs 1.50 ± 1.11 ng/mL, P = 0.094;
T60: 0.82 ± 0.35 vs 1.11 ± 0.79 ng/mL, P = 0.090; T90: 1.75 ±
0.84 vs 1.60 ± 1.01 ng/mL, P = 0.464). In order to minimize
Paolo Andreozzi, et al
Journal of Neurogastroenterology and Motility
516
Figure 4. Cholecystokinin (CCK) release after standard meal. Data are
expressed as difference vs basal level (ΔT90 vs T0) and vs pre-meal level
(ΔT90 vs T60). Significantly higher ΔT90 vs T0 and ΔT90 vs T60 were
found when the subjects received a capsule containing hydrochloride
quinine vs those taking placebo. *P = 0.033 and **P = 0.026.
Table 2. Calorie Intake in Phenylthiocarbamide Tasters and Phenylthiocarbamide Non-tasters After the Administration of HCl Quinine and
Placebo (Number of Subjects)
Calorie intake (mean ± SD, kcal) P-value
After HCl quinine After placebo
PTC tasters (n = 11) 526 ± 275 659 ± 320 0.005
PTC non-tasters (n = 9) 499 ± 227 519 ± 231 0.525
PTC, phenylthiocarbamide.
intra-individual variability, we evaluated the difference in CCK
with respect to basal values. Significantly higher ΔT90 vs T0 and
ΔT90 vs T60 were found when subjects received HCl quinine
compared to placebo (ΔT90 vs T0: 0.70 ± 0.69 vs 0.10 ± 0.86
ng/mL, P = 0.033; ΔT90 vs T60: 0.92 ± 0.75 vs 0.50 ± 0.55
ng/mL, P = 0.026) (Fig. 4).
On the contrary, ghrelin plasma levels were not significantly
different with HCl quinine or placebo, either when considered as
absolute concentrations (T0: 2.48 ± 0.66 vs 2.57 ± 0.52 ng/mL,
P = 0.538; T60: 2.50 ± 0.48 vs 2.43 ± 0.39 ng/mL, P = 0.493;
T90: 2.65 ± 0.69 vs 2.45 ± 0.50 ng/mL, P = 0.239) or delta val-
ues (ΔT90 vs T0: 0.17 ± 0.62 vs -0.13 ± 0.46 ng/mL, P =
0.231; ΔT90 vs T60: 0.15 ± 0.49 vs 0.01 ± 0.37 ng/mL, P =
0.319).
Effect of Bitter on Gastric Emptying Rate
The evaluation of gastric emptying revealed that in a subset
of 8 subjects (5 tasters) HCl quinine was not able to significantly
modify the rate of emptying in comparison to placebo (87 ± 14
vs 88 ± 12 minutes; P = 0.842).
Phenylthiocarbamide Status Assessment
According to the PTC paper test, 11 out of 20 subjects were
identified as “tasters,” while 9 subjects were not able to identify
any bitter sensation and were classified as “non-tasters.”
Most interestingly, a further analysis revealed that PTC tast-
ers ingested a significantly lower amount of calories when they re-
ceived HCl quinine compared to placebo (526 ± 275 vs 659 ±
320 kcal; P = 0.005), whereas no significant differences were
found for PTC non-tasters (499 ± 227 vs 519 ± 231 kcal; P =
0.525) (Table 2). This finding was even more evident when the
difference of calorie intake between the 2 experiments was con-
sidered, since PTC tasters presented a significantly different Δ
Kcal compared to non-tasters (−134 ± 124 vs −20 ± 89 kcal;
P = 0.034) (Fig. 5).
Moreover, the linear regression showed a negative associa-
tion between PTC bitter intensity and ΔKcal (r = −0.579; P =
0.008) (Fig. 6).
Regarding CCK release, in PTC tasters we observed that
ΔT90 vs T0 and ΔT90 vs T60 after the administration of HCl qui-
nine was higher compared to placebo but these differences were
not statistically significant (ΔT90 vs T0: 0.84 ± 0.72 vs 0.12 ±
0.88 ng/mL, P = 0.103; ΔT90 vs T60: 1.08 ± 0.88 vs 0.53 ±
0.57 ng/mL, P = 0.083). Similarly, no significant differences for
ghrelin levels were found between the 2 groups in the 2 sessions
of the study (data not shown).
Discussion
The gut “senses” the food, although the molecular mecha-
nisms of GI chemosensitivity are not fully understood. This study
showed for the first time that the direct intra-luminal admin-
istration of a bitter taste receptor agonist is able to significantly re-
duce calorie intake in healthy subjects, likely acting on gut bitter
taste receptors and altering gut hormone levels.
Bitter Taste and Food Intake
Vol. 21, No. 4 October, 2015 (511-519) 517
Figure 5. Phenylthiocarbamide (PTC) status assessment. PTC tasters ingested a significantly lower amount of calories when they received
hydrochloride quinine compared to placebo (526 ± 275 vs 659 ± 320 kcal; P = 0.005), whereas no significant differences were found for PTC
non-tasters (499 ± 227 vs 519 ± 231 kcal; P = 0.525). PTC tasters presented a significantly higher difference in calorie intake between 2 experiments
(Δ Kcal) compared to non-tasters (−134 ± 124 vs −20 ± 89 kcal; P = 0.034). *P = 0.005 and **P = 0.034.
Figure 6. Correlation between bitter phenylthiocarbamide (PTC)
intensity and ΔKcal (calorie intakeplacebo - calorie intakequinine). Linear
regression showed a negative association between ΔKcal and bitter PTC
intensity (r = −0.579, P = 0.008).
In our hands, the administration of HCl quinine in healthy
subjects was able to significantly reduce calorie intake in an ad
libitum food intake test. In particular, the subjects ingested about
15% less calories when compared to placebo, and this occurred
without a significant change in meal duration. This result is in
agreement with previously published data showing that quinine
per se strongly reduces food intake in rats, independently of its
aversive taste.27,28 In addition, we found that the quinine-medi-
ated reduction of food intake is associated to the individual ability
to percept PTC, since a significantly lower amount of calories was
observed in “tasters” than in “non-tasters” subjects.
We used quinine, an anti-malarial drug extracted from the
bark of the cinchona tree, able to trigger bitter taste by the activa-
tion of several members of T2R family receptors.29 The drug was
administered by an acid-resistant capsule for 2 main reasons: (1)
in order to prevent the activation of the taste receptors of the oral
cavity and (2) to prevent the release of ghrelin, produced by the
X/A-like cells of the oxyntic glands of the stomach and to allow
its release into the duodenum where CCK-releasing EECs are
located.19
The role of bitter taste in the control of digestive functions is
controversial: bitter compounds are naturally unpleasant. From a
classical point of view, the bitter taste evolved in order to refuse
potentially toxic compounds, such as plant alkaloids or microbial
toxins. However, bitter compounds are also used to stimulate ap-
petite or digestion in several parts of the world. For example, the
habit of drinking aperitif before a meal is justified by the belief
that the bitter taste stimulates appetite and increases gastric
secretion.
Gustatory signals likely play a role in the cephalic phase of
food intake to prepare the digestive tract to receive food.30
However, previous studies have shown that the activation of in-
testinal T2R results in the release of gut peptides, suggesting a
role in the post-ingestion phase of food intake.18,19,31 In particular,
the release of ghrelin from gastric EECs may explain, at least par-
tially, that bitter compounds could increase appetite feeling.
The process that limits calorie intake is the result of a coordi-
nated series of neural and humoral signals that originate from the
gut. In particular, hormonal signaling is strongly influenced by
intra-luminal chemicals (lipid, carbohydrates, and amino acids).32
According to our preliminary hypothesis, quinine failed to affect
Paolo Andreozzi, et al
Journal of Neurogastroenterology and Motility
518
ghrelin release while significant postprandial CCK increase
occurred. These data could explain, at least partially, the lower
calorie intake in comparison to placebo. To the best of our knowl-
edge, this is the first report indicating that quinine, a T2R ago-
nist, is able to modify postprandial CCK-release in humans.
Taking into account that the gastric motility may affect food
intake,33 we studied in a subset of volunteers the effect of quinine
on gastric emptying rate. Our results failed to find any significant
change of quinine compared to placebo, further suggesting that
the reduced calorie intake is independent of gastric motility.
Although bitter compounds in rats were able to delay gastric
emptying, our result is in agreement with a previous study show-
ing that quinine did not affect the rate of gastric emptying in
humans.34,35
The sensitivity to bitter compounds is genetically grounded
and individuals can be classified as “tasters” or “non-tasters” be-
cause of their ability to recognize PTC. Chang et al36 have dem-
onstrated a positive relationship between PTC/PROP taster sta-
tus and oral taste sensitivity to sucrose or quinine. To verify that
both oral and gut taste mechanisms are comparable, we also
screened the oral taste status of PTC in each subject. We found
that the effects of quinine on calorie intake affected only subjects
who were able to discriminate PTC, but not PTC non-tasters.
However, only a non-significant trend in CCK increase was ob-
served after the administration of quinine in PTC tasters.
Our results suggest that quinine-mediated CCK release is
likely unable per se to explain the observed effects on food intake
and other mechanisms could be involved in the food intake
reduction. To date, it is known that bitter stimuli trigger the re-
lease of gut hormones in EECs through a mechanism appearing
to involve T2Rs, -subunits of the G protein gustducin, phos-
pholipase and Ca2+ influx.18 Given the complexity of bitter stim-
uli transduction pathways in GI cells, we can speculate that other
mechanisms (ie, via gustducin, calcium or phospholipase) may
play a role in quinine-related effects on food intake, activating
neural reflexes and/or acting in a paracrine or endocrine manner.
Various studies have tried to elucidate the physiological role
of gut taste receptors in humans. Gerspach et al37 demonstrated
that lactisole, a T1R1/T1R3 (sweet) antagonist, induced a sig-
nificant reduction in GLP-1 and PYY but not CCK secretion, in
response to intragastric and intraduodenal glucose administration.
Our study demonstrates, for the first time, that bitter taste re-
ceptors are also involved in the physiological mechanisms that
control appetite in humans. Therefore, while the role of the T2Rs
in the oral cavity would prevent the intake of hazardous chem-
icals, the bitter taste receptors in the GI tract may act as a second
level mechanism which is able to further limit the intake of poten-
tially toxic bitter chemicals. Jeon et al31 showed that T2R signal-
ing stimulates the secretion of CCK from EECs and induces the
expression of ATP-binding cassette B1, a transporter expressed
on the apical membrane of intestinal epithelial cells able to limit
absorption of toxic substrates, both in Caco-2 cells and mouse in-
testine, through a CCK-dependent mechanism.
Summarizing, we showed that the intra-luminal release of
quinine, a bitter taste receptor agonist, significantly reduces the
calorie intake in an ad libitum test in healthy subjects, by a mecha-
nisms likely involving CCK. Furthermore, the reduced calorie
intake was positively associated with bitter PTC status. Our re-
sults suggest that T2R signaling in the human gut may have a
role in energy intake and appetite control, likely through the re-
lease of gut hormones. However, further studies are needed to
elucidate the ability of bitter taste receptors to control food intake.
In particular, it could be of extreme interest to test whether the
stimulation of bitter receptors is able to limit calorie intake, and
whether bitter compounds, other than quinine, are able to exert
similar effects. In conclusion, if our hypothesis will be confirmed,
we can speculate that the modulation of T2R activity could turn
out to be a novel therapeutic approach to over-nutritional diseases
and obesity.
Acknowledgements
The authors would like to express gratitude to Eleonora
Effice for her help in the administration of the capsule to volun-
teers and Rosanna Scala for her help with the preparation of the
manuscript.
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