Assignment 2- Research Paper Summary and Critique
Humans Can Taste Glucose Oligomers Independent of the hT1R2/hT1R3 Sweet Taste Receptor
Authors: Trina J. Lapis, Michael H. Penner and Juyun Lim
Summary
Introduction
The
main function of taste is to identify substances in the oral cavity, including
food which provides energy or harmful toxins. Currently there are 5 generally
excepted tastes; sweet, sour, bitter, salty and umami. Each flavour is known to
use its own receptors and transduction pathways. Umami uses G-protein coupled
receptors (T1R family) and metabotropic glutamate receptors, bitter uses G
protein coupled receptors (T2R family), sour uses cyclic nucleotide-gated
channels (HCNs), acid sensing ion channels (ASICS), and transient receptor
potential channels (PDK2L1 and PKD2L3), and salty uses epithelial sodium channels
(ENaC). The flavour with the most focus in this research is sweet, which
utilizes G protein coupled receptors T1R2 and T1R3 (hT1R2/hT1R3 in humans). Recently
the idea of the 5 main flavours has been changing with evidence towards flavour
detecting of chemicals such as calcium, fat and starch products.
It
would be beneficial to animals and humans if starch had its own flavour and
could be detected (due to the nutritional value), but it is unlikely as starch
is a very long polymeric molecule. However, it is possible for starch hydrolysis
products to be detected in the oral cavity after salivary amylase hydrolyzes it
into simple sugars and oligomers. Previous studies have demonstrated the
ability of humans to detect maltodextrin (polymer mixture) independent of sweet
taste and rats to detect polycose (mixture of glucose oligomers and polymers)
and differentiate it from sucrose. However, all previous studies have used
mixtures and have not used accurate controlling factors to ensure accurate
results.
This
experiment separated maltodextrin into 3 groups based on their degree of
polymerization (number of monomers in the chain). Sample 1 (S1) had a DP of 7
(oligomer), sample 2 (S2) had a DP of 14 (oligomer), and sample 3 (S3) had a DP
of 44 (polymer). The objectives of the study were to determine the lengths of
glucose chains that can be detected, investigate a potential gustatory mechanism,
establish glucose oligomer taste qualities, and create a dose-response curve
for glucose oligomers. Each objective was the focus of one experiment in a
series of 4 completed during this study.
Materials and Methods/Results
Experiment 1A – Taste Discrimination of Glucose Oligomer and
Polymer Stimuli as equivalent % w/v solutions
It was
required to stop the hydrolysis of the oligomers and polymers by alpha-amylase
into simple sugars in order to get accurate results. In order to do this, an
alpha-amylase inhibitor (acarbose) was used. Pilot experiments were completed
to ensure that acarbose truly inhibited alpha-amylase (in average salivary
concentrations, 5mM) when S1, S2, and S3 were present. This
test was completed to determine what samples were able to be detected by human
gustation, and it was predicted that S1 would have the greatest detection rate and
S3 would have the lowest detection rate. 22 subjects (male and females) with an average age of 25
were used. The criteria for entering the experiment were that they were
non-smokers, not pregnant, not medication takers, no history of smell or taste
loss, no past oral disorders, no oral piercings, no recent dental work, no
alcohol consumption in 12h, no food or beverage containing dairy within 4h, no
food or beverage at all in 1h, and no products containing menthol within 1h. Solutions
of 6% and 8% (w/v) were created, stored at 4-6 degrees Celsius, and returned to
room temperature before testing. Each subject was put through 2 sessions (one
6% and one 8%) on different random days. The test consisted of 3 sip-and-spit
tests (2 controls and 1 sample), where they sampled all 3 and had to identify
the one with different flavours. In order to prevent olfactory input, they wore
nose clips as well. This continued 3 times (one for each sample) and they
rinsed with water in between, and the orders were counterbalanced and
randomized. The
results were added by number of correct identifications, which were
statistically converted to d' values that accurately represented the stimulus
and subtracted background results (noise). The prediction was shown to be accurate
as the subjects could distinguish S1 and S2 from blanks, but not S3. d' values
were also higher for S1 than S2, and were higher for the 8% samples than the 6%
in both cases.
Experiment 1B- Taste discrimination of glucose oligomer and
polymer stimuli as equivalent mM solutions
Experiment 2- Taste Discrimination of sugars and glucose
oligomers in the absence and presence of lactisole
It was
also important to test if similar mechanisms of detecting simple sugars were used to
detect glucose oligomers. In order to do this a chemical called lactisole was
used (tasteless, sweet taste blocker that binds to hT1R3). 25 subjects were
used with an average age of 25, with the same criteria as previous experiments.
S1 and S2 samples were used, along with glucose, maltose and sucralose
(artificial sweetener) to test for lactisole effects. Acarbose was added, and
all target and blank stimuli were prepared with and without lactisole. 5
stimuli were provided, with and without lactisole (total of 10 tests), and
pseudo-randomization was used as it was impractical to do all the possible
combinations. The same cotton swab triangle tests were completed exactly as
before, and results were analysed into d’ values. All 5 stimuli were equally
detected in the absence of lactisole, but when lactisole was present it blocked
the taste of glucose, maltose and sucralose. However, lactisole did not
compromise the ability to detect the glucose oligomers.
Experiment 3- Determination of taste quality of glucose
oligomer through a focus group discussion
This
experiment was aimed at subjectively describing the taste quality of the
glucose oligomers, which had already been established as using different
mechanisms of detection in experiment 2. 7 subjects who had already participated in one of the
studies were used. Different amounts of equally intense aqueous solutions of
sucrose, sucralose and S2 were provided using the cotton swab technique. In a focus
group the subjects tasted the unknown stimuli and were asked to describe the
flavour and, as a group, come up with a one word descriptor for the taste they
were experiencing. It was seen that sucrose was “sweet” like sugar water,
sucralose was “sweet” like artificial sweetener, and the glucose oligomer was “starchy”
like a root vegetable, corn, bread, or pasta. Sucrose and sucralose were also
identified as being much more similar than either sucrose or sucralose to the
S2 oligomer.
Experiment 4- Establishing dose-response curves for sugars
and glucose oligomers
Once
the new flavour was characterized, it was of interest to create a dose-response
curve which could be compared to natural sweeteners. 20 subjects (mean age of
25) were recruited again with the same criteria. Sucrose, glucose and S2 were
provided at 3 concentrations (45, 100, and 224mM) with acarbose present in all.
The cotton swab technique was used again, and the subjects were asked to rate
the intensity of a general version of the Labeled Magnitude Scale (gLMS) which
they were previously trained to use. The data was plotted on a dose response
curve based on molar and %w/v concentrations. These results showed that glucose
and the glucose oligomer had almost indistinguishable curves based on moles,
however, when using %w/v ratios, it was seen that glucose oligomers were
shifted to the right of the glucose curve.
Discussion
Based
on the results from experiment 1, it was seen that humans can discriminate
glucose oligomers from water, but not glucose polymers from water. It is
important to note that taste is not the only sense responsible for detection of
glucose oligomers, although the other sense were controlled in these experiments (nose clips to reduce
olfactory clues and similar textures to reduce somatosensory clues). It is also important to
note that the breakdown of starch and glucose oligomers to simple sugars can
contribute to the recognition of glucose oligomers in vivo, but this was
constantly controlled for in every experiment using acarbose. A conclusion can
be made that, in the absence of confounding effects, glucose oligomers can be
sensed by the human gustatory system. Although these results conflict with
previous studies which stated that rats can taste glucose polymers, the previous studies
did not control for glucose polymer hydrolysis.
Based
on the results from experiment 2, it was seen that subjects lost the ability to
taste glucose, maltose, and sucralose, in the presence of lactisole but did not
lose the ability to taste glucose oligomers. As lactisole (which blocks T1R3
sweet receptors) did not stop the sensation of glucose polymers, it is clear
that glucose oligomers use a different taste receptor. These results are consistent
with previous finding where T1R2/T1R3 knockout mice could still taste glucose
oligomers.
These
findings were backed up by evidence of experiment 3, where subjects rated the
flavour of oligomers as “starchy”. In the past, other receptors have been
thought to be used for detection of sugars and glucose oligomers, such
as T1R-independent pathways. However, the findings in this study also disprove
this theory, and it is now thought that there is a completely novel receptor
used for starch hydrolysis products. The main function of this receptor is
likely to identify incoming starch which has begun being broken down by amylase
while entering the digestive system.
Personal Critique
This
paper did an excellent job at covering all of the bases when determining the
detection of starch hydrolysis products on the tongue. It reviewed many pieces
of previous work done on this topic, and improved upon them by adding more
controlling factors and doing multiple experiments to back up their theory. The
use of acarbose as an amylase blocker was what differentiated this paper from
the rest and seemed to produce more accurate effects and results. Experiment 1
was also repeated using different measurements in order to fully ensure that
the results they obtain were accurate. Using step by step experiments
(determining if humans could taste starts, and then determining if they could
taste them without sweet receptors) was a very intelligent thing to do. If they
relied on previous work, which stated that rats could taste glucose polymers,
it may have skewed the results when they blocked the sweet receptors with
lactisole. The experimenters also made it very clear that they completed a pilot study (ensures the chemicals were functioning accurately) whenever it was required. Although the results of this experiment were not
specific (they did not find a receptor responsible for glucose oligomer detection),
they did their best to lay down the groundwork for further research on this
topic and ruled out some possible paths of research.
In my opinion, experiments 1 and 2 were enough
information to produce a paper on their own. The research completed in
experiments 3 and 4 seemed unnecessary and generally not important to the main
idea of the paper. Experiment 3 produced subjective flavour characteristics of
the glucose oligomers which were deemed “starchy”, however this idea had
already been demonstrated in the first 2 experiments. Experiment 4 created a
dose-response curve for the concentrations needed to detect glucose oligomers,
which could have been done in a separate paper, especially because the results
were not even discussed in the discussion section. One important thing to note about
the first 2 experiments are the small sample sizes that were used (about 25
people). Even though the data was statistically significant, it could have
increased the validity of the research if more subject were used in each
experiment. Other than these factors, I think the paper was well written and
organized in a way that made sense. The results they obtained could open up a
door into more research about flavour detection of not only glucose oligomers,
but many other potential chemicals.
Reference
Lapis, T. J., Penner, M. H., & Lim, J. (2016). Humans Can Taste
Glucose Oligomers Independent of the hT1R2/hT1R3 Sweet Taste Receptor. Chemical Senses, 41(9), 755-762. doi:10.1093/chemse/bjw088
Note: All Figures and Images in this section were retrieved from this paper which can be found here.
