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INTRODUCTION
The Oral gustatory perception during a meal ful-
fills very important physiological roles in food selec-
tion, subsequently oral functions such as salivation,
which essential for mastication, and swallowing, and
nutrient availability through pre-ingestive responses
as a cephalic phase. From time immemorial, human
beings have used their sense of taste to identify
which foods are good to eat in order to ensure sur-
vival. Each taste portray a different meaning for
the selection of food. Therefore, in the field of taste
physiology research, each sensation of the five ba-
sic tastes (saltiness, sweetness, sourness, bitterness
and umami taste) are thought to have a physiologi-
cal meaning to ingest and maintain each nutrient
homeostasis. A sweet taste indicates that a foodstuff
contains carbohydrates. Sourness tells us whether
a fruit or a vegetable is ripe. Bitterness warns us of
the presence of poisonous substances, and saltiness
helps us to choose foods that contain sufficient min-
erals, particularly sodium ion. Umami taste indicates
the presence of amino acids, i.e. proteins. We can
distinguish a slight change in each nutrient com-
position among daily meals by perception for soluble
taste materials of foods. Purified carbohydrates or
proteins are generally tasteless due to their high
molecular sizes.
Cephalic-phase responses triggered by sensory
properties of food shape the coordinated response
of an incoming nutrient load and enhance food di-
gestion, nutrient absorption and utilization (1). Par-
ticular attention has focused on the cephalic-phase
of exocrine (gastric and pancreatic juices) and en-
docrine (insulin) secretions because of their cru-
cial roles in regulating food digestion and limiting
postabsorptive hyperglycemia (2). Umami as well
as sweet taste stimulation evokes an increase of gas-
tric juice section (pepsinogen and gastric acid) and
pancreatic juice secretion in animals (3, 4).
MINI-REVIEW
Contribution of umami taste substances in human sali-
vation during meal
Hisayuki Uneyama, Misako Kawai, Yuki Sekine-Hayakawa, and Kunio Torii
Physiology and Nutrition Group, Institute of Life Sciences, Ajinomoto Co., Inc., Kawasaki, Japan
Abstract : The oral gustatory perception during a meal has very important physiological
roles such as inducing appetite, smoothing mastication and swallowing, promoting di-
gestion and each nutrient availability. One hundred years ago, L- glutamate was discov-
ered as a new taste substance in Japan. Since then, Japanese taste physiologists have lead
the research to establish L-glutamate as the prototype molecule for the fifth basic taste
(umami taste), in addition to saltiness, sweetness, bitterness and sourness. Meanwhile,
various lines of evidence demonstrated that taste perception is linked to taste stimuli -
oral/pharyngeal reflexes. In this review, we focus on the efficacy of L-glutamate for hu-
man salivation and discuss the possible application of umami taste simulation to the nu-
tritional management for the elderly due to amelioration of their quality of life (QOL).
J. Med. Invest. 56 Suppl. : 197-204, December, 2009
Keywords :dietary L-glutamate, umami taste, human salivation, elderly nutrition.
Received for publication November 19, 2009 ; accepted Novem-
ber 26, 2009.
Address correspondence and reprint requests to Kunio Torii,
D.V.M., Ph.D., Physiology and Nutrition Group, Institute of Life
Sciences, Ajinomoto Co., Inc., Suzuki-cho 1 -1, Kawasaki-ku,
Kawasaki 210- 8681, Japan and Fax : +81 -44 -210 -5893.
The Journal of Medical Investigation Vol. 56 Supplement 2009
197
One hundred years ago, the new taste molecule,
umami taste substance (L-glutamate), was discov-
ered in Japan from the extract of dried sea tangle.
Since then, Japanese taste physiologists have lead
the research that establish L-glutamate as the fifth
basic taste (umami) (5). Meanwhile, many lines of
evidence demonstrated that there is a taste stimuli-
oral/pharyngeal reflex with taste perception. Now
we just began to apply the effect of taste stimulation
to improve oral functions, such as salivation, mas-
tication and swallowing. In this paper, we would
like to review recent clinical advancements on be-
tween umami taste sensation and human salivation.
We will discuss the importance of taste stimulation
during a meal to maintain nutrient homeostasis due
to our health, and propose the possible applications
of umami taste substances, particularly L-glutamate
for the nutritional management in the elderly.
UMAMI TASTE SUBSTANCES IN FOODS
Extracts of seaweed (konbu) have been used as
a soup stock for over a thousand years in Japan. In
1908, moved by the desire to improve the nutritional
status of Japanese, Dr. Kikunae Ikeda at the Tokyo
imperial university discovered that salts of the amino
acid L-glutamic acid were the taste -active compo-
nents of the konbu stock, and named this taste the
umami taste (6). He also invented the production
process for the monosodium salt of L- glutamic acid
(monosodium glutamate : MSG), which consisted
on the hydrolysis of wheat gluten with HCl. Aftter
his discovery and invention, MSG became commer-
cially available as an umami seasoning for the first
time in the world. In recent, it was appreciated that
the molecular discovery of umami receptors in the
taste buds established umami taste as one of the
five basic tastes, distinct from the other basic tastes
such as saltiness, bitterness, sourness and sweet-
ness. Even though it was only recently recognized,
many foods and seasonings popular throughout his-
tory have appeared to be naturally high in umami
taste materials. One such flavoring agent used in
ancient Greece and Rome was a pickled fish sauce
called Garum. This condiment dates back 2500
years, making it the oldest recorded umami season-
ing. Various traditional seasonings around the world,
such as soy and oyster sauce, tomato ketchup, nam
plaaandothersaswellasfoodingredientssuchas
tomatoes and cheese have been found to be rich
sources of free glutamate, and L- glutamate in amino
acid form contributes to the deliciousness of foods
(7, 8).
In 1913, Shintaro Kodama, a colleague of Kikunae
Ikeda, examined the components of katsuo-bushi
(dried bonito) and reported that 5’-inosinate was
also involved in umami taste (9). Many years later,
during a study of ribonucleotide production through
biochemical degradation of yeast RNA, Dr. Akira
Kuninaka, another Japanese scientist, identified 5’-
guanylate to be one more important umami sub-
stance (10). This nucleotide is naturally present in
a soup stock “dashi”stuff and comes from dried
Japanese black mushroom, shiitake.Now,theseL-
glutamate and 5’-ribonucleotides are considered
typical umami taste substances (Fig. 1).
The Ministry of Health and Welfare in Japan (the
Ministry of Health, Labor and Welfare) reported a
daily free L-glutamate intake for people over 20
years of over 1.6 g (12). We wanted to know the
real content of free glutamate to evaluate the prac-
tical free glutamate intake through daily meals.
We measured free amino acid contents in a typical
Japanese lunch box as shown in figure 2. Interest-
ingly, the concentration of total free amino acids in
this lunch was about 1,500 mg from which only 450
mg was free glutamate. Obviously, the total daily
ingestion of free L-glutamate has to be divided into
three meals through breakfast, lunch and dinner.
Fig. 1 Discoveryofmajorthreeumamitastesubstances
The amino acid L- glutamate is the major umami taste component
in Konbu (Japanese sea kelp). 5’- nucloeotides, 5’- inosinate and
5’-guanylate, are major umami taste substance in Katsuo-bushi
(dried bonito) and shiitake (dried Japanese mushroom), respec-
tively. Those three umami substances are commercially available
as monosodium L-glutamate (MSG), 5’- inosine monophosphate
(IMP) and 5’-guanosine monophosphate (GMP). Figure was
quoted from Uneyma & Yamada (11).
H. Uneyama, et al. Umami taste and human salivary secretion.
198
SENSORY PERCEPTION OF UMAMI TASTE
IN THE ELDERLY
The nutritional management of the elderly, due
to the maintenance of their appetite, mastication,
swallowing, food digestion and each nutrient absorp-
tion, is very important to regulate their body nutri-
tional status during their daily meals. Taste is a fun-
damental sensory system because it regulates food
selection, the hedonic and sensory experience of
food, and related metabolism efficiency. At the end
of all meals, these factors help to preserve a good
quality of life in people, especially the elderly. Taste
also regulates fundamental physiological functions
through cephalic phase reflex, such as exocrine (sa-
liva, gastric and pancreatic juices) and endocrine
(gut hormones) activities during meals. Neck can-
cer patients treated with radiation therapy show
sometimes taste dysfunction including umami taste
loss and umami insensitivity. Among the five basic
tastes, perception of umami taste dysfunctions in
patients is the one presents the strongest co - rela-
tionship with appetite loss (14), suggesting that
umami taste is an important taste quality for appe-
tite in humans. Thinking together, umami taste
stimulation is also expected to normalize oral and
gut functions, when some retardation occurs.
There are several reports describing the taste
threshold of MSG in the old people of the Western
world (15, 16), whereas there is little knowledge
rather than them in Eastern one. Thus, we exam-
ined that relationship between umami taste sensitiv-
ity and preference in the Japanese elderly (Fig. 3).
In this study, we used rice gruel as taste medium,
because it is a familiar ingredient for Japanese
elderly who are used to combine gruel with tradi-
tional savory pickles. As a result, the taste thresh-
old for MSG in these old subjects was 0.5% (39
Fig. 2. Amino acid contents in a typical Japanese lunch box.
Amino acids were measured by an amino acid analyzer in hot -water soluble components. Data were quoted from Uneyama (13).
Fig. 3. Umami taste sensitivity and preference of Japanese
elderly
Two - alternative forced choice test was performed using 0.2% NaCl
containing rice gruel as medium in old women (n=39, 84 !
6.1
year-old). The ratio of correct answer (filled circle) means the
ratio of subjects who correctly chose the gruel with MSG as the
stronger taste comparing gruels with and without MSG. For the
calculation of the preference ratio (open triangle), the preference
in subjects that gave the wrong answer in the sensitivity test was
ignored. Asterisks show statistical significance by binomial test
(* : p!
0.05, *** : p!
0.001). Data were modified from Hayakawa
et al. (17).
The Journal of Medical Investigation Vol. 56 Supplement December 2009 199
women, 84.3!
6.1 year -old), higher than those for
the middle-aged adults (40 women, 49.6!
5.6 year-
old) in which was less than 0,063%. Accordingly,
the optimal preferred concentration of MSG in old
Japanese was around 0.5 % showing a bell-shaped
concentration-preference curve (17). Thus, as ob-
served in other gustatory perceptions, the amount
of ingested L-glutamate seems to be self -limiting
by the intensity of umami taste in food.
UMAMI TASTE STIMULATION IMPROVES
SALIVA SECRETION IN THE ELDERLY
Saliva has many essential functions as shown in
the figure 4. It is the first digestive juice in the ali-
mentary canal and is secreted in response to food
assisting mastication, swallowing and initiation of
the oral digestion of starch and lipids (18). During
this process, saliva acts as a solvent of taste sub-
stances allowing the sensation of taste substances
in variety of foods. Clinically, the most important
role of saliva is the formation of the food bolus and
maintenance of oral health, including the protection
of teeth and mucosa from infections, maintenance
of the milieu of taste receptors and communication
through speech.
Beyond oral health, considerable evidence now
demonstrates that saliva and its components have
multiple functions in the esophageal and gastric
mucosa. Saliva aids the formation of the bolus ; it
lubricates, protects and cleanses the pharyngeal and
esophageal mucosa. Salivary bicarbonate buffers
esophageal acid in common reflux. Salivary epider-
mal growth factor (EGF) stimulates gastrointestinal
(GI) mucosal proliferation via a direct lumenal ef-
fect in the esophagus and stomach (20). Normal
salivary flow decreases the duration of acid contact
with esophageal mucosa, an important factor in the
development of gastro- esophageal reflex disease
(GERD). For instance, the esophageal mucosal bar-
rier is significantly enhanced by the quantity and
the quality of salivary organic components such as
salivary mucin, nonmucin protein, salivary EGF and
salivary prostaglandin E2(21). If salivary flow is de-
pressed or if the esophago- salivary reflex is lost, a
patient may be predisposed to develop GERD.
Ingestion of palatable foods yields the greatest
pleasures in life in general. In the elderly it is most
important to satisfy their appetite and maintain their
dignity and quality of life (QOL). Since the normal
physiological functions in the elderly are compro-
mised and often take various drugs, their taste sensi-
tivity, salivary secretion, chewing and swallowing,
Fig. 4. Physiological functions of saliva
Saliva has many essential functions regulating appetite, mastication, swallowing, digestion, and maintenance of the gastrointestinal
mucosa. The figure was modified from ref. 19. See the text for details.
H. Uneyama, et al. Umami taste and human salivary secretion.
200
as well as the sanitary condition of the oral cavity,
are often prone to deterioration. The effectiveness
of umami taste se sensation for salivation in humans
was first reported by Dr. Kawamura and colleagues
from Osaka university, Japan (22). Recently, it has
been re-confirmed that a typical umami taste ma-
terial (MSG) at most preferable concentration (0.5%)
induces salivary secretion in the healthy adults (23).
We investigated the temporal pattern of MSG-in-
duced salivation in the healthy adults (Fig. 5). The
time-course of salivation after the gustatory stimu-
lation with umami taste (MSG) was compared with
the effect of sour taste (citric acid) at the same in-
tensity. The reason why we used sourness as a con-
trol taste stimuli is that sourness is often used for
promotion of salivary secretion in hospital for the
oral care. Interestingly, sour taste was a strong sali-
vary inducer but during a short period of time
(within 2 min after taste stimulation in our condi-
tion). In contrast, umami taste sensation induced a
long-lastingsalivarysecretion(morethan10min).
Therefore, the total amount of saliva induced by
umami taste (MSG) stimulation became significantly
larger than the case of sour taste stimulation (24).
As considering the function of saliva in mastication,
swallowing and cleanness of the oral cavity, this
feature of umami taste seems to be important for
normal digestion. The use of umami taste materials
in oral care might be helpful to maintain the oral
mucosal integrity in patients with dry mouth. In-
deed, an effective salivation with umami taste stimu-
lation was observed in the elderly with otherwise
hyposalivation (25). Shiffman reported previously
that the supplementation of foods with MSG im-
proved salivary flow and increased the total amount
of secreted IgA in the elderly (25) (Fig. 6). As
Fig. 6. Effects of umami taste stimulation on the salivary IgA in the elderly.
Secretion rate of sIgA (left), and concentration of sIgA (right) of collected saliva secreted were shown after taking chicken soup with
(black bar) and without (white bar) MSG. Each column and vertical bar indicates mean!
s.e.m., respectively. Data were quoted from
Schiffman et al. (25).
Fig. 5. Effects of umami taste stimulation on human salivation.
After subjects tasted 3 ml of each stimulus solution for 30 sec, then they spat content in their mouth at every 30 sec for 10 min (n =
24). The weight of the content except stimulus solution was regarded as the weight of secreted saliva. Salivary flow per min (left)
and total saliva for 10 min (right) were presented with mean!
s.e.m.. In the graph for salivary flow, same symbol in each time
means that there is no significant difference. Data was modified from Hayakawa et al. (24).
The Journal of Medical Investigation Vol. 56 Supplement December 2009 201
mentioned before, IgA is one of key molecules to
maintain the mucosal environment as a barrier sys-
tem against bacterial contamination in the oral cav-
ity. Thus, umami-fortified meals might contribute
to the oral care management in the elderly.
POSSIBLE CONTRIBUTION OF UMAMI
TASTE SUBSTANCES IN THE ELDERLY
QOL
Recent animal evidence using L- glutamate indi-
cate that dietary free L- glutamate is sensed by the
alimentary tract and helps gastric digestion, nutrient
absorption and utilization via visceral glutamate in-
formation through vagal afferent pathways (26- 29).
Altogether, umami taste substance, especially MSG,
might be used for its properties in gustatory and
visceral perception to improve the management of
nutritional stasus (Fig. 7). We ingest certain amount
of free L-glutamate, which is a residue of peptides
and proteins, every day through our daily meals be-
cause a great variety of foods (vegetables, meats
and traditional seasonings) contain free L- glutamate.
During ingestion, free glutamate is sensed by the
umami taste receptors on the tongue and the umami
sensation to be yielded, which is thought to indicate
protein intake. The specific appetite for proteins
allows maintaining the body protein (L-amino acid)
homeostasis. At the same time, the sensation of
umami taste induces taste reflexes such as salivary
secretion that is required for mastication and swal-
lowing of meals, and the cephalic phase responses
to prepare the gastrointestinal tract for protein di-
gestion. At the same time, saliva keeps the oral hy-
giene after each meal. After swallowing the food bo-
lus, the free glutamate in the bolus is sensed again
by the visceral afferents that innervate the gastro-
intestinal tract. Glutamate-mediated visceral nutri-
ent information might increase the efficiency of pro-
tein digestion, absorption and utility within the body,
by stimulating the gut function. Free glutamate it-
self is consumed in the mucosa as energy during
protein digestion. The visceral sensation through
post-ingestive process is expected to control the
appetite for protein intake and the visceral afferent
inputs to the brain stem can contribute to the main-
tenance of the basal brain activity. The input of pe-
ripheral sensory stimuli plays very important role
for the activation of brain circuits. Old people has
special difficulties to maintain a proper oral and gut
function, which reduces their QOL with diseases
such as anorexia, aphasia, diarrhea, constipation and
memory dysfunction. Umami taste substances have
the potential to improve these conditions in the eld-
erly with some disorders of GI functions. After MSG
fortification trials in hospitalized elderly, some of the
expected outcomes were reconfirmed (improve the
redox status of plasma albumin, immunity and con-
sciousness) (30-32).
Fig. 7. A new hypothesis based on the scientific evidence for the umami taste substance glutamate in nutritional management.
Scientific evidence for contributing the nutritional management for the elderly was summarized. See text for details.
H. Uneyama, et al. Umami taste and human salivary secretion.
202
CONCLUSIONS
The elderly usually suffers from hyposalivation
due to a reduction of sensory perception such as
taste and smell. Compared to other taste stimuli
such as sourness, umami taste induces long - lasting
salivation in humans. Recently, Sato-Kuriwada and
colleagues reported that oral stimulation with MSG
increased salivary flow in minor salivary glands (33).
It is essential to encourage the ingestion of food in
bedridden old people. With the proper nutritional
management the appetite, mastication, and swallow-
ing can improve in this elderly subjects. A better
understanding of the umami taste physiology espe-
cially in taste reflex will help develop new methods
or new treatments for eating-related disorders such
as disgeusia, dysphagia, dry mouth, and anorexia.
ACKNOWLEDGEMENT
The authors thank for Dr. Ana San Gabriel for
helpful assistance to prepare this review.
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