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Tryptophan (Trp) is an amino acid and an essential component of the human diet. It plays a crucial role in many metabolic functions. Clinicians can use Trp levels in the course of diagnosing various metabolic disorders and the symptoms associated with those diseases. Furthermore, supplementation with this amino acid is considered in the treatment of depression and sleep disorders, mainly due to the Trp relationship with the synthesis of serotonin (5-HT) and melatonin. It is also used in helping to resolve cognitive disorders, anxiety, or neurodegenerative diseases. Reduced secretion of serotonin is associated with autism spectrum disorder, obesity, anorexia and bulimia nervosa, and other diseases presenting peripherals symptoms. The literature strongly suggests that Trp has a significant role in the correct functionality of the brain-gut axis and immunology. This information leads to the consideration of Trp as an essential dietary component due to its role in the serotonin pathway. A reduced availability of Trp in diet and nutraceutical supplementation should be considered with greater concern than one might expect. This paper constitutes a review of the more salient aspects gleaned from the current knowledge base about the role of Trp in diseases, associated nutritional disorders, and food science, in general.
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How important is tryptophan in human health?
Joanna Ka»u_
zna-Czapli
nska
a
, Paulina G ˛atarek
a
, Salvatore Chirumbolo
b
, Max Stanley Chartrand
c
,
and Geir Bjørklund
d
a
Department of Chemistry, Institute of General and Ecological Chemistry, Lodz University of Technology, Lodz, Poland;
b
Department of Neurological
and Movement Sciences, University of Verona, Italy;
c
DigiCare Behavioral Research, Casa Grande, AZ, USA;
d
Council for Nutritional and Environmental
Medicine, Mo i Rana, Norway
ABSTRACT
Tryptophan (Trp) is an amino acid and an essential component of the human diet. It plays a crucial role in
many metabolic functions. Clinicians can use Trp levels in the course of diagnosing various metabolic
disorders and the symptoms associated with those diseases. Furthermore, supplementation with this
amino acid is considered in the treatment of depression and sleep disorders, mainly due to the Trp
relationship with the synthesis of serotonin (5-HT) and melatonin. It is also used in helping to resolve
cognitive disorders, anxiety, or neurodegenerative diseases. Reduced secretion of serotonin is associated
with autism spectrum disorder, obesity, anorexia and bulimia nervosa, and other diseases presenting
peripherals symptoms. The literature strongly suggests that Trp has a signicant role in the correct
functionality of the brain-gut axis and immunology. This information leads to the consideration of Trp as
an essential dietary component due to its role in the serotonin pathway. A reduced availability of Trp in
diet and nutraceutical supplementation should be considered with greater concern than one might
expect. This paper constitutes a review of the more salient aspects gleaned from the current knowledge
base about the role of Trp in diseases, associated nutritional disorders, and food science, in general.
KEYWORDS
Tryptophan; serotonin;
metabolic disorder;
neurodegenerative disease;
food technology; human diet
Introduction
The role of the essential amino acid tryptophan (Trp) is gaining
in interest relative to dietary and nutritional sciences. Recent
research has demonstrated that this amino acid exerts a protec-
tive action in the intestine, as it contributes to the enhanced
expression of the tight junction proteins claudin-3 and zonula
occludens (ZO-1) in the jejunum of experimental animals (Liu
et al., 2017). Its fundamental importance appears mostly in its
relationship with serotonin, which is important in food-to-
nutrition synthesis. Past research has suggested a direct connec-
tion between serotonin production and the available circulating
Trp, recently proposed as a hallmark as a possible marker of
psychiatric serotonin-related disorders (Comai et al., 2016).
From a food technology viewpoint, the importance of Trp in
human physiology would suggest recommendations and guide-
lines by worldwide experts to supplement or enrich foods with
Trp. Due to the complexity of the relationship of serotonin and
melatonin and circadian rhythms, its hard to ascertain or attri-
bute the needed levels of Trp in a given diet (Hulsken et al.,
2013; Silva et al., 2017).
However, several researchers have suggested that Trp sup-
plementation in a daily diet might improve pharmacotherapy
in some diseases (Figure 1). Because of the tryptophan has
comparatively low tissue storage and their concentration in the
body is low, compared to other amino acids, for healthy nutri-
tion are needed only small amounts (Richard et al., 2009).
Some food products containing tryptophan are presented in
Table 1(Rambali et al., 2002; Richard et al., 2009; USDA Food
Composition Databases, 2017). The recommended daily dose
for adults is estimated to be between 250 mg and 425 mg, which
results in a dietary intake of 3.5 to 6.0 mg/kg of body weight per
day (Richard et al., 2009).
Tryptophan is an essential component of the diet. It plays a
key role in protein synthesis, and is a precursor of biologically
active compounds such as serotonin, melatonin, quinolinic
acid, kynurenic acid, tryptamine, and also coenzymes impor-
tant for electron transfer reaction (redox balance of metabo-
lism), such as nicotinamide adenine dinucleotide (NADC).
This compound, which are nal product tryptophan metabo-
lism, might be produced from ingested tryptophan but also
vitamin B3 (niacin) (Richard et al., 2009; de Figueiredo et al.,
2011; Palego et al., 2016). To people health is detrimental both
deciency and excessive intake. Tryptophan has been used to
treat variety disorders, but in most countries has been with-
drawn. During the treatment of tryptophan preparations have
been observed undesirable symptoms including a variety of
pulmonary, cutaneous, and neurologic symptoms, and also
eosinophilia-myalgia syndrome, and disease associated with
muscle pain. Many different diseases and disorders have been
linked with tryptophan and its metabolites (Table 2). An
increased metabolism of Trp, or adverse effects of low Trp such
as decreased absorption or intake, have been observed in differ-
ent types of pathology. It should be mention disease and disor-
ders such as premenstrual syndrome (PMS) (tryptophan plays
CONTACT Joanna Ka»u_
zna-Czapli
nska joanna.kaluzna-czaplinska@p.lodz.pl Department of Chemistry, Institute of General and Ecological Chemistry, Lodz Uni-
versity of Technology, Zeromskiego 116, 90924 Lodz, Poland.
© 2017 Taylor & Francis Group, LLC
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a role in increased activation of Trp catabolism), pellagra
(caused by a deciency of niacin which precursor is Trp),
chronic kidney disease (is observed alterations in Trp metabo-
lism in the case of kynurenine pathway) (Karu et al., 2016), coe-
liac disease (availability of Trp to the brain is low, especially in
subjects with depression), Parkinsons disease (large neutral
amino acid compete with Trp), mental disorders (reduced
availability of Trp what is consequence is low level of serotonin)
(Sainio, Pulkki, and Young, 1996; Russo et al., 2009), sleep dis-
orders (abnormal level of melatonin, which is synthesize from
serotonin) (Kaczor and Skalski, 2016), schizophrenia (dysfunc-
tional serotonin transmission), bulimia and anorexia (depletion
of Trp) (Sainio, Pulkki, and Young, 1996; Russo et al., 2009).
It is well known that Trp is only available through the die-
tary process, as its precursors allow gut microora to synthesize
the essential amino acid in humans. Tryptophan can be metab-
olized through the methoxyindole and kynurenine pathways.
The kynurenine pathway, which takes up about 95% of the bio-
logically available Trp, is controlled by the rate-limiting
enzymes indoleamine 2,3-dioxygenase (IDO) and Trp 2,3-diox-
ygenase (TDO).
Stress hormones and Trp induce TDO synthesis and activa-
tion, while IDO can be induced by pro-inammatory, inter-
feron-gamma (IFN-g), tumor necrosis factor-alpha (TNF-a),
and Th-1 type cytokines, i.e. during an innate response of the
immune system. IDO suppresses the activity causing the induc-
tion of TDO, and vice versa, while the ratio of kynurenine
(products) to Trp (substrate) gives information about IDO
activity. Upregulation of the IDO activity caused by chronic
inammation of the immune system could be a major factor in
the initiation and propagation of obesity and associated meta-
bolic syndrome (Mangge et al., 2014).
Tryptophan supplementation could promote synthesis and
neurotransmission of serotonin. Moreover, it may be effective
in treating disorders of serotonin deciency by increasing the
precursor for 5-HT synthesis and normalizing its release
(Haleem, 2012). Levels of Trp, and hence its circulating bio-
availability, does not seem to be directly linked to cognition
and mood improvements, as recent reports suggest that excess
of Trp impairs cognition, rather than improving it (Hulsken
et al., 2013). On the other side, a deciency in Trp, caused by
malnutrition, may affect the central and peripheral serotoniner-
gic pathways, although further nutrition-derived hormonal
molecules may rescue some of this deciency (Patrick and
Ames, 2014).
With serotonergic dysfunction have been associated with
symptoms of panic, depression, aggression and suicidality.
Because the serotonin system is involved in the various psychi-
atric disorders, but not only, serotonin system is also involved
in the regulation of satiety, it can be concluded that the activity
of serotonin can be important in the pathophysiology of eating
disorders such as anorexia nervosa. A fundamental concern for
nutritionists and food technologists is to focus on the role of
Trp in neurological and immune disorders, to achieve a deep
awareness and knowledge of the risks and potentials associated
with the supplementation use of this amino acid. The aim of
this paper is based on the currently existing and very recent lit-
erature to present a more focused viewpoint relative to the role
of Trp in diseases associated with human nutritional disorders.
1. Tryptophan and irritable bowel syndrome
One of the most common alimentary tract illnesses in humans
is irritable bowel syndrome (IBS). This disease also constitutes
a signicant social problem. IBS is a bowel function disorder.
Pain associated with defecation and defecation frequency or
stool consistency characterizes this disease. Still unclear is etio-
pathology of the illness. Pathological factors include, among
others, disturbances in the functions of serotonin at this level of
Figure 1. Tryptophan and different diseases.
Table 1. Tryptophan amount per 100 g in common foods.
Tryptophan (mg)
milk 42
eggs 165
wheat our 110
sausage 93
potato 28
chees 325
beef 230
banana 10
soybeans 160
bread, oat bran, toasted 140
chia seeds, dried 440
chicken, breast, skinless, boneless, meat only 400
cocoa 290
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the digestive process (
_
Zelowski et al., 2013). IBS is a functional
gastrointestinal disease, because these disorders arise from dys-
function of the organ, excluding morphological changes within
it (Fitzgerald et al., 2008).
There are four clinical forms of this syndrome: IBS with
constipation (IBS-C), IBS with diarrhea (IBS-D), mixed IBS
(IBS-M) and unsubtyped IBS (Longstreth et al., 2006).
Throughout the world, about 1020% of adolescents and adults
exhibit symptoms associated with IBS (Fitzgerald et al., 2008).
Irritable bowel syndrome is more common in women than in
men. The disease develops most often in the third decade of life
(Drossman et al., 2002). The factors causing this disease
include, among others, environmental, genetic, depression,
inammatory predispositions, and chronic stress (Fitzgerald
et al., 2008;
_
Zelowski et al., 2013).
Fitzgerald et al. (2008) examined patients with IBS and
healthy, of comparable age and body mass index (BMI) and
sex-matched controls. They observed the higher concentration
of kynurenine in the blood of patients with IBS in comparison
with the control group and positive correlation between the
kynurenine/tryptophan (Kyn/Trp) ratio and IBS symptom
severity (Fitzgerald et al., 2008).
Keszthelyi et al. (2012) demonstrated a relationship between
the amount of serotonin, synthesized in the brain, and the
amount of Trp supplied to the body with diet. Their random-
ized placebo-controlled study suggested that the sudden short-
age of the precursor of serotonin obtained by administering an
amino acid-enriched beverage lacking Trp to patients resulted
in the dramatic reduction of the concentration of serotonin in
the blood, as well as, a decrease in the level of 5-hydroxyindole-
acetic acid in urine. In contrast, there was no change in the con-
centration of these compounds in the intestinal mucosa. The
researchers concluded that 5-HT synthesis in the brain is highly
dependent on the availability of Trp in plasma, which is inu-
enced by the competitive uptake of other large neutral amino
acids (LNAAs) and Trp across the blood-brain barrier
(Keszthelyi, 2012).
Other scientists measured serum serotonin concentration in
individuals with irritable bowel syndrome, compared with con-
trol group, and also evaluated the urine concentration of 5-
hydroxyindole acetic acid (5-HIAA), which is a metabolite of
serotonin. They examined participants aged 1950 years,
including healthy subjects, patients with predominant constipa-
tion (IBS-C) and patients with predominant diarrhea (IBS-D).
The results pointed to a reduction of serotonin concentration
in patients with IBS-C and IBS-D. In all patients with IBS, a
decrease in urinary excretion of 5-hydroxyindole acetic acid
was observed. These results indicate that disturbed metabolism
of serotonin could play a role in the pathogenesis of functional
bowel diseases (Moskwa et al., 2007).
Studies conducted by Atkinson et al. (2006), showed that
patients with IBS-D (aged 1952 years) have a higher concen-
tration of serotonin in the blood than healthy patients, both in
the fasting and during the postprandial time. However, the dif-
ferences were not detected as IBS with constipation (IBS-C,
aged 1952 years). Food intake signicantly increased levels of
serotonin in the blood of patients with IBS with diarrhea, with
respect to healthy subjects (n D35, aged 1846 years). The
results assessed the concept that an impaired release might
characterize IBS-C, whereas IBS-D is characterized by reduced
serotonin reuptake (Atkinson et al., 2006).
Reports by Dunlop et al. (2005) indicated an increase in the
postprandial levels of serotonin in the blood of patients with a
diarrhea form of IBS (IBS-D) and a signicant reduction in
patients with constipation-predominant IBS (IBS-C), compared
to healthy controls (Dunlop et al., 2005). Dunlop et al. (2005)
examined 15 patients with IBS-D, 15 patients with IBS-C and
15 healthy control participants. This study compared postpran-
dial serotonin release and mucosal serotonin metabolism in
various types of IBS. The results demonstrated that patients
with IBS-C showed impaired postprandial serotonin release.
Houghton et al. (2003) examined 39 female patients with
IBS-D aged 1952 years, and 20 healthy females aged 20
46 years. Obtained data suggested that postprandial symptom-
atology could be connected with increased plasma serotonin
concentration in IBS-D patients (Houghton et al., 2003; Dunlop
et al., 2005). Furthermore, sleep disorders are frequently associ-
ated with women affected by IBS. In this circumstance, it has
been observed that a reduction in the early nighttime ratio of
melatonin: Trp may be related to the altered sleep status in IBS
cases (Heitkemper et al., 2016).
Current therapy of IBS should involve Trp biology and
metabolism, either by improving diet panels, herbal therapy
and/or using pharmacotherapeutic drugs able to prevent or
reduce Trp catabolism and chemical degradation (Grundmann,
Yoon and Moshiree, 2010; Catanzaro et al., 2014; Shi et al.,
2015). At the same time, the controversial role of the excess of
Trp that has been reported in past studies on gut mucosa can-
not yet be dismissed (Madara and Carlsso, 1991).
2. Obesity, overweight and tryptophan metabolism
In the last four decades, obesity has increased dramatically
throughout the world. In 1980 the number of obese and over-
weight people were 857 million, whereas by 2013 this number
had increased to 2.1 billion (Youssef, 2015). Obesity is a very
complex, multifactorial metabolic disorder, which is often
related to an immune-mediated systemic inammation of the
adipose tissue and to insulin resistance and hyperlipoproteine-
mia, where a major role is exerted by NF-kB (Catrysse and Van
Loo, 2017). The basic determinants of obesity can be both
over-nutrition and lack of physical exercise. Simple reasoning
on a diet should suggest that the excessive intake of food might
even lead to an excess intake of Trp precursors and of food-
derived Trp.
Furthermore, Trp is responsible for the calorie intake regula-
tion (Mangge et al., 2014). Recent data suggests that obesity is
associated with altered Trp and tyrosine (Tyr) metabolism
(Strasser, Berger, and Fuchs, 2015). As previously reported,
these compounds also play a role in neuropsychiatric symp-
toms (Andr
e et al., 2014). As the primary pathway of Trp
metabolism is the kynurenine pathway, and indoleamine-2,3-
dioxygenase (IDO) is the rst enzyme of the pathway, these
proinammatory molecules that stimulate IDO may cause or
exacerbate obesity (Andr
e et al., 2014; Mangge et al., 2014;
Strasser, Berger, and Fuchs, 2015). Yet, apparently contradic-
tory issues do exist, particularly regarding activity of Trp based
on its circulating levels in obese subjects, or in those
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circumstances where metabolic syndrome may have conse-
quences in other anatomic regions of the human body (Oxenk-
rug, 2013; Mangge et al., 2014; Oxenkrug, 2015; Yu et al., 2017).
In overweight adults, Strasser, Berger, and Fuchs, 2015)
investigated the effect of a two-week caloric restriction weight
loss diet (CRWLD) on the circulating levels of leptin, on further
inammatory biomarkers and assessing the short-term dietary
effects of Trp and inammatory biomarkers in overweight
adults (Strasser, Berger, and Fuchs, 2015), researchers found an
impairment in the biosynthesis of serotonin from its natural
precursor. This may be related to the increased susceptibility
for mood disorders and carbohydrate craving observed in the
study. Leptin is a hormonal peptide, produced by adipocytes,
which is correlated to body fat homeostasis and satiety, as it
contributes in regulating food intake and energy balance but
the functionality of which is also strongly associated with the
neurological activity (Zhou et al., 1997; Van Doorn et al.,
2017). High levels of leptin characterize most of the patients
with obesity (L
onnqvist et al., 1995; Hundal et al., 2000; Savino
et al., 2013), suggesting the involvement of a peripheral and
central resistance. Recent papers showed that a reduction of
body weight has a dramatic impact on the circulating levels of
leptin (Klempel and Varady, 2011; Musil et al., 2015). In the
research carried on by Strasser, Berger, and Fuchs (2015), the
reduction of leptin concentrations in the circulation can
improve insulin sensitivity, blood pressure, and blood lipid lev-
els. Concentrations of Trp and Kyn decreased signicantly by
15 and 17% for the low caloric diet (LCD) group and by 21 and
16% for the very low-calorie diet (VLCD) group, while leptin
was reduced by 46% (Strasser, Berger, and Fuchs, 2015).
Reducing body weight by increasing metabolic activity and
accelerating the onset of satiety may involve a serotoninergic-
driven mechanism. For this reason, it might be very useful in
the treatment of obesity to consider the supplementation of
Trp during caloric restriction diet (Yu et al., 2017). Also,
increased availability of Trp can increase the production of
serotonin and reduce the symptoms of depression in people
struggling with overweight (Oh, Park, and Kim, 2016). Thus,
Trp supplementation could prove very useful in the treatment
of uncontrolled weight gain or prevent neuropsychiatric symp-
toms (Strasser, Berger, and Fuchs, 2015).
Despite the fact that weight loss in obese patients showed an
improvement or prevention of changes in the ratio of the
intake/bioavailable Trp and other signals related to obesity that
activates the immune system and inammation, in obese
patients with bariatric surgical intervention a reduction in this
ratio or of immune markers was not found (Brandacher et al.,
2007). Current literature reports several studies dealing with
obesity and Trp metabolism, which are often related to
immune-mediated inammation, with notorious differences
between juveniles and adults (Mangge et al., 2014; Reininghaus
et al., 2014; Raheya et al., 2015).
In a study from a group with Mangee et al. (2014), 527 par-
ticipants aged between 1065 years were analyzed. Results
showed that Kyn serum levels and Kyn/Trp ratio to over-
weight/obese adults (age from 18, to 65 years), signicantly
increased in comparison to controls. Data for ow/ob juvenile
males (age 18 years) showed decreased Kyn/Trp ratio values
compared to controls. Furthermore, juveniles fullling the
criteria of the metabolic syndrome exhibited constant Kyn/Trp
ratio and Kyn, whereas adults with MetS had signicantly
increased Kyn and Kyn/Trp ratio. Trp serum levels decreased
in adult ow/ob females but were not markedly different from
normal weighted patients in the ow/ob groups or between ow/
ob group with or without MetS (Mangge et al., 2014).
The results from these researchers suggested that Trp
metabolism and obesity vary signicantly between juveniles
and adults. This indicates that early onset low-grade inamma-
tion, which can be found in obese adolescents, is different from
adults, and that juveniles are more likely to suffer from a pro-
cess driven by a Th2-mediated response, contrarily to obese
adults, where a Th1 immune mechanism is prevalent. These
facts have potential clinical signicance, because, rst, conser-
vative treatment of obesity through lifestyle changes include
more prevalent physical activity during childhood and adoles-
cence, and can prevent the critical transition to more aggressive
immunology before there is irreversible clinical damage. Sec-
ond, a simple analytical determination of the concentration of
Kyn/Trp may provide more reliable diagnostic evidence of the
presence of Th-1 proinammatory markers regardless of age,
particularly in obese patients (Mangge et al., 2014).
Furthermore, scientists have indicated that potential patho-
genic links do exist between serotonin levels, chronic immune
activation and in decreased IDO-mediated Trp in obesity (Ritze
et al., 2015; Ritze et al., 2016). Immune activation and systemic
inammation are associated with obesity comorbidity, while at
the same time it is connected with synthesized and released
proinammatory cytokines (like TNF-a, INF-g, hormones-
leptin and others) in adipose tissue. IDO is inducible by IFN-g;
and is also involved in the regulation of immune responses and
degraded Trp to form N-formyl kynurenine, which subse-
quently can convert to niacin. Furthermore, IDO can reduce
Trp plasma levels in morbidly obese patients. Serotonin pro-
duction may be reduced by Trp metabolic changes, and this
can in turn contribute to depression, mood disturbances, and
impaired satiety leading to increased caloric uptake and nally
obesity.
Obesity has shown to be associated with a reduced concen-
tration of Trp in the plasma, independently from dietary intake
or weight reduction (Namkung et al., 2015; Zhang et al., 2015).
As stated earlier, Trp is a precursor for the biosynthesis of 5-
hydroxytryptamine (5HT, serotonin). Serotonin is a neuro-
transmitter and biochemical regulator, which contributes to
satiety and hunger balance. Gustatory information during the
act of eating is transmitted to the nucleus accumbent, which is
typically considered the reward center. This leads to the release
or the up-regulation of serotonin and opiates, which are called
the reward mediators. Likewise, appetite-controlling neurons
are connected to specialized brain regions (Halford and Blun-
dell, 2000).
Overeating and obesity are the results of diet including pal-
atable food, where the time spent eating will be prolonged due
to suppressed satiety (Brandacher et al., 2007). In this context,
serotonin as neurotransmitter may be involved in the control
of food intake, which is a satiety signal (Halford and Blundell,
2000). Moreover, this process is responsible for the inhibition
of the expression of neuropeptides Y, which occur in the hypo-
thalamus, through depressing hunger and control of body
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Table 2. Summary of studies regarding amount of tryptophan and its metabolites in body uids in various diseases or disorders.
No. Disease or disorder Metabolites Study population Sex Sample Relation to disease Level of metabolite Methods Reference
1 Irritable Bowel
Syndrome (IBS)
kynurenine (Kyn)
tryptophan (Trp)
IFN-g
41 IBS 33 controls female plasma Increase of Trp along the Kyn catabolic
pathway caused increased sensivity of the
IDO to IFN-gand association between IFN-
gand Kyn/Trp ratio contribute to
serotonergic dysfunction, via decit 5-HT,
which may explain generation
gastrointestinal symptom and increase
incidence of anxiety and depression.
Kyn "Trp $Kyn/Trp "
IFN-g$
Trp, Kyn- HPLC IFN-g-
electrochemiluminesc
ence multiplex system
Fitzgerald
et al. 2008
2 Irritable Bowel
Syndrome (IBS)
kynurenic acid (KYNA)
quinolinic acid (QA)
37 IBS 20 controls both serum Control of the gut motility and enteric
neurnal excitability is involved balance
between quinolinic and kynurenic acid.
KYNA #QA "HPLC Wollny et al.
2006
3 Irritable Bowel
Syndrome (IBS)
tryptophan (Trp)
serotonin (5-HT) 5-
hydroxyindoloacetic
acid (5-HIAA)
14 IBS 14 controls both plasma Serotonergic modulation by ATD
a
affects
visceral perception and cognition in IBS
and control.
Trp #5-HIAA#HPLC Kilkens et al.
2004
4 Irritable Bowel
Syndrome (IBS)
serotonin (5-HT) 5-
hydroxyindoloacetic
acid (5-HIAA)
23 IBS-C 23 IBS-D 25
controls
both serum
urine
Metabolism of 5-HT and secretion may be
disturbed in irritable bowel syndrome
(IBS). Disturbed metabolism of serotonin
probably play a role in pathogenesis of
functional bowel diseases.
5-HT "5-HIAA#ELISA Moskwa et al.
2007
5 Irritable Bowel
Syndrome (IBS)
tryptophan (Trp) 8 IBS-C 10 IBS-D 11
control
both plasma Rise level of tryptophan affects on
gastrointestinal symptoms in IBS and also
decreases anxiety symptoms.
ATD
a
: Trp #ATI
b
: Trp #HPLC
Shufebotham
et al. 2006
6 Irritable Bowel
Syndrome (IBS)
serotonin (5-HT) 29 IBS-C 55 IBS-D 35
controls
both plasma Modulating of different 5-HT receptors are
involving in IBS. Reduced 5-HT reuptake
connected with IBS-D, impaired release
may be linked with IBS-C.
IBS:C: 5-HT #IBS-D: 5-HT
"
HPLC Atkinson et al.
2006
7 IBS-D serotonin (5-HT) 39 IBS-D 20 controls female plasma Symptom exacerbation following meal
ingestion in patients with IBS-D is
connected with increased levels of plasma
5-HT, together with a reduction in 5-HT
turnover.
5-HT "HPLC Houghton
et al. 2003
8 Irritable Bowel
Syndrome (IBS)
serotonin (5-HT) 5-
hydroxyindoloacetic
acid (5-HIAA)
15 IBS-C 15 IBS-D 15
controls
both plasma IBS-C patients show impaired postprandial 5-
HT release.
IBS-C: 5-HT "5-HIAA #HPLC Dunlop et al.
2005
9 Chronic Kidney
Disease (CKD)
tryptophan (Trp) 10
metabolites of Trp
c
27 both serum Decline in kidney function is associated with
metabolism of tryptophan via the
kynurenine pathway, without evident
elimination of tryptophan metabolism via
the 5-HT pathway.
KYNA "were associated
with #cognitive
function IAA "was
correlated with
anxiety and
depression
LC-MS/MS Karu et al.
2016
10 Obesity tryptophan (Trp) Trp
/LNAA
d
ratio
9 obese 8 controls both plasma Brain Trp uptake is correlated with the
plasma Trp/LNAA ratio. This determine
brain serotonin synthesis. Serotonin-
mediated regulation of food intake may
contribute to blunted Trp/LNAA, which
response to carbohydrate intake in the
obese.
Trp #HPLC Caballero
et al. 1988
(Continued on next page)
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Table 2. (Continued)
No. Disease or disorder Metabolites Study population Sex Sample Relation to disease Level of metabolite Methods Reference
11 Obesity tryptophan (Trp)
kynurenine (Kyn)
Kyn/Trp ratio
359 ow/ob
e
212
controls
both serum Induction of the Trp-Kyn pathway are
associated to development of the
metabolic syndrome in obesity. Obesity
and Trp metabolism differs between
juveniles and adults.
adult: Trp #HPLC Mangge et al.
2014Kyn "
Kyn/Trp "
juvenile: Trp "
Kyn #
Kyn/Trp #
12 Obesity tryptophan (Trp)
kynurenine (Kyn)
Kyn/Trp ratio
27 overweight 11
obese
both serum Disturbed metabolism of Trp inuents on
biosynthesis of serotonin and might be
associated with increased carbohydrate
craving and susceptibility for mood
disturbances.
VLCD
f
LCD
g
HPLC Strasser et al.
2015Trp ##
Kyn ##
Kyn/Trp $$
13 Obesity/Depression tryptophan (Trp) 973 individuals both plasma Lower Trp levels in overweight/obese woman
suggests that low Trp (low serotonin
synthesis) may contribute to either
vulnerability to depression in obese
women or vulnerability to obesity in
depressed women.
ow /ob
e
women men HPLC Raheja et al.
2015Trp #"
14 Type 2 Diabetes
(T2D)
tryptophan (Trp)
kynurenine (Kyn)
kynurenic acid
(KYNA)
30 T2D 24 controls both plasma Increased plasma levels of Kyn and KYNA of
T2D patients might conrm kynurenine
hypothesisof insulin resistance and its
progression to T2D.
Trp "Kyn "KYNA "GC-MS Oxenkrug,
2015
15 Bipolar Disorder
(BD)
kynurenine (Kyn) Kyn/
Trp ratio
78 BD (54 overweight,
24 normal weight),
156 controls (76
overweight, 80
normal weight)
both serum Increased level of kynurenine and Kyn/Trp
ratio in the overweight patients with BD
could be connected between short
periods of euthymia and worsening of
illness course in overweight patients with
BD.
Overweight patients
with BD: Kyn "Kyn/
Trp "
HPLC Reininghaus
et al. 2014
16 Anorexia Nervosa
(AN)
tryptophan (Trp) 32 acAN
h
32 recAN
i
32
controls
female plasma In acAN
h
patients observed lower Trp levels,
which also inuence in diminished 5-HT.
Reduced availability of the 5-HT may
account for the poor response to
treatment AN patients.
acAN
h
recAN
i
HPLC Ehrlich et al.
2009Trp ##
17 Anorexia Nervosa
(AN)
5-hydroxyindoloacetic
acid (5-HIAA)
14 AN 10 controls female cerebrospinal
uid (CSF)
Central nervous system serotoninergic
metabolism is associated with weight loss
and malnutrition in AN.
5-HIAA #GC-MS Kaye et al.
1988
18 Anorexia Nervosa
(AN)
tryptophan (Trp)
serotonin (5-HT)
LNAA
d
Trp/LNAA
ratio
42 AN 42 controls both plasma Decrease in depressive symptoms and
anxiety, which encountered in the course
of re-feeding in AN may be bio-availability
of tryptophan.
Trp #5-HT #LNAA
d
#
Trp/LNAA #
HPLC Gauthier et al.
2014
19 Anorexia Nervosa
(AN)
tryptophan (Trp) LNAA
d
Trp/LNAA ratio
13 AN 21 controls female plasma Mood disturbances have been connected
with reduce serotonergic function. In AN
individuals observed reduced central
serotonin metabolism (brain tryptophan
availability decreased).
Trp: after protein meal "
after carbohydrate
meal #Trp/LNAA:
after protein meal #
after carbohydrates
meal "
HPLC Schweiger
et al. 1986
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20 Bulimia (BN) and
Anorexia
Nervosa (AN)
tryptophan (Trp) LNAA
d
Trp/LNAA ratio
13 BN 10 AN 15
controls
female plasma Decreased Trp/LNAA ratio may cause
consequences such as disturbances of
mood and neuroendocrine regulation in
AN individuals.
AN: Trp/LNAA #vs
control
HPLC Schreiber
et al. 1991
BN: Trp/LNAA ratio
$vs controls
21 Anorexia Nervosa
(AN)
tryptophan (Trp)
serotonin (5-HT) Trp/
LNAA ratio
19 AN 12 controls both blood In pathology could be involved all measured
biological indices except 5-HT. AN is
associated with impulsivity and anxiety.
5-HT "total Trp #free
Trp #total Trp/LNAA
#
HPLC Askenazy et al.
1998
22 Anorexia Nervosa
(AN)
tryptophan (Trp)
serotonin (5-HT) 5-
hydroxytryptophan
(5-HTP)
16 AN 25 controls female serum Based on these studies can be distinguished
two different subgroups of AN patients.
One of group characterized by a markedly
lower Trp level and higher levels of 5-HTP
and 5-HT.
Trp #5-HT #HPLC Comai et al.
20105-HTP "
23 Anorexia Nervosa
(AN)
tryptophan (Trp) 20 AN 20 controls serum High level of Trp may trigger AN because Trp
is a precursor of 5-HT. Serotonin is
responsible for mood regulation, and it
high level may cause depression and
decreased eating which leads to AN.
Trp "HPLC Naureen et al.
2014
24 Bulimia Nervosa
(BN)
tryptophan (Trp) Trp/
LNAA ratio
22 BN 16 controls female plasma Participants with BN can be more vulnerable
to the mood lowering effects of ATD
a
.
Acute changes in 5-HT activity are linked
with mood BN subjects.
Trp "uorometric method of
Denkla and Dewey
Kaye et al.
2000
25 Autism (ASD) tryptophan (Trp) 37 ASD 28 controls
adults
plasma Abnormal Trp-serotonin metabolism in the
brain might be responsible for the clinical
manifestations and behavioral
abnormalities of autism. High free Trp
level is responsible for lower mental
development and hyperactivity.
total Trp $free Trp "uorometric method of
Denkla and Dewey
Hoshino et al.
1986
12 controls child
26 Autism (ASD) tryptophan (Trp) 20 ASD adults both plasma Changes in behavior (increasing whirling,
banging, hitting self, rocking, and toe
walking) are triggered by depleting Trp in
adult patients with ASD.
HPLC McDougle
et al. 1996
27 Autism (ASD) tryptophan (Trp) 55 ASD children 44
controls
both plasma Decrease level of Trp in ASD could impair
serotonin synthesis, and this lead to a
worsening in behavior in ASD subjects.
Lower level of Trp might be due to
reduced protein intake and/or dysfunction
in synthesizing protein into amino acids in
the digestive tract.
Trp #HPLC-MS/MS Adams et al.
2011
28 Autism (ASD) tryptophan (Trp) 138 ASD children 138
controls
both plasma Lower level of Trp may deterioration in the
behavior of autistic children. Trp and other
LNAA
d
compete for brain serotonin
synthesis and when is low level of Trp
then is low brain serotonin synthesis.
Trp #HPLC Naushad et al.
2013
29 Autism (ASD) tryptophan (Trp) 33 ASD children 21
controls
both urine Abnormal Trp-serotonin metabolism in the
brain might be responsible for the
worsening of autistic symptoms. Reduce
level of Trp may cause increased irritability
and mild depression.
Trp #GC-MS Ka»u_
zna-
Czapli
nska
et al. 2010
30 Autism (ASD) tryptophan (Trp) 14 ASD children 10
controls
both urine Lower level of Trp may lead to the worsening
of autistic symptoms (increased irritability
and mild depression).
Trp #GC-MS Ka»u_
zna-
Czapli
nska
et al. 2014
(Continued on next page)
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Table 2. (Continued)
No. Disease or disorder Metabolites Study population Sex Sample Relation to disease Level of metabolite Methods Reference
31 Autism (ASD) tryptophan (Trp) 48 ASD 53 controls both urine Abnormal amino acid metabolism (including
Trp metabolism), increased oxidative
stress, and altered gut microbiomes in
ASD.
Trp #UPLC/MS/MS GC-MS Ming et al.
2012
32 Parkinsons Disease
(PD)
tryptophan (Trp)
kynurenine (Kyn)
Kyn/Trp ratio
22 PD 11 controls both serum, cerebrospinal
uid (CSF)
Increased Trp degradation in peripheral
blood in PD patients substantiates that in
this disease participate immunological
abnormalities. Interferon-mediated IDO
activity is responsible for the increased Trp
degradation rate as is evident by the
increased Kyn/Trp ratios.
Trp #Kyn #Kyn/Trp "HPLC Widner et al.
2002
33 Parkinsons Disease
(PD)
tryptophan (Trp) 20 PD 20 controls both cerebrospinal uid
(CSF)
Degradation of Trp can lead to the generation
of 3-HKA, a compound leading to
increased oxidative stress in preclinical PD
studies.
Trp #GC-TOFMS Trupp et al.
2014
34 Parkinsons Disease
(PD)
3-hydroxykynurenine (3-
HK)
48 PD 57 controls cerebrospinal uid
(CSF)
3-HK linked with potent excitotoxicity
properties. Block production of 3-HK
(through Trp catabolism) allows
neuroprotective strategy and therapeutic
intervention against 3-HK formation.
3-HK "UHPLC-MS/MS GC-MS Lewitt et al.
2013
35 Parkinsons Disease
(PD)
tryptophan (Trp) 92 PD 65 controls both urine Degradation of Trp may be connected with
the activated cell-mediated immune
response typical of PD.
Trp #GC-MS LC-MS Luan et al.
2015
36 Alzheimers Disease
(AD)
tryptophan (Trp)
kynurenine (Kyn)
Kyn/Trp ratio
21 AD 20 controls serum Increased Trp degradation in AD patients is
associated with signs of a chronic immune
activation, while increased Kyn/Trp was
associated with reduced cognitive
performance.
Trp #Kyn "Kyn/Trp "HPLC Widner et al.
1999
37 Alzheimers Disease
(AD),
Huntingtons
disease (HD)
tryptophan (Trp)
kynurenine (Kyn)
Kyn/Trp ratio
24 AD 12 HD serum Systemic chronic immune activation in
patients with AD and HD is associated
with signicant degradation of Trp, which
is most likely due to activation of IDO by
immunologic stimuli.
Trp #Kyn #Kyn/Trp "HPLC Widner et al.
2000
38 Alzheimers Disease
(AD)
tryptophan (Trp)
kynurenine (Kyn)
Kyn/Trp ratio
43 AD both serum Increased blood concentration of Kyn/Trp is
associated with immune activation and
inammation represent critical factors in
the pathogenesis of AD.
Trp #Kyn "Kyn/Trp "HPLC Wissmann
et al. 2013
39 Alzheimers Disease
(AD)
tryptophan (Trp) 16 AD 17 controls both plasma Acute Trp depletion had no effect on cortisol
secretion for subjects with AD and healthy
controls.
Trp #HPLC Porter
Marshall,
and
OBrien
2002
40 Alzheimers Disease
(AD)
kynurenic acid (KYNA) 19 AD 20 controls both cerebrospinal uid
(CSF)
No signicant alterations in CSF KYNA levels
in AD patients compared to controls. In
AD the inconsistency of KYNA alterations
could be because of the heterogeneity of
the disease.
KYNA $KYNA "female
AD vs male AD
HPLC Wennstr
om
et al. 2014
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41 Sleep Disorders
(SD)
melatonin 94 individuals male saliva The combined intervention on breakfast,
morning sunlight and evening-lighting
seems to be effective to keep higher
melatonin secretion at night. Higher level
of melatonin is responsible for easy onset
of the night sleep and higher quality of
sleep.
melatonin "in G3 vs G1
and G2
j
ELISA Wada et al.
2013
42 Delayed Sleep
Phase
Syndrome
(DSPS)
melatonin 56 individuals both saliva Examination of the melatonin secretion
prole can reveal several key differences
between individuals with and without
circadian rhythm disruptions.
The time of melatonin
secretion are
signicantly delayed
in DSPS patients.
ELISA Rahman et al.
2009
43 Sleep Deprivation tryptophan (Trp)
serotonin (5-HT)
109 individuals plasma The increased levels of 5-HT and Trp may
explain the antidepressive effect of acute
sleep deprivation.
Trp "5-HT "LC-MS Davies et al.
2014
$no differences in levels metabolite between subjects with disease/disorder and controls
"increase level of metabolite in subjects with disease/disorder compared to controls
#decrease level of metabolite in subjects with disease/disorder compared to controls
a
ATD- Acute Tryptophan Depletion
b
ATI- Acuter Tryptophan Increase, subjects consumed an amino acid drink that either containing 2.3 g Trp
c
10 metabolites of Trp:
serotonin (5-HT)
5-hydroxy-3-indole acetic acid (5-OH IAA)
kynurenine (Kyn)
kynurenic acid (KYNA)
quinolinic acid
xanthurenic acid
quinaldic acid
3-OH anthranilic acid
indoxyl sulfate
indole-3-acetic acid (IAA)
d
LNAA- Large Neutral Amino Acids
e
ow/ob- overweight/obese subjects
f
VLCD- Very Low Caloric Diet
g
LCD- Low Caloric Diet
h
acAN- participants with acute anorexia nervosa (AN)
i
recAN- participants were previously treated for anorexia nervosa (AN)
j
G1- no intervention
G2- have protein-rich foods and vitamin B-6-rich foods at breakfast and sunlight exposure after breakfast
G3- the same content as G2 and incandescent light exposure at night
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adiposity (Wurtman and Wurtman, 1995; Manousopoulou
et al., 2016). Also, it has been found that serotonin specically
regulates fat and/or carbohydrate intake (Blundell and Lawton,
1995; Bray, 2001).
3. Anorexia nervosa and bulimia nervosa
Eating disorders (EDs) are widespread and serious diseases
throughout the world, with a chronic course and potentially
fatal outcome (Winkler et al., 2016). Genetic and environmen-
tal factors contribute to the development of many complex eat-
ing disorders (Haleem, 2012). The most common eating
disorders may include anorexia nervosa (AN) and bulimia
nervosa (BN). Both diseases are disorders of considered to arise
from unknown etiology. They usually begin during adolescence
in women, but may also be seen in men (Becker et al., 2003;
Kaye et al., 2005,2008; Haleem, 2012).
A study conducted by Haleem (2012) suggested that females
are more vulnerable to food restriction, which may start with a
chronic deciency in Trp levels and bioavailability. Monoamin-
ergic neurotransmitters such as serotonin (5-HT), noradrena-
line (NA) and dopamine (DA) contribute to the regulation
feeding behavior, while the accessibility of precursor amino
acids in the blood powerfully inuences the synthesis of those
monoamines in the brain (Ehrlich et al., 2009).
The most common symptoms of eating disorders are
restricted eating, body image distortions, and denial of emacia-
tion, binge-purge behaviors, and resistance to treatment. They
are also characterized by aberrant patterns of weight regulation
and feeding behavior, but also by different perceptions towards
shape and body weight, dysphoric mood exhibit behaviors,
such as perfectionism and obsessivecompulsiveness. AN and
BN are relapsing and often are chronic disorders, whereas AN
has the highest death rate compared to other psychiatric
disorders.
Patients with AN accompanied an obsession with body
weight and inexplicable fear of weight gain, even in the face of
the increasing destruction of the body, characteristically exhibit
motor restlessness and excessive exercise (Kaye, Gendall, and
Strober, 2001). The main role in behavioral changes observed
in a patient with anorexia nervosa presents anomalies occurring
in the serotoninergic pathway. In AN an individuals serotonin
level is involved in almost all the behavioral changes. Patients
with AN show higher frequency of compulsive exercising rela-
tive to those with BN patients (Haleem, 2012). After a period of
food restriction, the sufferer usually emerges with BN. They
may or may not have been linked with weight loss. After surfeit
followed by self-induced vomiting or different way compensa-
tion of surfeit, people suffering from BN also have a fear of
weight gain and distorted view of their body shape. Individuals
with BN are impulsive, and often sensation seeking, whereas
individuals with AN tend toward emotional expressiveness and
constriction of affect, and may exhibit great constraint.
The above-described characteristics of anorexia nervosa and
bulimia nervosa often begin in childhood, are premorbid and
often persist even after recovery. This would suggest that such
behavior caused by underfeeding is not secondary. Dysregula-
tion of impulse control and appetite or mood in AN and BN
contributes modied brain serotonin function. The occurrence
of AN precedes the disturbance of neuronal serotonin modula-
tion, which contributes to premorbid symptoms of inhibition,
anxiety, and obsessionality. In patients with BN, it has been
observed that dietary depletion of Trp is associated with Trp-
associated mood irritability and increased food intake, which is
caused by dysfunction of serotonergic tone (Kaye et al., 2005,
2008). Because Trp is the precursor to serotonin, Trp decit
could signicantly alter serotoninergic neurotransmission. In
the refeeding, the Trp/LNAA ratio increases, as it associated
with a decrease in depressive symptoms. This fact provides an
argument for a possible impact of the AN mood symptoms
with the serotoninergic pathway through a normalization of
the biological markers. The increase in the Trp/LNAA ratio is
possible by the intake of related essential amino acids. The
transport of Trp is predictive through the blood-brain barrier
towards the cerebrospinal uid (CSF) in the evaluation of the
Trp/LNAA ratio. Then, Trp can be used for the synthesis of
cerebral serotonin. In these particular ways, the serotoninergic
transfer, which leads to a decrease in depressive symptoms, is
restored (Gauthier et al., 2014). For all patients with EDs, com-
mon features include dysfunctional cognizance relating to
shape and weight and result in restrained eating behaviors.
In the literature, there is lots of evidence of anxiety, appe-
tite dysregulation, extremes of impulse control and obses-
sional behaviors, caused by disturbances 5-HT in those
suffering from EDs. Enhancement of the brain serotonin
release, which can affect appetite regulation, can determine
meal consumption, depending on the amount of protein and
carbohydrate in the meal. Carbohydrate consumption causes
depletion of the large neutral amino acids valine, leucine,
isoleucine, phenylalanine and tyrosine. This LNAA competes
with Trp for uptake into the brain. Such elevates the plasma
Trp/LNAA ratio, thereby the amount of Trp in the brain,
causing rapid synthesis and release of 5-HT. In contrast, a
diet rich in proteins can block those effects, resulting in the
large amounts of LNAA in the blood (Fernstrom et al., 1979;
Kaye et al., 2005,2008). The results of many studies indicate
a decrease in plasma of the Trp/LNAA ratio and Trp levels
in patients with acute underweight (Schreiber et al., 1991;
Askenazy et al., 1998; Ehrlich et al., 2009;Comaietal.,2010;
Gauthier et al., 2014).
A broader viewpoint is shown by Gauthier et al. (2014),
where they show links between serotonin biomarkers, nutri-
tional status and psychological states in anorexia nervosa con-
jointly. For the rst time, they were able to highlight the role of
the low level of Trp in plasma, blood serotonin, and LNAA and
Trp/LNAA ratio with malnutrition (Gauthier et al., 2014).
They also found a positive correlation between anxiety, depres-
sion score, and total blood serotonin levels in a group of AN
individuals classied as an impulsive. These results are consis-
tent with the results obtained by other teams. Acute Trp deple-
tion in AN patients was found to lead to an increase in anxiety.
Through the restrictive dietary behaviors, individuals may
decrease cerebral serotonin synthesis. Additionally, reduced 5-
HT concentrations in hypothalamic causes hyperactivity, which
intensies behaviors leading to weight loss. The emergence of
BN symptoms and amplied impulsivity appear to be related to
low serotonin levels. This may explain the frequent overlap
between the restrictive forms of AN and the bulimic state, and
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behaviors are alternating between these eating disorders
(Gauthier et al., 2014).
4. Autism spectrum disorder
Autism spectrum disorder (ASD) is a developmental, multi-
factorial disorder, characterized by symptoms that evolve
with age adversely affecting the development of the child
(Bara, Bucciarelli, and Colle, 2001). The incidence of ASD
has grown worldwide by 600% since the 1970s. In the US,
currently at least one of 68 children has ASD (Zablotsky
et al., 2015; Christensen et al., 2016). Patients with ASD
demonstrate the problem with interacting and exhibit little
interest in others and lack of social awareness (Ka»u_
zna-
Czapli
nska and B»aszczyk, 2012). Etiology and pathogenesis
of ASD are not still fully known. However, evidence points
to nutritional deciencies or overloads, complex genetic
interactions, maternal age and health state, exposure to
chemicals or viruses, heavy metal toxicity, immunological
overload from early vaccinations, certain food additives,
and dysfunctional immune systems or allergies. Deciencies
in the levels of amino acids occur for many children with
developmental disorders.
In recent years, observations relative to metabolic bio-
markers have shed light on the inuence of amino acids on var-
ious developmental disorders. In some cases, the neurological
function could be specied just by studies of amino acids. Some
researchers have suggested that a pivotal role in ASD might be
found with Trp metabolism. A metabolite of Trp is the neuro-
transmitter serotonin (5-HT), which is, among other things,
responsible for regulating humor and behavior, and also facili-
tating calmness, feeling of well-being, relaxation, personal secu-
rity, concentration, and self-condence. Hence, reduced
serotonin levels have been demonstrated to inuence many
developmental disorders. For this reason, it is reasonable to
posit a connection between escalation of autistic symptoms and
abnormal levels of serotonin (Adams and Holloway, 2004).
Likewise, a dysfunctional serotonergic system could be involved
with ASD. As stated above, Trp is converted into serotonin in
the brain, where it competes for transport with nine other large,
neutral amino acids (LNAA) (Beretich, 2009).
Many children with ASD exhibit a deciency in Trp due to
signicant food selectivity and self-imposed diet restriction.
This often leads to reduced levels of serotonin and a worsening
of autistic behaviors. The literature mentions that urinary
excretion of Trp might be caused by a low concentration of die-
tary proteins. In 1986, research suggested a link between some
problems in children with ASD and abnormal Trp metabolism.
The results indicated that free plasma Trp levels were evaluated
in ASD children compared to healthy children and adult con-
trols (Hoshino et al., 1986). Therefore, biochemical abnormali-
ties were associated with a signicantly lower level of Trp in
urine.
More recently, researchers examined 54 children aged
410 years, 33 ASD children (4 female and 29 males) and 21
normal children as healthy controls (8 females and 13 males)
the gender ratios in the subjects under study, incidentally, were
about the same as found in other studies. The ASD children
were divided into a group of 10 children with ASD on the
restricted diet low casein and gluten, and 23 ASD children
without restricted diet. The highest values of Trp in urine were
observed in control group. Signicantly lower concentration
levels of Trp were reported in the samples from 23 ASD chil-
dren that were on the restricted diet. Low levels of Trp might
also cause intensication of the symptoms of ASD, such as
increased irritability and mild depression (Ka»u_
zna-Czapli
nska,
Michalska, and Rynkowski, 2010). McDougle et al. (1993) have
received similar results about low diet in Trp. The researchers
also suggested that by depleting Trp in an adult with ASD they
might induce signicant changes in behavior, which were not
seen in that control group such as increasing whirling, banging,
hitting self, rocking, and toe walking (McDougle et al., 1996).
Other scientists focused on the relationship between devel-
opmental disorders and metabolic disturbances. Researchers
examined 55 children ages 516 years with ASD and 44 healthy
controls of similar age, gender, and geographical distribution.
The study was aimed at comparing the metabolic and nutri-
tional status of ASD children with that of control children and
investigated autism severity to the Trp-related biomarkers.
Results showed signicantly decreased Trp in the children with
ASD. This might be due to reduced protein intake, and dys-
function in synthesizing protein into amino acids in the diges-
tive tract. Decreased Trp could further impair serotonin
synthesis. A deciency of Trp and thus serotonin lead to a sig-
nicant worsening in behavior in ASD participants (Adams
et al., 2011).
Similar results were also found in other studies. For exam-
ple, Naushad et al. (2013) examined 138 autistic children, 120
males and 18 females, and 138 non-autistic controls, 120 males
and 18 females. Children were matched for age, gender, ethnic-
ity and geographical area. Researchers observed markedly lower
levels of Trp in the ASD children (Trp levels decreased by an
average of 48%), compared to healthy controls (Naushad et al.,
2013).
5. Parkinson and alzheimer diseases
The pathogenesis of neurodegenerative disorders such as Par-
kinsons (PD) and Alzheimers diseases (AD) are not entirely
known. However, it is believed that in this pathogenic process
are involved immunologic mechanisms. In the development
and progression of PD and AD, a cause has been ascribed to
stimulate immunocompetent cells and a signicant number of
(proinammatory) cytokines (Widner et al., 2002). Parkinsons
disease categorically belongs to chronic, progressive, and irre-
versible neurodegenerative diseases, which are caused mainly
by the progressive degeneration of the dopaminergic pathway.
The second most common neurodegenerative disease among
older adults is PD. So far, the disease has long been considered
a disease of old age (over 60 years of age), but it also occurs
increasingly in younger people. When the damage of dopami-
nergic neurons reaches 5060%, and the striatum does not
reach adequate dopaminergic input, there arise characteristic
motor symptoms and behaviors (Andersen et al., 2017).
Symptoms and signs of PD are resting tremor, curved pos-
ture, bradykinesia, rigidity, depression, and postural instability,
shufing gait. For severe disability progressively lead to long
term complications of dopaminergic treatment, which focuses
CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 11
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on minimizing the symptoms like motor blocks and dyskinesia.
The close relationship might observe between neurodegenera-
tive diseases and nutritional status (Barichella, Cereda, and Pez-
zoli, 2009). PD is very difcult, if not impossible, to diagnose
before motor symptoms have begun developing. Other symp-
toms associated with early stage disease are very unspecic
including obstipation, olfactory deciency, depression, and
sleep disorders (Andersen et al., 2017).
Increased risk of PD can relate to many factors, such as
consumption of (processed) dairy products, past traumatic
brain injury, heavy metal toxicity, certain food additives,
polypharmacy, certain parasites, and exposure to pesticides
and even history of melanoma (although this latter factor
appears strictly correlational). In contrast, the factors that
reduce the risk of occurring PD are associated with caffeine
consumption, smoking, physical activity, and higher serum
urate concentrations of NSAIDs. Caffeine, nicotine, and
urate may be neuroprotective and give benets in patients
with early PD. Whether this mild evidence is offset by the
other more serious detriments implicated by caffeine, nico-
tine, and NSAIDs is another matter. Researchers are looking
for possible ways to identify this disease in its early stages
and possibly using (healthy) neuroprotective interventions
before the presentation of motor symptoms (Ascherio and
Schwarzschild, 2016).
Tryptophan is the precursor not only of serotonin, but also
is degraded to the kynurenic acid, 3-hydroxykynurenine, and
quinolinic acid. Kynurenine pathway that regulates the synthe-
sis of these neuroactive metabolites. The human immune sys-
tem controls the kynurenine pathway. Hyperfunction or
hypofunction of neuroactive metabolites is caused by dysregu-
lation of the kynurenine pathway, which relates closely to neu-
rological and neurodegenerative disorders. The concentration
of 3-hydroxykynurenine (3-HK) is increased in the basal gan-
glia of PD patients, whereas kynurenic acid (KYNA) and
kynurenine levels are slightly reduced (Ogawa et al., 1992;
Schwarcz et al., 2012). The strongest quinolinic acid (QUIN) is
found in glial cells. This fact suggests that QUIN might partici-
pate in the pathogenic process in Alzheimers disease (Guille-
min et al., 2005; Schwarcz et al., 2012). Interferon g(INF- g)
product large amounts of neopterin, wherein IFN-ginduces
indoleamine 2,3-dioxygenase (IDO), which causes degradation
L-tryptophan to kynurenine.
In the literature, there are reports on high concentrations of
neopterin, Trp, and kynurenine found in serum and CSF sam-
ples. Widner,Leblhuber, and Fuchs (2002) examined 22
patients with PD (15 females and seven males) and 11 age-
matched controls group, without obvious neuropsychiatric
symptoms (6 females and ve males). From eight patients with
PD were collected cerebrospinal uid specimens. The results
showed signicantly higher concentrations of neopterin and
kynurenine/Trp ratio (kyn/trp ratio) and lower Trp concentra-
tions in serum samples of PD patients compared to healthy
controls. Similar relationships were found in CSF from eight
PD patients. Comparing the two body uids, serum neopterin
concentrations were higher than in CSF. It can be assumed that
reduced dietary intake of Trp could signicantly contribute to
Trp depletion in PD patients (Widner et al., 2002). Similar
results were obtained by Ogawa et al. (1992) who observed in
PD patients increased the level of 3-hydroxykynurenine (3-
HA), while the level of KYNA decreased.
The search for new biomarkers is always scientic interest.
Lewitt et al. (2013) employed targeted metabolomics, using
CSF from PD patients and controls. They observed changes in
the ratio of 3-hydroxykynurenine (3-HK)/kynurenic acid
(KYNA). This variation in the ratio 3-HK/KYNA is signicant
because 3-HK is a precursor of the quinolinic acid and by gen-
eral hydroxyl radicals might cause oxidative damage. Whereas
KYNA has neuroprotective potential. Promote neurodegenera-
tion in the brain might cause an increased ratio of 3-HK/
KYNA (Lewitt et al., 2013). In another study, Mollenhauer and
Zhang (2013) tried to unveil the candidate metabolic pathway
related to PD. They examined 35 patients with PD without
dementia, and as a control group, 15 healthy age-matched par-
ticipants without PD. The results showed that metabolomic
proles of patients with PD were substantially different from
control groups. PD proles had signicantly lower levels of
Trp. Decreased serum Trp levels appear to be signicantly
related to psychiatric problems in patients with PD (Mollenha-
uer and Zhang, 2013).
Alzheimers disease was rst described more than a century
ago. This disease affects approximately 35.6 million people
worldwide in 2010 and by the year 2050 estimated 115 million
people (Van Wijngaarden et al., 2017). One of the major causes
of dementia is AD. So far, the pathogenesis of this disease is
not completely understood. It is, however, well known that the
kynurenine pathway is the principal route for the metabolism
of the Trp. Among other metabolic pathways, Trp is the kynur-
enine pathway involved in AD pathogenesis (Kincses, Toldi,
and V
ecsei, 2010). Hence, changes in AD behaviors have been
observed in the kynurenine pathway. These changes are based
on a reduction in the serum concentration of KYNA and Trp
and in increased concentrations of 3- hydroxykynurenine (3-
HA) and kynurenine (OFarrell and Harkin, 2017).
The mechanism of AD is similar to other neurodegenerative
diseases such PD. Mechanisms of AD are associated with the
kynurenine pathway (KP). Many proinammatory cytokines
activate kynurenine pathway, and then they create metabolites
associated with the pathogenesis of AD. For limiting kynure-
nine pathway responsible is indoleamine-2-3 dioxygenase
(IDO). The expression of IDO is markedly increased with the
proliferation of proinammatory cytokines INF-g. Overexpres-
sion of IDO is induced by INF-gin the presence of amyloid
plaques, which leads to dysregulation of KP. In dysregulation
of KP is also involved interleukin-18 (IL-18), which induces
strong inammatory reactions. IL-18 appears to be responsible
for the production of neurotoxic QUIN, wherein is promoted
neurodegeneration. Furthermore, QUIN may cause an
increased level of lipid peroxidation in oxidative stress and may
provoke neuronal death by cytotoxicity. In AD patients,
unchanged levels of QUIN in cerebral and CSF have been
observed, whereas the level of KYNA was increased in the stria-
tum, but in CSF and plasma, the level of KYNA was decreased.
At this time, there is still no clear explanation concerning to
how decrease KYNA levels that contribute to Alzheimers dis-
ease (Tan, Yu, and Tan, 2012).
(Kincses, Toldi, and V
ecsei, 2010) presented evidence of
theparticipationofthekynureninepathwayinthe
12 J. KAºU
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pathogenesis of AD. They measured the kynurenine, Trp
and KYN/Trp ratio in the plasma. Examined were ten
patients with AD (six females and four males) and 15
healthy controls (11 females and four males). The results
showed that Trp concentration was signicantly lower in
AD patients than controls, while KYN showed no signi-
cant differences between AD patients compared to controls,
and the KYN/Trp ratio was considerably higher in patients
with AD (Kincses, Toldi, and V
ecsei, 2010).
In the pathogenesis of AD, inammation and immune acti-
vation are factors related to increased blood concentration of
certain biomarkers, such as the kynurenine to Trp ratio (KYN/
Trp) and neopterin. Wissmann et al. (2013) examined 43 AD
patients (26 females and 17 males, range aged 5799 years),
they measured neopterin, Trp, and kynurenine concentration.
They observed lower Trp levels, higher kynurenine levels, and a
higher KYN/Trp ratio, which is correlated with the higher con-
centration of neopterin. (Wissmann et al., 2013). Similar results
were obtained by Widner et al. (1999), which examined 24
patients with AD and observed lower Trp levels, higher kynure-
nine levels, and KYN/Trp ratio.
Other scientists examined the concentrations of the com-
pound in CSF of patients with AD. They explored correlations
between KYNA levels, well-established AD, cognitive decline
and proinammatory markers. Then they measured KYNA lev-
els in 19 AD patients, aged 7279 years, and 20 healthy con-
trols, age matched. The results showed that AD patients have
signicantly KYNA levels versus the healthy controls. Addi-
tionally, they observed that female AD patients had signi-
cantly higher KYNA levels compared to male AD patients,
wherein this result was not observed in the healthy control
group (Wennstr
om et al., 2014).
6. Tryptophan and sleep disorders
Sleep disorders are a serious problem in industrialized soci-
eties and concerns not only adults but also children and
young people. It is estimated that sleep disorders affect at
least 2040% of adults, and half of them consider it to be
important. Likewise, various types of sleep difculties con-
cern 2562% of the population of children, depending on
their stage of development (Blader et al., 1997;Kaczorand
Skalski, 2016).
In recent years, experts were interested in the relation-
ship between sleep and diet. The basis for the discussion of
the problem is the enzyme pathway of melatonin synthesis,
which precursor for melatonin is serotonin. This, in turn, is
synthesized by enzymatic transformation of Trp (Kaczor
and Skalski, 2016).
Hormones produced in the brain, such as serotonin and
melatonin, control sleep and circadian rhythms in humans.
Melatoninisanactivebiologicalcompoundthatisrespon-
sible for regulating diurnal rhythms and inuences the
immune and reproductive system, and gastrointestinal
motility and other digestive processes. The pineal gland
secretes melatonin during periods of darkness. Its task is to
regulate circadian rhythms and sleep patterns (Richard
et al., 2009; Szczepanik, 2007). Tryptophan is often used for
the treatment of sleep disorders. In the diet Trp produces
therapeutic effects through melatonin. A crucial feature of
Trp treatment is that it does not directly reduce cognitive
ability (Richard et al., 2009).
One hypothesis is that sunlight accelerates serotonin synthe-
sis. Studies in Japan suggest that combined intervention of a
breakfast rich in Trp, regular morning sun exposure, and even-
ing lighting combine to improve higher melatonin secretion at
night. Associated with this was found improved sleep quality
and reduced time required to fall asleep (Nakade et al., 2012;
Wada et al., 2013).
Infant sleep problems constitute a serious disorder and
might affect brain development (and be implicated in other
more serious health problems, as well). In the literature, there
is a report about the impact of diet on improving the conditions
of sleep. Cubero et al. (2009) examined 30 children with sleep
problems, aged 816 months old. In the evening meal, they
administrated to infants were cereals with varying content of
Trp over a ve-week period. Feeding of enriched cereals led to
the maintenance of calmer children and restored sleep. They
concluded that regulation of circadian cycle can be inuenced
by diet (Cubero et al., 2009).
One of many sleep disorders is night terrors. Sharp waking
from sleep characterizes night terrors, accompanied by persis-
tent terror and fear or increased heart rate, sweating, and
screaming. Promising results have been demonstrated that Trp
supplementation for night terrors. Bruni et al. (2004) examined
the inuence L-5-hydroxytryptophan on sleep terrors. They
studied 45 children (34 males and 11 females, aged 3.2
10.6 years) safer from sleep terrors. The studied group was sup-
plementation by L-5-hydroxytryptophan. Within a month,
they observed a reduction of more than half night terror epi-
sodes in over 93% of children. These results conrm that
arousal level might be positively inuenced by treatment with
L-5-hydroxytryptophan, resulting in reduced sleep terror
behaviors in children (Bruni et al., 2004).
7. Conclusion
The interest in Trp is growing throughout research and health
community worldwide (Figure 2).TheroleofTrpinthe
Figure 2. Diagram of the frequency of scientic reports on the use of tryptophan
supplementation in the study of different diseases in 20072016. The literature
review was based on PubMed sources, sorted by best match, for the phrase: tryp-
tophan supplementation or Trp supplementation and diseases.
CRITICAL REVIEWS IN FOOD SCIENCE AND NUTRITION 13
Downloaded by [Umeå University Library] at 09:39 01 September 2017
relationship serotonin-Trp uptake with diet is particularly
intriguing and deserves much more insightful data to achieve a
forthright and clearer conclusion. Certainly, research in the
nutritional elds must be further investigated and implemented,
to elucidate the role of supplemented Trp in foods and meals
that improve human health and prevent many serotonin-related
pathologies. We believe that an optimized and personalized diet
can help to minimize the symptoms of illness, which will result
in improved health.
ORCID
Salvatore Chirumbolo http://orcid.org/0000-0003-1789-8307
Geir Bjørklund http://orcid.org/0000-0003-2632-3935
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... Food restriction is shown to decrease the availability of Trp, an essential amino acid. 1 Following which, brain 5-HT levels are also decreased. 2 Extreme food restriction leads to hyperactivity 3 and hormonal dysregulation. 4 Studies on animal models show that excessive food restriction results in weight-loss and stress. 5 Studies suggest that most of the behavioral symptoms associated with diet restriction are due to decreased 5-HT neurotransmission in the brain. ...
... In view of an important role of 5-HT1A auto-receptor in altering the availability of 5-HT in terminal regions, 5-HT1A receptor expression in the raphe nuclei is also determined. 2 International Journal of Tryptophan Research chow diet and tap water. Temperature of the housing room was kept at 25°C and 12 hours light and dark cycle was also maintained. ...
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To determine the effect of long-term restricted feeding schedules on behavior, serotonergic responses, and neuro-endocrine functions, metabolism of serotonin (5-HT) in the striatum, expression of serotonin-1A (5-HT1A) auto-receptor in the raphe nuclei and circulating levels of leptin and corticosterone were determined in female Wistar rats kept on excessive food restriction schedule. Due to a role of dietary deficiency of tryptophan (Trp) in influencing serotonergic neurotransmission, circulating levels of Trp were also determined. Estimations were done in 2 different restricted feeding models: time-restricted feeding (TRF) and diet restricted (DR). TRF animals were given access to food ad libitum only for 2 hours/day. The DR animals were given a small calculated amount of food each day. We found that chronic food restriction for 5 weeks cause a significant decrease in the body weight and produced hyperactivity in both, TRF and DR animals. Levels of Trp were declined in circulation and in the striatum. Similarly, the levels of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were decreased in the striatum. Also, the expression of 5-HT1A auto-receptor was declined in the raphe nuclei. These changes in 5-HT metabolism and 5-HT1A auto-receptor expression were more profound in DR animals as compare to TRF animals. Similarly, hypoleptinemia and increased corticosterone found in both models was higher in DR animals. Effect of dietary deficiency of Trp in the modulation of striatal 5-HT metabolism and its consequences on circulating leptin and corticosterone are discussed.
... Inhibition of the FTO gene, however, can inhibit macrophage activation, inhibit NLRP3 inflammasome in a mouse model (Luo et al., 2021). Tryptophan can influence energy homeostasis and obesity (Kałuzna-Czaplińska et al., 2019) and the tryptophan kynurenine pathway is also related to inflammation. The relationship between the FTO gene and the tryptophan kynurenine pathway has been less explored. ...
... Tryptophan exerts many biological effects, including those related to obesity and energy homeostasis (Kałuzna-Czaplińska et al., 2019). We found in the present study that genetic variation in the FTO gene is likely to influence the kynurenine pathway, the main metabolic pathway of tryptophan. ...
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Background Genome-wide association studies have identified the alpha-ketoglutarate dependent dioxygenase gene (FTO) as the first susceptibility gene of obesity. In the present study, we utilized targeted metabolomics in an attempt to further elucidate mechanisms underlying the action of the FTO gene. Methods This study was part of a health survey of employees of the Electricity Generating Authority of Thailand ( n = 79, 10 female and 69 male). Targeted metabolomics was performed by liquid chromatography–mass spectrometry using Biocrates AbsoluteIDQ-p180 kit. Genotyping of FTO rs9939609 was performed by real-time PCR (TaqMan™ MGB probes). Results Using OPLS-DA variable importance in projection (VIP), tryptophan was found to be among the metabolites with the 10 highest VIP scores. Pearson’s correlation analysis showed that kynurenine and tryptophan were positively correlated only in subjects with the rs9939609 A allele ( n = 32, r = 0.56, p < 0.001) and the correlation coefficients were significantly higher in subjects having the A allele than in those without the A allele ( p < 0.05). Moreover, the kynurenine/tryptophan ratio was significantly associated with the presence of the A allele, independently of body mass index and sex. Conclusions The FTO gene is likely to influences the conversion of tryptophan to kynurenine.
... Isoleucine aids in the detoxification of nitrogenous wastes such as ammonia, which are then expelled by the kidneys [24]. The critical significance of tryptophan in various metabolic activities has been reported by [25]. Clinicians can utilize tryptophan levels to diagnose a variety of metabolic problems and the symptoms that go along with them [26]. ...
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The scotch bonnet pepper (Capsicum chinense), which is grown in Ado Ekiti, Nigeria, is frequently used to spice up foods and soups in addition to a variety of therapeutic benefits. They contain vitamins and amino acids and can be used to treat pain disorders including headaches, osteoarthritis, rheumatoid arthritis, and severe diabetic neuropathy. The concentration of amino acids and vitamins B 1 , B 2 , and B 6 in the flesh and seed of red and green scotch bonnet peppers, on the other hand, remains unknown, which prompts this inquiry. On a nearby farm in Ado Ekiti, Nigeria, red and green scotch bonnet peppers were picked, split into flesh and seed, mixed, air-dried at room temperature and then ground into powder. With the aid of an HPLC-UV detector, the samples were tested for their amino acid content and vitamin B 1 , B 2 , and B 6 identification. For the red scotch bonnet pepper, 17 amino acids were identified at different retention times and classified as essential and non-essential. Eighteen amino acids were discovered in the seed and eighteen in the flesh. However, serine was only found in the seed and not in the flesh. Furthermore, eighteen acids were identified in the flesh and seed of green pepper, eighteen in the flesh, and fifteen in the seed. Serine has only been detected in the flesh and in the flesh and seed. The vitamins B 1 , B 2 , and B 6 were all present in the seed and flesh of the red and green peppers studied with varying heights and retention times. The abundance of serine in the green pepper seed and flesh, as well as the availability of eighteen amino acids shows that it is more nutritious than the red pepper.
... Autism spectrum disorder, a severe neurodevelopmental condition, has also been linked to altered GM and Trp metabolism (Kaluzna-Czaplinska et al., 2019). Polymorphisms in the gene encoding the AhR nuclear translocator have been associated with the severity of autism spectrum disorder (Fujisawa et al., 2016). ...
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Acne vulgaris is a chronic inflammatory skin disease in which the influence of gut microbiota has been implicated but without clarification of mechanisms. Gut microbiota may exert such an influence via metabolites, particularly those of tryptophan. End metabolites of tryptophan activate receptors, including aryl hydrocarbon, G protein-coupled, and pregnane X receptors to stabilize the immune microenvironment and intestinal mucosal homeostasis. Any impact on the pathogenesis of acne vulgaris remains unclear. The current review collates recent advances concerning potential roles of tryptophan metabolism in mediating skin inflammation, follicular sebaceous gland function and intestinal permeability, all of which influence the pathogenesis of acne vulgaris. The aim was to improve understanding of the pathogenesis of acne vulgaris and to expose therapeutic opportunities.
... July 2022 | Volume 13 | Article 922707 8 functions (Kałużna-Czaplińska et al., 2019). There are three metabolic pathways for tryptophan: 1) The kynurenine pathway metabolizes 95% of tryptophan. ...
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Traditional Chinese medicine (TCM) is an important complementary and alternative branch of chronic kidney disease (CKD) therapy. Jian-Pi-Yi-Shen formula (JPYSF) is a TCM formula used for treating CKD with good efficacy. However, the underlying mechanisms of JPYSF in treating CKD remain to be elucidated. The purpose of the present study was to investigate the renoprotective effect and potential mechanism of JPYSF in treating CKD. CKD rat model was induced by feeding a diet containing 0.75% w/w adenine for 4 weeks. JPYSF was given by gavage every day, starting from the 3rd week of the adenine-containing diet and continuing for 4 weeks at the dose of 10.89 g/kg. Renal injury was evaluated by serum creatinine (Scr), blood urea nitrogen (BUN), histopathology, and fibrotic markers expression. Serum levels of tryptophan metabolites were detected by ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Aryl hydrocarbon receptor (AHR) signaling was tested by Western blot analysis. The results found that JPYSF treatment significantly lowered Scr and BUN levels, improved renal pathological injury, and down-regulated fibrotic markers expression in CKD rats. Furthermore, JPYSF significantly reduced the levels of 10 tryptophan metabolites in the serum of CKD rats and restored the level of tryptophan. Additionally, the kidney expression of AHR signaling was enhanced in CKD rats and was further suppressed in JPYSF treated rats. These results suggested that JPYSF protected against adenine-induced CKD via modulating tryptophan metabolism and AHR activation.
... Changes in TRP metabolism can alter the availability of TRP for protein and 5-HT biosynthesis and dysregulate the levels of immuno-and neuro-active KYN metabolites. Recently, research has begun to assess KYN metabolites as biomarkers to many neuro-and immune-associated illnesses, including Alzheimer's Disease [43][44][45], ALS [46], and Major Depressive Disorder [47]. ...
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Tryptophan (TRP) is an essential dietary amino acid that, unless otherwise committed to protein synthesis, undergoes metabolism via the Tryptophan-Kynurenine (TRP-KYN) pathway in vertebrate organisms. TRP and its metabolites have key roles in diverse physiological processes including cell growth and maintenance, immunity, disease states and the coordination of adaptive responses to environmental and dietary cues. Changes in TRP metabolism can alter the availability of TRP for protein and serotonin biosynthesis as well as alter levels of the immune-active KYN pathway metabolites. There is now considerable evidence which has shown that the TRP-KYN pathway can be influenced by various stressors including glucocorticoids (marker of chronic stress), infection, inflammation and oxidative stress, and environmental toxicants. While there is little known regarding the role of TRP metabolism following exposure to environmental contaminants, there is evidence of linkages between chemically induced metabolic perturbations and altered TRP enzymes and KYN metabolites. Moreover, the TRP-KYN pathway is conserved across vertebrate species and can be influenced by exposure to xenobiotics, therefore, understanding how this pathway is regulated may have broader implications for environmental and wildlife toxicology. The goal of this narrative review is to (1) identify key pathways affecting Trp-Kyn metabolism in vertebrates and (2) highlight consequences of altered tryptophan metabolism in mammals, birds, amphibians, and fish. We discuss current literature available across species, highlight gaps in the current state of knowledge, and further postulate that the kynurenine to tryptophan ratio can be used as a novel biomarker for assessing organismal and, more broadly, ecosystem health.
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If life developed in hydrothermal vents, it would have been within mineral membranes. The first proto-cells must have evolved to manipulate the mineral membranes that formed their compartments in order to control their metabolism. There must have occurred a biological takeover of the self-assembled mineral structures of the vents, with the incorporation of proto-biological molecules within the mineral membranes to alter their properties for life's purposes. Here, we study a laboratory analogue of this process: chemical-garden precipitation of the amino acids arginine and tryptophan with the metal salt iron chloride and sodium silicate. We produced these chemical gardens using different methodologies in order to determine the dependence of the morphology and chemistry on the growth conditions, as well as the effect of the amino acids on the formation of the iron-silicate chemical garden. We compared the effects of having amino acids initially within the forming chemical garden, corresponding to the internal zones of hydrothermal vents, or else outside, corresponding to the surrounding ocean. The characterization of the formed chemical gardens using X-ray diffraction, Fourier transform infrared spectroscopy, elemental analysis, and scanning electron microscopy demonstrates the presence of amino acids in these structures. The growth method in which the amino acid is initially in the tablet with the iron salt is that which generated chemical gardens with more amino acids in their structures.
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Background: The gut microbiome influences host physiology and cardiometabolic diseases by interacting directly with intestinal cells or by producing molecules that enter the host circulation. Given the large number of microbial species present in the gut and the numerous factors that influence gut bacterial composition, it has been challenging to understand the underlying biological mechanisms that modulate risk of cardiometabolic disease. Scope of the review: Here we discuss a systems-based approach that involves simultaneously examining individuals in populations for gut microbiome composition, molecular traits using "omics" technologies, such as circulating metabolites quantified by mass spectrometry, and clinical traits. We summarize findings from landmark studies using this approach and discuss future applications. Major conclusions: Population-based integrative approaches have identified a large number of microbe-derived or microbe-modified metabolites that are associated with cardiometabolic traits. The knowledge gained from these studies provide new opportunities for understanding the mechanisms involved in gut microbiome-host interactions and may have potentially important implications for developing novel therapeutic approaches.
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Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers. Although extensive research has been conducted on chemotherapy and immunotherapy for PDAC patients, the efficacy of these treatments varies, partially due to immunosuppressive environments and the limited infiltration of effector cells into tumors. Here, we highlight the potential application of iron (Fe) chelators to inhibit immunosuppressive environments in PDAC. This assumption is based on data from published articles indicating that 1) tryptophan (TRP) plays a role in immunosuppressive microenvironments; 2) metabolism of TRP is regulated by tryptophan hydroxylase (TPH), indoleamine-2,3-dioxygenase (IDO), and tryptophan-2,3-doxygenase (TDO), all of which are Fe-dependent enzymes; and 3) wide varieties of both synthetic Fe chelators and natural products are available for clinical application. In light of these data from the literature, we hypothesize that combining Fe chelators with immunotherapeutic or chemotherapeutic agents in PDAC patients can improve treatment efficacy by inhibiting tryptophan depletion in the surrounding mesenchymal stromal cells, allowing effector cells to infiltrate the tumor, and enabling cytotoxic drugs to access the targeted cells.
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Problem/condition: Autism spectrum disorder (ASD). Period covered: 2012. Description of system: The Autism and Developmental Disabilities Monitoring (ADDM) Network is an active surveillance system that provides estimates of the prevalence and characteristics of ASD among children aged 8 years whose parents or guardians reside in 11 ADDM Network sites in the United States (Arkansas, Arizona, Colorado, Georgia, Maryland, Missouri, New Jersey, North Carolina, South Carolina, Utah, and Wisconsin). Surveillance to determine ASD case status is conducted in two phases. The first phase consists of screening and abstracting comprehensive evaluations performed by professional service providers in the community. Data sources identified for record review are categorized as either 1) education source type, including developmental evaluations to determine eligibility for special education services or 2) health care source type, including diagnostic and developmental evaluations. The second phase involves the review of all abstracted evaluations by trained clinicians to determine ASD surveillance case status. A child meets the surveillance case definition for ASD if one or more comprehensive evaluations of that child completed by a qualified professional describes behaviors that are consistent with the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision diagnostic criteria for any of the following conditions: autistic disorder, pervasive developmental disorder-not otherwise specified (including atypical autism), or Asperger disorder. This report provides ASD prevalence estimates for children aged 8 years living in catchment areas of the ADDM Network sites in 2012, overall and stratified by sex, race/ethnicity, and the type of source records (education and health records versus health records only). In addition, this report describes the proportion of children with ASD with a score consistent with intellectual disability on a standardized intellectual ability test, the age at which the earliest known comprehensive evaluation was performed, the proportion of children with a previous ASD diagnosis, the specific type of ASD diagnosis, and any special education eligibility classification. Results: For 2012, the combined estimated prevalence of ASD among the 11 ADDM Network sites was 14.5 per 1,000 (one in 69) children aged 8 years. Estimated prevalence was significantly higher among boys aged 8 years (23.4 per 1,000) than among girls aged 8 years (5.2 per 1,000). Estimated ASD prevalence was significantly higher among non-Hispanic white children aged 8 years (15.3 per 1,000) compared with non-Hispanic black children (13.1 per 1,000), and Hispanic (10.2 per 1,000) children aged 8 years. Estimated prevalence varied widely among the 11 ADDM Network sites, ranging from 8.2 per 1,000 children aged 8 years (in the area of the Maryland site where only health care records were reviewed) to 24.6 per 1,000 children aged 8 years (in New Jersey, where both education and health care records were reviewed). Estimated prevalence was higher in surveillance sites where education records and health records were reviewed compared with sites where health records only were reviewed (17.1 per 1,000 and 10.4 per 1,000 children aged 8 years, respectively; p<0.05). Among children identified with ASD by the ADDM Network, 82% had a previous ASD diagnosis or educational classification; this did not vary by sex or between non-Hispanic white and non-Hispanic black children. A lower percentage of Hispanic children (78%) had a previous ASD diagnosis or classification compared with non-Hispanic white children (82%) and with non-Hispanic black children (84%). The median age at earliest known comprehensive evaluation was 40 months, and 43% of children had received an earliest known comprehensive evaluation by age 36 months. The percentage of children with an earliest known comprehensive evaluation by age 36 months was similar for boys and girls, but was higher for non-Hispanic white children (45%) compared with non-Hispanic black children (40%) and Hispanic children (39%). Interpretation: Overall estimated ASD prevalence was 14.5 per 1,000 children aged 8 years in the ADDM Network sites in 2012. The higher estimated prevalence among sites that reviewed both education and health records suggests the role of special education systems in providing comprehensive evaluations and services to children with developmental disabilities. Disparities by race/ethnicity in estimated ASD prevalence, particularly for Hispanic children, as well as disparities in the age of earliest comprehensive evaluation and presence of a previous ASD diagnosis or classification, suggest that access to treatment and services might be lacking or delayed for some children. Public health action: The ADDM Network will continue to monitor the prevalence and characteristics of ASD among children aged 8 years living in selected sites across the United States. Recommendations from the ADDM Network include enhancing strategies to 1) lower the age of first evaluation of ASD by community providers in accordance with the Healthy People 2020 goal that children with ASD are evaluated by age 36 months and begin receiving community-based support and services by age 48 months; 2) reduce disparities by race/ethnicity in identified ASD prevalence, the age of first comprehensive evaluation, and presence of a previous ASD diagnosis or classification; and 3) assess the effect on ASD prevalence of the revised ASD diagnostic criteria published in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition.
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