Past Experiences for Future Applications of Metabolomics in Critically Ill Patients with Sepsis and Septic Shocks

A disruption of several metabolic pathways in critically ill patients with sepsis indicates that metabolomics might be used as a more precise tool for sepsis and septic shock when compared with the conventional biomarkers. This article provides information regarding metabolomics studies in sepsis and septic shock patients. It has been shown that a variety of metabolomic pathways are altered in sepsis and septic shock, including amino acid metabolism, fatty acid oxidation, phospholipid metabolism, glycolysis, and tricarboxylic acid cycle. Based upon this comprehensive review, here, we demonstrate that metabolomics is about to change the world of sepsis biomarkers, not only for its utilization in sepsis diagnosis, but also for prognosticating and monitoring the therapeutic response. Additionally, the future direction regarding the establishment of studies integrating metabolomics with other molecular modalities and studies identifying the relationships between metabolomic profiles and clinical characteristics to address clinical application are discussed in this article. All of the information from this review indicates the important impact of metabolomics as a tool for diagnosis, monitoring therapeutic response, and prognostic assessment of sepsis and septic shock. These findings also encourage further clinical investigations to warrant its use in routine clinical settings.


Introduction
Sepsis is a clinical syndrome defined as a life-threatening organ dysfunction caused by a dysregulation of the host's response to infection [1]. Septic shock is a worse condition than sepsis, in which patients need vasopressor administration to maintain their mean arterial pressure over 65 mmHg, in a combination with serum lactate level greater than 2 mmol/L, despite adequate volume resuscitation [1]. Sepsis and septic shock result in systemic abnormalities involved in circulatory, cellular, and metabolic failure [1], and are associated with high mortality rate, which is 25-35% of critical illness [2]. Therefore, early identification and management within a golden period, according to hour-1 bundle [3], are considered crucial to improve patients' survival. Unfortunately, conventional biomarkers, including procalcitonin and interleukin-6, have a limitation in their sensitivity and accuracy [4]. For this reason, new biomarkers are needed, and metabolomics is currently considered as a new hope for sepsis biomarkers [5].
Metabolomics is one of the omics sciences providing information about low molecular weight chemical compounds inside human biological specimens. This has become increasingly acknowledged in critically ill patients with sepsis and septic shock. However, the complicated alterations of various metabolic pathways in sepsis make it difficult for every single metabolite to serve as the sole biomarker for the clinical practice of sepsis. This article aims to provide an overview of metabolomic-assisted tools for critically ill patients with sepsis and septic shock in order to spot the possibility of applying metabolomics for diagnosis, monitoring of the treatment response, and prognostic evaluation of these two serious conditions. The information from this comprehensive review can contribute to further studies, integrating metabolomics with other molecular modalities and clinical manifestation, leading to the establishment of the use of a metabolomic approach in the clinical settings of sepsis and septic shock, which ultimately results in a reduction in sepsis-related mortality.
The pertinent evidence of metabolomic-assisted tools for critically ill patients with sepsis and septic shock was searched in correspondence with the assigned keywords, including 'metabolomics, adult critically ill patients, and sepsis OR septic shock' from PubMed, from its inception to May 2021. The relevant studies from this search were extracted and grouped into five issues, including the application of metabolomics for (1) sepsis diagnosis, (2) septic shock diagnosis, (3) prognostication in sepsis, (4) prognostication in septic shock, and (5) monitoring treatment response in sepsis and septic shock.

Metabolomics for Sepsis Diagnosis
Despite limited investigation, six studies reported several metabolites which differentiated sepsis patients from their non-septic counterparts ( Table 1). The elderly patients were prone to have sepsis than the young patients, and sepsis patients were more severe than those who did not have sepsis, as indicated by a higher APACHE-II score. Serum and plasma are the two common biomaterial samples [8][9][10][11][12][13]; however, one study [13] used erythrocytes for investigation in addition to plasma. Samples were mostly gathered within 24-36 h after the admission to ICU. According to those six reports, the common metabolic pathways that facilitated sepsis diagnosis were (1) amino acids and amines, (2) fatty acid (FA)-related metabolites, and (3) phospholipids.
Critically ill patients with sepsis had a wide range of amino acid spectral changes that differ from SIRS [11]

Alterations of Amino Acids and Amines in Sepsis
It has been shown that most amino acids were decreased during sepsis, as triggered by catabolic hormones, inflammatory mediators, and ubiquitin proteasome system [14]. Using a metabolomic approach, the changes in amino acids as a result of sepsis are comprehensively summarized in Table 1. In sepsis, the levels of cysteine and lysine were decreased [10,11], whereas the levels of glycine, serine, polyamines, and amino acid-derived carnitines were increased due to the body's response to infection [9,10,12].

A Decrease in Cysteine and Lysine
The level of cysteine and lysine were found to be decreased in sepsis patients [10]. Since cysteine is used for the synthesis of antioxidants glutathione and taurine, a reduction in cysteine suggests an increased requirement of cysteine for antioxidants synthesis to counteract oxidative stress in sepsis [15].
Indeed, lysine is involved in physiological functions for cytokines synthesis, lymphocytes proliferation, nitric oxide (NO) synthesis regulation, anti-viral infection, and antiinflammation [11,16]. In mice with sepsis, lysine supplementation revealed a lesser degree of inflammation and less hypotensive episodes than those received placebo [17]. However, the knowledge regarding the roles of lysine supplementation in patients with sepsis remains very limited. Future clinical studies are needed to clarify its use in septic patients.

An Increase in Glycine, Serine, Polyamines, and Amino Acid-Derived Acylcarnitines
Glycine has been shown to attenuate inflammatory response [18], and has a cytoprotective property [19]. This metabolite was increased in patients with sepsis [10]. Interestingly, glycine administration was shown to reduce hepatic injury in endotoxic shock [20]. Despite its promising benefits, limited clinical study is available. Therefore, its protective mechanism and safety dose remain to be elucidated [19].
Serine, a precursor for glucose and glycine synthesis, was found to be increased during sepsis [12]. Both glycine and serine are involved in the generation of glutathione, and are necessary for the physiological function of macrophages [21] and T lymphocytes proliferation [22]. An increment of serine in sepsis patients could be a compensatory process to counteract sepsis-induced oxidative stress [16,21].
The level of polyamines, including spermidine and spermine, were increased in patients with sepsis [12]. Two studies revealed serum [23] and polyamines, originally synthesizing from the arginine-ornithine pathway [24], to be involved in the RNA synthesis, transcription, and translation of human cells [25,26], as well as in several pathogens [24,27]. However, the interaction between host cells and the pathogen's polyamine requires elucidation. Moreover, polyamines might be markers for sepsis severity. Therefore, the correlation between polyamines levels and sepsis severity should be further investigated.
Patients with sepsis were also found to have increased levels of short-chain acylcarnitines (ACs), including C3 to C5 carnitines [9]. These ACs are derived mainly from branched-chain amino acids (BCAAs), which suggest an enhancement of BCAA oxidation [28] and are markers of insulin resistance [29]. Interestingly, the interplay between sepsis and insulin resistance is most commonly encountered in order to provide sufficient glucose for a cellular consumption [30]. Thus, short-chain ACs may represent the state of insulin resistance induced by sepsis as well.

Alterations of Fatty Acids and Their Related Metabolites in Sepsis
In sepsis, eicosanoid FAs were decreased [13]. In contrast, free monounsaturated fatty acids (MUFAs) that degrade from the cell membrane phospholipids were increased [13]. Moreover, free FA-derived ACs and ceramides were altered [8,9,12].

A Decrease of Eicosanoids
The circulating phospholipase A2 (PLA2) mediates a release of proinflammatory lipids called eicosanoids from the cell membrane phospholipids [31]. Eicosanoids are composed of arachidonic acid (AA) and polyunsaturated fatty acids (PUFAs) that have multiple regulatory functions involved in the inflammatory process [31]. A significant decrease in some n-3 PUFAs, including docosapentaenoic acid (C22:5 n-3) and docosahexaenoic acid (C22:6 n-3), was revealed [13]. These are the precursors of anti-inflammatory mediators, called resolvins. Hence, a greater reduction in eicosanoids may reflect the degree of sepsis severity, on which a future study should be conducted to identify this relationship.

An Increase in Free Fatty Acids
A noticeable increase in MUFAs in patients with sepsis was exhibited, particularly oleic acid (18:1 n-9) [13]. This increment may be as a result of sepsis-induced lipolysis [32].

An Alteration of Fatty Acid-Derived Acylcarnitines and Ceramides
The major function of carnitines is to facilitate the transportation of long-chain fatty acids (LCFAs) from cytoplasm into the mitochondria for β-oxidation [28]. Moreover, carnitines can remove an overwhelming amount of incomplete oxidized FAs in order to prevent the intoxication of acyl-CoA [28]. In other words, increased medium-chain ACs represent increased incomplete β-oxidation. It was observed that there were alterations of ACs in patients with sepsis compared to those without sepsis. These changes include a decrease in long-chain ACs-C16:2(OH) carnitine [12], and an increasing level of medium-chain ACs-C6 [9,12], C8 [9], and C10 carnitines [9]. These findings suggested that sepsis is associated with decreased FA uptake into the mitochondria and increased incomplete β-oxidation.

A Decrease in Sphingomyelines and Lysophosphatidylcholines
The level of SMs were found decreased in both plasma and erythrocyte of patients with sepsis, which included C34 to C44 SMs [13]. It is well known that inflammation triggers SM catabolism via the activation of acid sphingomyelinase enzymes, leading to increased downstream metabolites of SM, such as ceramides and PCs [36].

Metabolomics for Septic Shock Diagnosis
Two studies revealed serum and plasma [23,42] metabolites that could discriminate septic shock patients from sepsis patients without shock, as listed in Table 2. These metabolites were amino acids and amines, as well as glycolysis-related metabolites.

A Decrease in Branched-Chain Amino Acids, Glutamine, Glutamate, Arginine, and Proline
BCAAs, including leucine, isoleucine, and valine, are categorized as essential amino acids that have a protein anabolic effect, and elicit wound healing promotion, hepatic encephalopathy prevention, and insulin secretion stimulation [43]. Two studies reported a reduction in BCAAs in their septic shock patients [23,42]. However, current evidence does not support BCAAs' supplementation in sepsis patients owing to controversial outcomes [43,44]. Therefore, BCAA supplementation in critically ill patients may be resolved by metabolomics study, since metabolomics may help identify the metabolism mediating the effect of BCAA supplementation on sepsis.
Glutamine is an essential amino acid that plays a role in immune function, glutathione production, and the biosynthesis of purines and pyrimidines [45]. A decrement in glutamine in septic shock patients was demonstrated from a single study [23]. This finding may reflect increased glutamine consumption that exceeds the rate of biosynthesis in sepsis [43,45].
Glutamate has functions for preserving nitrogen balance in skeletal muscle [45] and promoting the clearance of nitrogen waste products in the liver before excretion via the kidney [46,47]. Moreover, glutamate can convert into α-ketoglutarate, which is one of the energy sources in the TCA cycle. Both studies revealed a decrease in the glutamate level of their septic shock patients [23,42], which is likely to be a result of low dietary intakes and hepatic glutamate synthesis, as well as an increase in glutamate consumption [46,48].
Arginine, one of intermediate amino acids in the urea cycle, has a pertinent role in the biosynthesis of protein, NO, creatine, urea, and polyamines [49,50]. A decrease in the arginine levels of septic shock patients was demonstrated in both studies [23,42]. Although a low level of arginine is associated with sepsis, its supplementation remains a controversial issue [51]. Indeed, some benefits were shown in trauma and critically ill surgical patients [52]. However, in sepsis shock patients, worsening hemodynamic instability is aggravated, as arginine can turn into NO [49,50].
Proline functions to regulate the cellular redox state, promote lymphocytes proliferation, eradicate pathogens via superoxide formation, advocate wound healing, and is involved in ornithine and polyamine formation via pyrroline-5-carboxylate (P5C) [16]. Proline was decreased in septic shock patients from both studies [23,42]. This result might represent the promotion of tissue repair during sepsis [53].
Septic shock patients had different patterns in amino acids, fatty acids, and TCA cycle metabolites [23] Fatty acids - Continuous data are presented in median (IQR1-3). Abbreviations: 1 H-NMRS, 1 H-Nuclear Magnetic Resonance Spectroscopy; APACHE-II score, Acute Physiology and Chronic Health Evaluation-II score; TCA, tricarboxylic Acid.

An Increase in Aromatic Amino Acids
Phenylalanine, tyrosine, and tryptophan are categorized as AAAs [54,55]. Phenylalanine has an important function for producing tetrahydrobiopterin (BH4) cofactor, which is involved in arginine-NO synthesis [56,57]. Although the increment in phenylalanine was reported in septic shock [23,42], the information of tyrosine and tryptophan alteration remained unknown. For this reason, a metabolomics study quantifying a whole AAA metabolic pathway in sepsis and septic shock should be established.

Alterations of Glycolysis-Related Metabolites in Septic Shock
Hypoglycemia was found in both studies [23,42]. This phenomenon could be a result of low dietary intake, a decrease in gluconeogenesis, the depletion of glycogen, or an increase in peripheral consumption [32]. Furthermore, a decrease in mannose and an increase in sucrose and myo-inositol were found [23,42]. Although it is difficult to explain the exact role of these sugars, the alterations may be modulated by the organism, rather than the body's response to infection.
Acetylcarnitine (C2 carnitine) represents a cycling of acetyl-CoA, which is a product of glycolysis [58]. C2 carnitine was found elevated in patients with septic shock [23,42], owing to an excess acetyl group from metabolic stress that may intoxicate the cell [59].

Alterations of Amino Acids and Amines in Sepsis Non-Survivors
It was shown that taurine, tryptophan, glutamate, arginine, and serine were decreased in non-surviving patients with sepsis [11]. In contrast, intermediate metabolites of amino acids, including S-(3-methyl-butanoyl)-dihydrolipoamide-E [10], amino acidderived carnitines [61,62], symmetric dimethylarginine (SDMA) [61,66], and asymmetric dimethylarginine (ADMA) [66], were found to be increased in non-surviving patients with sepsis. The details are shown in Table 3. Fatty acids - Fatty acids - Fatty acids - Amino acids and amines - Fatty acids - At H24 Amino acids and amines - Non-surviving 28-D sepsis patients had specific changes in amino acids, fatty acids, glycolysis, and bile acids' metabolic pathways, as well as an increase in aromatic microbial metabolites [62] Fatty acids -    [65] Continuous data are presented in mean ± SD, otherwise reported as median and IQR1-3. $ Sequential Organ Failure Assessment (SOFA) score. Abbreviations Taurine, a sulfur-containing amino acid that most abundant in leukocytes, is recognized as an antioxidant, and exerts an antimicrobial effect [16,67]. Moreover, a decreased level of taurine was associated with hyperlactatemia and cardiopulmonary dysfunction [68]. One study reported a reduction in taurine in non-surviving patients with sepsis at day 28 [11]. Nonetheless, taurine supplementation in sepsis patients revealed promising results in terms of decreasing IL-6 and improving clinical outcomes [69].
One study showed a decreased level of tryptophan associated with sepsis mortality at day 28 [11]. A reduction in tryptophan reflects the increased tryptophan degradation to be kynurenine owing to cytokines release [70], and is considered to be one of the main mechanisms of hypotension in sepsis [71]. Correspondingly, a study showed an increased level of kynurenine in patients with sepsis non-survivors at day 28 [62]. Taken together, a decrease in tryptophan and an increase in kynurenine level may be potential factors with which to determine the severity of sepsis. Moreover, tryptophan can be metabolized to be serotonin and melatonin [72], which attenuate the inflammatory response and play a significant role in the regulation of mood, circadian rhythm, and sleep [73]. A supplementation of tryptophan in critical illness may decrease the incidence of sleep deprivation and delirium, which are common problems in critically ill patients [74]. Thus, future studies are needed to answer this question.
Glutamate, arginine, and serine were decreased in sepsis non-survivors at day 28 [11]. Interestingly, serine alteration was different between study populations. It was decreased in non-surviving patients with sepsis at day 28 [11] but was increased in patients with sepsis, as previously mentioned in Section 3.1.2 [12]. An overwhelming utilization of serine in sepsis non-survivors might explain this finding.

An Increase in S-(3-Methyl-butanoyl)-dihydrolipoamide-E, Amino Acid-Derived Acylcarnitines, and Symmetric Dimethylarginine and Asymmetric Dimethylarginine
S-(3-methyl-butanoyl)-dihydrolipoamide-E is an intermediate metabolite of leucine degradation [75]. One study demonstrated an increase in this metabolite in non-surviving patients with sepsis within 48 h of admission [10]. Moreover, an increase in short-chain ACs: C3, C4, C5, C5-OH, and C5:1 carnitines were observed in non-surviving patients with sepsis at day 28 [61,62]. These increments may be related to an abnormality in BCAA synthesis and degradation. However, further metabolomic studies covering all metabolites in BCAA synthesis and short-chain ACs production are needed to identify which steps of BCAAs are disrupted in septic non-survivors.
SDMA and ADMA are residual from a broken-down arginine process [76] and are considered as competitive inhibitors of NO synthesis [77]. Based on mortality at day 28, prior studies demonstrated a higher level of SDMA and ADMA at day 1 [61,66], 3 [66], and 7 [66] in sepsis non-survivors than the survivors. These results might suggest an excessive production of NO in septic shock non-survivors.

Alterations of Fatty Acids and Their Related Metabolites in Sepsis Non-Survivors
Non-surviving patients with sepsis at day 7 had an increase in several lipid mediators [60]. Specifically, AA-derived FAs, including prostaglandin F2α and leukotriene B4, and specialized pro-resolution lipid mediators including 17R-protectin D1, resolvin D5, and resolvin E1, were persistently elevated along a study period of 7 days [60]. It is well known that lipid mediators, particularly prostaglandin and leukotriene, enhance vasodilatation and inflammation [78], which may ameliorate by non-steroidal, anti-inflammatory drugs [79,80]. However, a vulnerability to acute kidney injury in critically illness [81,82] limits the feasibility of using NSAIDs in the case of most septic patients. Further study is needed to establish the appropriate medications to counteract the devastated lipid mediators.

Alterations of Glycolysis-Related Metabolites in Sepsis Non-Survivors
Pyruvate and lactate were increased in sepsis non-survivors at day 28 [64]. Instead of entering into the TCA cycle, pyruvate accumulation can convert to lactate when tissue is confronted with poor perfusion [84].

Alterations of Aromatic Microbial Metabolites in Sepsis Non-Survivors
It was demonstrated that plasma 3-(4-hydroxyphenyl) lactic acid was positively associated with death from sepsis at day 28 [62]. This metabolite is classified as an aromatic microbial metabolite that has been shown to be involved in the pathogenesis of septic shock [85]. Therefore, these reports emphasize the impact of microbiota on the development and progression of sepsis.

A Decrease in Dimethylamine and Citrulline
One study revealed a decreased level of dimethylamine in septic shock non-survivors [42]. Dimethylamine is originated from fish and seafood consumption and it is a byproduct of the post-translation modification of arginine that involved in the inhibition of NO synthesis [76,92]. Therefore, future research might be conducted to identify the benefit of dimethylamine supplementation in septic shock patients.
Another study reported that a decrement of citrulline at baseline could predict septic shock non-survivors at day 7 [87]. This low level of citrulline is likely due to a decrease in citrulline absorption through the gastrointestinal tract [49]. Or it might be related to a low level of ornithine and arginosuccinate [87], which involved in citrulline metabolism in the urea cycle pathway. These results suggested that the disturbance of urea cycle can be a potential indicator of septic shock with non-survival.

An Increase in Symmetric Dimethylarginine, Total Dimethylarginine, and Tyrosine
A prior study revealed that SDMA and total DMA, competitive inhibitors of NO synthesis, were increased in septic shock non-survivors at day 28 [89]. Hence, measuring these metabolites could be used prognosticating the patients with septic shock outcomes.
Tyrosine is a precursor for catecholamine biosynthesis under the activity of specific enzyme-phenylalanine hydroxylase (PAH) [16,55]. Two studies demonstrating an increase in tyrosine level as a predictor of the septic shock non-survivors at 24 h after ICU admission [86] and at day 28, respectively [89]. This phenomena might be explained by the fact that PAH function is impaired during sepsis [55].

An Alternation of Phenylalanine and Methionine
The alternation of phenylalanine in septic shock non-survivors were controversial. Indeed, previous studies revealed a decreased phenylalanine in septic shock non-survivors at day 30 [90] and at one year [91]. On the contrary, other studies showed an increased phenylalanine in the non-survivors at 24 h [86] and at day 7 [87]. Since phenylalanine and tyrosine are involved in the same pathway, a whole pathway study may be further investigated in order to justify how phenylalanine interacts in septic shock non-survivors.
Methionine, one of the sulfur-containing amino acids, is a precursor for the syntheses of homocysteine, cysteine, glutathione, creatine, and polyamines [93]. A prior study displayed a decrease in methionine in non-surviving patients with septic shock at day 30 [90]. Furthermore, a decrease in methionine day 7/day 1 ratio in sepsis non-survivors at day 28 were observed [89]. However, one study demonstrated an increased level of methionine in septic shock non-survivors at 24 h [86]. Therefore, a future study quantifying the levels of methionine and its related metabolites should be conducted in order to clarify the inconsistent findings of methionine among studies.

Alterations of Fatty Acid-Related Metabolites in Septic Shock Non-Survivors
In non-surviving patients with septic shock at day 7, the level of long-chain ACs, including C16 and C18 carnitines, were decreased [87]. On the contrary, the level of medium-chain ACs, including C6, C10, and C12 carnitines, were increased [87]. These results indicating a decrease in FA uptake into the mitochondria and incomplete β-oxidation of long-chain FAs, which can be one of the potential markers of septic shock mortality.

Alterations of Glycolysis-Related Metabolites and TCA Cycle Metabolites in Septic Shock Non-Survivors
Hyperglycemia is commonly present in sepsis to provide sufficient glucose for neurons and leucocytes [30]. Glucose was found to be increased in patients with septic shock nonsurvivors at day 30 [90]. However, hypoglycemia can also be detected in sepsis and is associated with a lethal outcome as well [32].
Lactate was found to be increased in septic shock non-survivors within 24 h [86] and at day 7 [87]. Moreover, pyruvate was also increased in the non-survivors at day 7 [87]. Acetylcarnitine (C2 carnitine) was decreased in patients with septic shock non-survivors at day 7 [87]. This finding is in contrast to that mentioned in Section 4.2, where septic shock patients had an increased acetylcarnitine compared to the non-septic shock patients [23,42]. These contradictory results may be explained by a failure of the metabolic flexibility of acetylcarnitine to switch as fuel sources during sepsis [28].
As a result of mitochondrial dysfunction in sepsis [95], multiple TCA cycle metabolites including citrate, α-ketoglutarate, succinate, fumarate, and malate were increased in septic shock non-survivors within 24 h and at day 7 [86,87]. These metabolites represent an augmentation of aerobic metabolism during sepsis, which promotes oxidative phosphorylation and releases of reactive oxygen species (ROS) [96,97]. The ROS may be one of several reasons for sepsis-induced mitochondrial dysfunction [98].

Metabolomics for Monitoring Treatment Response in Sepsis and Septic Shock
Two studies have revealed metabolomics that monitor the treatment response in sepsis population [99,100]. The first study aimed to identify patients who would reduce the use of vasopressor after L-carnitine supplementation using metabolomics study [99]. A good response was found in patients with a low ketotic state (3-hydroxybutyrate < 153 µM), where methionine, lysine, phenylalanine, and tyrosine were found to be increased after L-carnitine was given [99]. In addition, patients with a good response had a low level of carnitine and acetylcarnitine as well [99]. The second study characterized septic shock patients who had a sequential organ failure assessment (SOFA) score of lower than 8 or a drop of more than 5 within 48 h after treatment [100]. An untargeted metabolomics comparing the degree of metabolites changes between the responders and non-responders overtime (from baseline to 48 h) found that myristic acid and oleic acid were more decreased, whereas creatinine was less decreased in the responders than that of the non-responders [100]. From targeted metabolomics, alterations of several metabolites showed differently. When comparing the degree of metabolites changes overtime, kynurenine was increased in the responders but lower than of the non-responders. Interestingly, several phospholipids had a predictive ability to determine treatment response. Indeed, most SMs, SM(OH)s, and PCs were increased in the responders, whereas they were decreased in the non-responders. In addition, SM C16:1 and C16-C20 lysoPCs were both increased in both groups; however, a greater increment was found in the responders than in the non-responders [100]. These findings suggest that several metabolites can be potential markers for monitoring the treatment response in sepsis patients.

Conclusions
Metabolomics is about to change the world of sepsis biomarkers. Not only can it be utilized for sepsis diagnosis, but also for prognosticating and monitoring the therapeutic response. In this review, we found that sepsis can lead to the alteration of enormous metabolites. Some significant amino acids, according to the study populations, are summarized in Figure 1. Multiple amino acids can be used as operational markers for sepsis determination, for instance, arginine, proline, and kynurenine, whereas BCAAs can be used as predictive markers to identify patient survival.  (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table 3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A down-sided triangle (▼) represents a decreased level, whereas an up-sided triangle (▲) represents vice versa. (A) Aromatic amino acids (AAAs) and its down-stream amino acid are illustrated. Phenylalanine is converted to tyrosine before metabolizing to dihydroxyphenylalanine (DOPA) and catecholamines, respectively. Tryptophan is another AAA that can change to either kynurenine or serotonin and melatonin. (B) Branched-chain amino acids (BCAAs) including leucine, isoleucine, and valine are catabolized to S-(3-methylbutanoyl)-dihy-  (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table 3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A down-sided triangle ( ) represents a decreased level, whereas an up-sided triangle ( ) represents vice versa. (A) Aromatic amino acids (AAAs) and its down-stream amino acid are illustrated. Phenylalanine is converted to tyrosine before metabolizing to dihydroxyphenylalanine (DOPA) and catecholamines, respectively. Tryptophan is another AAA that can change to either kynurenine or serotonin and melatonin. (B) Branched-chain amino acids (BCAAs) including leucine, isoleucine, and valine are catabolized to S-(3-methylbutanoyl)-dihydrolipoamide-E and amino acid-derived acylcarnitines (C3-5), respectively. (C) A substrate of glutathione synthesis begins with methionine that converts into homocysteine, cystathionine, and cysteine, respectively. Moreover, serine is involved in the cystathionine production with an exchange of α-ketoglutarate. Cysteine can also turn into taurine, which has an anti-oxidant effect. (D) Glutamate is an intermediate substrate between glutathione production and urea cycle-related metabolites. Glutamate can be converted into glutamine and pyrroline-5-carboxylate (P5C). The latter metabolite is a precursor for proline synthesis. Moreover, glutamate can interchange with the urea cycle pathway metabolites.
Free FAs are summarized in Figure 2. These metabolites also present as sepsis biomarkers as well; however, a further study combining FAs with other connecting metabolic pathways, in particular FA-derived ACs and TCA cycle metabolites, with a step-by-step metabolomic approach in each of these pathways, is needed. Free FAs are summarized in Figure 2. These metabolites also present as sepsis biomarkers as well; however, a further study combining FAs with other connecting metabolic pathways, in particular FA-derived ACs and TCA cycle metabolites, with a step-bystep metabolomic approach in each of these pathways, is needed.  (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table 3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A down-sided triangle (▼) represents a decreased level, whereas an up-sided triangle (▲) represents vice versa. Monounsaturated fatty acids (MUFAs) are found increased in sepsis diagnosis, whereas polyunsaturated fatty acids (PUFAs) are found contractedly. Most of ceramides are increased, together with an increase in arachidonic acids. Long-chain fatty acids enter mitochondrial for fatty acid oxidation (β-oxidation) under the carnitine shuttle process. The final product of β-oxidation is acetyl-CoA that can enter TCA cycle for an energy production. However, mitochondrial dysfunction in sepsis can alter β-oxidation process, leading to an accumulation of medium-chain acylcarnitines in the cytoplasm and in the circulation, which indicates incomplete β-oxidation.  (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table 3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A down-sided triangle ( ) represents a decreased level, whereas an up-sided triangle ( ) represents vice versa. Monounsaturated fatty acids (MUFAs) are found increased in sepsis diagnosis, whereas polyunsaturated fatty acids (PUFAs) are found contractedly. Most of ceramides are increased, together with an increase in arachidonic acids. Long-chain fatty acids enter mitochondrial for fatty acid oxidation (β-oxidation) under the carnitine shuttle process. The final product of β-oxidation is acetyl-CoA that can enter TCA cycle for an energy production. However, mitochondrial dysfunction in sepsis can alter β-oxidation process, leading to an accumulation of medium-chain acylcarnitines in the cytoplasm and in the circulation, which indicates incomplete β-oxidation.
Phospholipids - In NR, H48 vs. H0; Amino acids and amines • Taurine  Figure 3 demonstrates phospholipid-related membrane phospholipids, as sepsis always interferes in this structure. For this reason, any interventions that can specifically protect the cell membrane injury by septic pathogens may be established under the guide of the levels of these metabolites. Figure 3 demonstrates phospholipid-related membrane phospholipids, as sepsis a ways interferes in this structure. For this reason, any interventions that can specificall protect the cell membrane injury by septic pathogens may be established under the guid of the levels of these metabolites.  (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table 3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A down-sided triangle (▼) represents a decreased level, whereas an up-sided triangle (▲) represents vice versa. Several kinds of the cell membrane phospholipids are involved in this setting. An alteration of phosphatidylcholines (PCs), phosphatidylserine (PS), phosphatidylglycerols (PGs), lysophosphatidylcholines (LysoPCs), lysophosphatidylethanolamines (LysoPEs), cardiolipins, and sphingomyelines (SMs) are demonstrated.
Besides hemodynamic resuscitation in sepsis patients, mitochondrial resuscitatio remains another issue for treatment response monitoring [101]. Although lactate is a com monly accepted use for the monitoring of tissue perfusion, the alternations of mitochon drial TCA cycle metabolites may also be used in mitochondrial resuscitation as well. complete understanding of whole glycolysis and TCA cycle metabolites, as in Figure  could (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table 3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A down-sided triangle ( ) represents a decreased level, whereas an up-sided triangle ( ) represents vice versa. Several kinds of the cell membrane phospholipids are involved in this setting. An alteration of phosphatidylcholines (PCs), phosphatidylserine (PS), phosphatidylglycerols (PGs), lysophosphatidylcholines (LysoPCs), lysophosphatidylethanolamines (LysoPEs), cardiolipins, and sphingomyelines (SMs) are demonstrated.
Besides hemodynamic resuscitation in sepsis patients, mitochondrial resuscitation remains another issue for treatment response monitoring [101]. Although lactate is a commonly accepted use for the monitoring of tissue perfusion, the alternations of mitochondrial TCA cycle metabolites may also be used in mitochondrial resuscitation as well. A complete understanding of whole glycolysis and TCA cycle metabolites, as in Figure 4, could be well-achieved by metabolomics study.   (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table  3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A down-sided triangle (▼) represents a decreased level, whereas an up-sided triangle (▲) represents the opposite. The alterations of several sugars are found, including glucose, sucrose, mannose, and myo-inositol. Glucose, a main energy source for human cells, converts to pyruvate and acetyl-CoA, respectively, before entering the TCA cycle. In addition, acetylcarnitine (C2 carnitine) can feed via  (Table 1), septic shock diagnosis ( Table 2), prognostication of sepsis (Table 3), prognostication of septic shock (Table 4), and monitoring the treatment response (Table 5). A downsided triangle ( ) represents a decreased level, whereas an up-sided triangle ( ) represents the opposite. The alterations of several sugars are found, including glucose, sucrose, mannose, and myo-inositol. Glucose, a main energy source for human cells, converts to pyruvate and acetyl-CoA, respectively, before entering the TCA cycle. In addition, acetylcarnitine (C2 carnitine) can feed via acetyl-CoA as well. Another anaerobic metabolite, lactate, are found to be increased in sepsis patients with poor prognosis. Citrate is the initial metabolite of the TCA cycle, which turns into isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate, respectively. The increases in TCA cycle-related metabolites represent an augmentation of aerobic metabolism during sepsis, which can promote oxidative phosphorylation and reactive oxygen species (ROS) production. An increase in the ROS level may be one of the potential mechanisms mediating sepsis-induced mitochondrial dysfunction.

Limitation and Future Direction of the Metabolomic Research in Sepsis
The current advanced technology of metabolomics is just shinning the light at the beginning of the tunnel in terms of providing more precise biomarkers for sepsis. An uncertain standardization of the metabolomic studies is one of the major issues for replicating the results. Moreover, the complexity and heterogeneity of sepsis create are a large variety of study populations. Therefore, a combination of conventional biomarkers and metabolomic profiling is likely a key solution. Moreover, multi-omics modalities and systems of biology should be simultaneously approached, since sepsis is also associated with the alterations of protein and gene expression [102,103].
According to septic shock resuscitation, besides the bed-sided hemodynamic monitoring, metabolic and cellular resuscitation are crucial strategic keys to improve patient outcomes. Currently, no definite metabolic abnormality is warranted beyond hyperlactatemia [1]. We highly expect that metabolomics can be one of the solutions used to examine mitochondrial function or even abnormalities of the microcirculation in sepsis. Future studies with the metabolomics approach, attempting to develop bedside laboratory kits, are incredibly significant for the clinical practice.
Metabolomics to foresee outcomes of patients with sepsis and septic shock are undergoing research. Death signaling metabolites' modulation is another remedy to improve patients' outcomes. The integral application of pharmacometabolomics to identify the appropriate drugs or to certify suitable, well-responsive patients is another approach for precision medicine.
Nowadays, a lot of information about metabolites in sepsis comes across without enough pathophysiological elucidation. Moreover, the overwhelming evidence of metabolomic information in sepsis is challenging. Therefore, artificial intelligence facilitating machine learning experiences may be a worthwhile solution to handle this enormous information. In the end, we believe that metabolomics can illuminate the light at the end of the tunnel of sepsis management.