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. 2025 Oct 30;10(44):53317–53326. doi: 10.1021/acsomega.5c08154

Monosodium Glutamate Consumption Disrupts Vitamin B6 Status in Rat Kidney

Manatsaphon Sukmak †,‡, Kanokwan Nahok †,‡, Chalongchai Chalermwat †,‡, Worachart Lert-itthiporn †,‡, Atit Silsirivanit †, Porntip Pinlaor ‡,§, Apisit Chaidee ∥, Sirirat Anutrakulchai ‡,∥, Somchai Pinlaor †,⊥, Carolyn M Slupsky #, Ubon Cha′on †,‡,*
PMCID: PMC12612882  PMID: 41244394

Abstract

Monosodium glutamate (MSG) is widely used as a food additive and flavor enhancer, but concerns have emerged regarding its potential effects on kidney function and vitamin B6 metabolism. This study investigated the impact of varying levels of dietary MSG intake on vitamin B6 status in a rat model. Male Wistar rats were divided into four groups and administered MSG in their drinking water at concentrations of 0, 0.5, 1.5, and 3.0 g % for 12 weeks. Three isoforms of vitamin B6, pyridoxal (PL), pyridoxal 5′-phosphate (PLP), and pyridoxic acid (PA), were measured in plasma, liver, kidney, and urine using HPLC-MS. Kidney metabolites were analyzed by 1H nuclear magnetic resonance spectroscopy to assess MSG-related metabolic changes. We found that plasma PL and PLP levels were elevated in all MSG-treated groups compared with controls, regardless of dose. This correlated with increased hepatic PLP levels and a decreased PLP catabolic ratio (PAr). In contrast to the liver, PLP levels in the kidney were significantly lower compared to controls. Metabolomics profiling of kidney tissue revealed elevated levels of various amino acids and metabolic intermediates in the MSG-treated rats. In conclusion, MSG consumption cannot induce vitamin B6 deficiency systematically, such as in the liver and plasma, as long as dietary vitamin B6 is adequate. However, depletion of PLP occurred locally in the kidney. Because MSG consumption may affect renal vitamin B6 status, there is a need for further studies to elucidate the underlying mechanisms and assess the potential health implications of habitual MSG intake.

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1. Introduction

Vitamin B6, a water-soluble vitamin, plays a critical role in numerous physiological processes, including amino acid metabolism, neurotransmitter synthesis, hemoglobin production, and immune function. Vitamin B6 exists in the human body in six interchangeable forms: pyridoxal (PL), pyridoxine (PN), pyridoxamine (PM), and their respective phosphorylated derivatives, including pyridoxal 5′- phosphate (PLP), pyridoxine 5′-phosphate (PNP), and pyridoxamine 5′-phosphate (PMP). , Humans obtain vitamin B6 from both exogenous sources, via dietary intake, and endogenous sources, via B6-producing gut bacteria that populate the large intestine. − After absorption, the nonphosphorylated forms of vitamin B6 (PL, PN, and PM) are transported to the liver, the major organ for vitamin B6 metabolism. The active form of vitamin B6, PLP, functions as a coenzyme in over 160 enzymatic reactions, ,, including those involved in amino acid metabolism, lipid metabolism, neurotransmitter synthesis, and other essential physiological processes. 4-Pyridoxic acid (4-PA), the major catabolic end-product of vitamin B6 metabolism, is produced in the liver through oxidative degradation of PL or PLP and is excreted in urine. These vitamin B6 vitamers can be measured directly in biological samples such as blood (both plasma and serum), urine, and erythrocytes, and can provide a direct assessment of the body’s vitamin B6 levels. Currently, PLP, PA, and the ratio of PA to (PLP + PL), also known as PAr, are the most commonly used direct biomarkers for assessing vitamin B6 catabolism. ,

Vitamin B6 deficiency is a condition that can have significant health implications, as vitamin B6 is essential for various physiological functions. , One of the most important cellular functions of vitamin B6 is its role in both the biosynthesis and degradation of amino acids. PLP participates in transamination reactions, leading to the formation of various organic acids, including intermediates of the TCA cycle and gluconeogenesis. In addition, PLP serves as a coenzyme in α-decarboxylation and racemization reactions. Key pathways for vitamin B6 include the metabolism of tryptophan and homocysteine, which are linked to inflammation and immune activation. Several studies have reported that lower serum PLP levels are associated with increased risks of cardiovascular disease. , In addition, PAr has been shown to correlate with biomarkers of inflammation, as well as with all-cause mortality and cardiovascular disease mortality.

Monosodium glutamate (MSG) is a widely used flavor enhancer, recognized for its ability to enhance umami taste and improve the overall palatability of food. Despite its recognized benefits in the food industry, concerns about its safety and potential health effects are still being debated. Some epidemiological studies have reported that MSG consumption is associated with metabolic syndrome, obesity, , and hypertension. However, conflicting evidence exists. , The metabolic effects of MSG either orally , or parenterally , appear to be consistent in animal models, with evidence showing impacts on metabolic organs including the liver, pancreas, and kidney. Using an animal study, we previously showed that several metabolites associated with vitamin B6 metabolism in the liver and kidney tissues of rats treated with 1 g % MSG for 2 weeks were significantly altered. Elevated PN levels in the liver and reduced PN levels in the kidney of MSG-treated rats suggested alterations of vitamin B6 metabolism. Notably, histamine, kynurenic acid, and nicotinamide metabolites, which are linked to tryptophan metabolism, were observed to be lower in MSG-treated rats.

To further understand the mechanism by which MSG affects vitamin B6 status in the production, transport, and excretion by organs, we conducted a 12 week animal study to evaluate the dose–response relationships between MSG intake and vitamin B6 levels in the plasma, liver, kidneys, and urine.

2. Materials and Methods

2.1. Reagents

Pure food-grade (99%) MSG was used for mixing with drinking water. Analytical grade chemicals were used for HPLC-MS analysis. Metaphosphoric acid (HPLC grade) was purchased from Supelco (Burlington, MA, United States). Heptafluorobutyric acid (HPLC grade) was purchased from Sigma-Aldrich (Darmstadt, Germany). Methanol (HPLC grade) and hexane (PR grade) were purchased from RCI Labscan (Bangkok, Thailand). Perchloric acid (AR grade) was purchased from Duksan (Gyeonggi-do, South Korea). Throughout the experiments, we used reagent-grade water from a Millipore Milli-Q system. Mobile phase A was prepared using ultrapure water and 5 mM heptafluorobutyric acid, and methanol served as mobile phase B. The column utilized was an Agilent C18, 250 × 4.6 mm 5 μm analytical column (Santa Clara, CA, United States). All vitamin B6 standard substances, PL, pyridoxal 5′-phosphate (PLP), and PA (purity > 98%) were purchased from Sigma-Aldrich (Darmstadt, Germany). Each standard substance was dissolved in water to generate a 1000 mg/L stock solution. Standard substance stock solutions were protected from light and stored at −80 °C. A mixture solution containing all internal standards was prepared by diluting the pure IS stock solutions in ultrapure water into 0.20, 0.10, 0.05, 0.01, 0.005, and 0.001 mg/L.

2.2. Animal Study

Adult male Wistar rats (N = 32) were obtained from the Northeast Laboratory Animal Center, Khon Kaen University, Thailand. Animals were acclimatized for 2 weeks before the experiment. All animals were housed individually with ad libitum access to drinking water and food (Perfect Companion Group, Thailand). The rats were kept in stainless steel cages in a temperature- and humidity-controlled room maintained with a 12 h light–dark cycle. All protocols for this study were approved by the guidelines of the Institutional Animal Care and Use Committee of Khon Kaen University under the Ethics of Animal Experimentation of the National Research Council of Thailand (IACUC-KKU-6/63).

Animals were divided into four groups, with 8 rats in each group (N = 8/group). The rats received different concentrations of MSG dissolved in their drinking water: 0.0 g % (control group), 0.5 g % (low group), 1.5 g % (medium group), and 3.0 g % (high group), administered over a 12 week period. Food and water intake were measured every day. Bodyweight was recorded every week. Urine output was recorded every 2 weeks. Metabolic cages were used for the collection of 24 h urine, and samples were kept at −20 °C until analysis. One rat in the control group was excluded due to illness. At the end of the experiment, the rats were euthanized by using carbon dioxide inhalation. Plasma samples were collected via cardiac puncture, with the use of heparin as an anticoagulant. Liver and kidney organs were collected and stored at −80 °C until analysis.

2.3. Sample Preparation for HPLC-MS Analysis

2.3.1. Plasma

The protocol for preparation of the plasma samples was modified from a previous study. Prior to analysis, 200 μL of heparinized plasma was mixed with 5% metaphosphoric acid (v/v) (1:1) to precipitate plasma proteins. The samples were centrifuged at 14,000 rpm for 4 min at 4 °C, and the supernatants were filtered through a 0.22 μm cellulose acetate filter (OnePlus, Thailand) before injection into the HPLC-MS system.

2.3.2. Urine

The protocol for the sample preparation of urine samples was based on a prior study. A total of 100 μL of urine were treated in a 1:1 ratio with 6% perchloric acid to precipitate proteins and impurities. After mixing and centrifugation (4 min, at 4 °C and 14,000 rpm), the supernatant was filtered through a 0.22 μm cellulose acetate filter (OnePlus, Thailand) before injection into the HPLC-MS system.

2.3.3. Liver and Kidney Tissues

The protocol for sample preparation of tissue samples is based on a prior study. For each rat, 0.5 g of frozen liver and 0.1 g of frozen kidney were cryoground separately into powders. To extract, 10 mL of methanol was added to the frozen powders, and samples were sonicated for 30 min. Tissue homogenates were centrifuged at 3,500 rpm for 10 min at 4 °C. The supernatant obtained after centrifugation was carefully transferred into a new tube. To this, 10 mL of hexane was added, and the mixture was shaken before centrifugation at 3,500 rpm for 10 min at 4 °C. The upper (hexane) layer was then separated and removed. The lower layer containing methanol was dried by using a rotary evaporator before resuspension in 1 mL of deionized water. The resuspended solution was filtered through a 0.22 μm cellulose acetate filter (OnePlus, Thailand) before being injected into the HPLC-MS system.

2.4. HPLC-MS Analysis

HPLC-MS analysis was performed using an Agilent 1100 HPLC-MS system equipped with a C18 column (250 mm × 4.6 mm, 5 μm particle size). The mobile phase consisted of A = 5 mM heptafluorobutyric acid in water and B = methanol. The HPLC gradient conditions are listed in Table . B6 vitamers, including PL, PLP, and PA, were separated using the following binary gradient program with positive mode (flow rate = 0.8 mL/min).

1. HPLC Gradient Conditions.

time (min) mobile phase A (%) mobile phase B (%)
0 97 3
5 85 15
14 10 90
15 97 3
20 97 3
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a

Mobile phase A: 5 mM heptafluorobutyric acid in water; mobile phase B: methanol.

Total time per sample for this gradient program was 20 min. The injection volume was 100 μL for all of the samples. Chromatographic separation was performed on an HPLC system connected to a mass selective detector (MSD1). The mass spectrometer was operated in selected ion monitoring (SIM) mode with the following extracted ion chromatograms (EICs): PLP: m/z 248 (EIC = 247.7–248.7), retention time 8.705 min; 4 PA: m/z 184 (EIC = 183.7–184.7), retention time 10.973 min; PL: m/z 168 (EIC = 167.7–168.7), retention time 13.437 min. The concentration of each B6 vitamer was calculated relative to the peak areas of the internal standard. Six-point standard curves were prepared from the injection of the following ranges of vitamers: PLP, PL, and PA (0.001, 0.005, 0.01, 0.05, 0.1, and 0.2 mg/L). Data acquisition and calculations were performed using Chromatography Software.

2.5. Metabolomics Analysis Using 1H NMR Spectro­scopy

The kidney tissues were prepared following the protocol described in the study by Beckonert et al. Prior to NMR acquisition, the polar phase of tissue extracts was resuspended in 580 μL of NMR buffer (100 mM sodium phosphate buffer, pH 7.4 in D2O, containing 0.1 mM sodium trimethylsilyl-[2,2,3,3-2H4]-propionate (TSP) and 0.2% NaN3). The samples were briefly vortexed and centrifuged at 12,000 xg for 5 min, and then 550 μL of the mixture was transferred to 5 mm NMR tubes for subsequent NMR analysis.

NMR spectra of kidney tissue were acquired using a Bruker Avance 600 MHz III HD NMR spectrometer (Bruker, Massachusetts, United States) at a temperature of 298.15 K equipped with an inverse cryoprobe operating at a proton NMR frequency of 600.13 MHz. Each sample was collected with 128 scans using the zg30 pulse sequence with a spectral width of 12019.23 Hz, a pulse width of 10 μs, an acquisition time of 4 s, and a recycle delay of 1.0 s. TSP was used as an internal reference (δ1H 0.00), and its peak area was used to aid in the quantification of metabolites. Raw spectra were phase, baseline, and shim corrected using Chenomx Processor version 12.0 (Chenomx Inc., Edmonton, Canada). The water peak was removed from the kidney spectrum (δ1H 4.60 and 5.10) using the region deletion tool. Metabolite identification and quantification (μM) were completed using Chenomx Profiler version 12.0, which contains a library of hundreds of reference compounds (Chenomx Inc., Edmonton, Canada).

2.6. Statistical Analysis

All data were tested for normality using the Descriptive Statistics Function and the Shapiro–Wilk test in SPSS. When data satisfied the assumption of normality, group comparisons were conducted using one-way ANOVA followed by Tukey’s HDS posthoc test. For data that deviated from normality, nonparametric tests (Kruskal–Wallis with Dunn’s multiple comparisons) were used. Significance level was considered at p < 0.05. All analyses were done using GraphPad Prism version 9.0 (GraphPad Software Inc., USA) and SPSS (version 28.0, SPSS Inc., Chicago, Illinois, USA).

3. Results

3.1. No Effects of MSG in Drinking Water on Animal Growth

To determine whether MSG in drinking water can impact growth, body weight was monitored weekly throughout the 12 weeks of the experiment. We found no significant difference in bodyweight across all groups at any time point (Table and Figure S1). MSG consumption was calculated by measuring the daily water intake of each animal, and animals were grouped according to the amount of MSG consumed, 0 (control), 0.25–0.27 (low), 0.83–1.05 (medium), and 1.70–2.01 (high) g/day, over the 12 week period. All rats consumed an animal standard diet that contained PN at a concentration of 20 mg/kg of food. Daily food intake was recorded, and the average B6 intake was calculated to be from 0.44 to 0.54 mg/day with no significant differences in B6 intake among the groups.

2. Body Weight, as Well as MSG and Vitamin B6 Intake During Experimental Period .

  control (n = 7) low-MSG intake (n = 8) medium-MSG intake (n = 8) high-MSG intake (n = 8)
body weight (g)        
week 0 425.3 ± 31.2 428.9 ± 31.5 426.3 ± 29.0 422.1 ± 27.4
week 4 482.5 ± 38.2 477.8 ± 39.4 480.8 ± 33.9 474.3 ± 33.5
week 8 518.0 ± 42.5 510.2 ± 41.1 515.5 ± 32.8 512.7 ± 38.4
week 12 546.4 ± 43.0 536.1 ± 46.4 544.3 ± 37.0 539.6 ± 43.0
MSG intake (g/day)        
week 0 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00
week 4 0.00 ± 0.00 0.27 ± 0.06acd 1.05 ± 0.24abd 1.95 ± 0.41abc
week 8 0.00 ± 0.00 0.26 ± 0.03acd 1.01 ± 0.20abd 2.01 ± 0.20abc
week 12 0.00 ± 0.00 0.25 ± 0.05acd 0.83 ± 0.25abd 1.70 ± 0.35abc
vit. B6 intake (mg/day)        
week 0 0.51 ± 0.04 0.52 ± 0.03 0.48 ± 0.04 0.48 ± 0.04
week 4 0.50 ± 0.08 0.47 ± 0.05 0.48 ± 0.04 0.47 ± 0.04
week 8 0.46 ± 0.07 0.44 ± 0.06 0.44 ± 0.05 0.48 ± 0.05
week 12 0.51 ± 0.04 0.54 ± 0.05 0.50 ± 0.03 0.51 ± 0.02
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a

Control: 0 g % MSG in drinking water, low-MSG intake: 0.5 g % MSG in drinking water, medium-MSG intake: 1.5 g % MSG in drinking water, and high-MSG intake: 3.0 g % MSG in drinking water. Data are presented as mean ± standard deviation (SD). Statistical significance between groups was determined using one-way ANOVA (*p < 0.05). Based on pairwise comparisons, superscript letters indicate significant differences: a (vs control), b (vs low-MSG intake), c (vs medium-MSG intake), and d (vs high-MSG intake).

3.2. MSG Intake Increases PLP in Plasma and Liver but Decreases PLP in the Kidney of the Rat

To explore whether MSG consumption impacted vitamin B6 metabolism, plasma, liver, kidney, and urine samples were analyzed to determine B6-vitamer levels using HPLC-MS analysis. Three major B6 isoforms, including the precursor (PL), active (PLP), and excretion (PA) forms, were analyzed in all sample types. The changes in vitamin B6 vitamers were observed consistently across all MSG-treated groups. We found plasma PL and PLP levels were significantly higher in MSG-treated groups compared to the control group (Figure A,B). However, no significant differences in PA levels among the experimental groups were observed (Figure C). This led to a lower catabolic ratio (PAr) in all of the MSG-treated groups (Figure D). In hepatic tissues where the majority of vitamin B6 metabolism occurs, the pattern of PLP was similar; however, there was no difference in hepatic PL between the control and the MSG-treated groups. Interestingly, PA was lower in the MSG-treated groups, leading to a lower PAr for the treatment groups compared to the control (Figure ).

1.

1

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Concentrations of vitamin B6 vitamers in the plasma of Wistar rats after 12 weeks. Pyridoxal (PL) (A), pyridoxal 5′-phosphate (PLP) (B), pyridoxic acid (PA) (C), and PA to PL + PLP (PAr) ratio (D) were measured in both the control group and all MSG-treated groups. Data are presented as mean ± SD, and p-values were determined through one-way ANOVA (*p < 0.05) and Tukey’s multiple comparisons test.

2.

2

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Concentrations of vitamin B6 vitamers in hepatic tissue of Wistar rats after 12 weeks. PL (A), PLP (B), and PA (C) were assessed in both the control group and all MSG-treated groups. Data are presented as mean ± SD, and p-values were determined through one-way ANOVA (*p < 0.05) and Tukey’s multiple comparisons test.

In contrast to the liver and plasma, PLP in the kidney was lower in the MSG-treated groups compared to the control. Interestingly, PL was higher for the low-MSG group but not different from the control for the medium or high MSG groups. The excretion form of vitamin B6 (PA) was similar between all groups, and thus similar renal PAr values were observed in this tissue (Figure ).

3.

3

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Concentrations of vitamin B6 vitamers in renal tissue of Wistar rats after 12 weeks. PL (A), PLP (B), and PA (C) were assessed in both the control and all MSG-treated groups. Data are presented as mean ± SD, and p-values were determined through one-way ANOVA (*p < 0.05) and Tukey’s multiple comparisons test.

Analysis of urinary PA revealed that PA was lower in all MSG-treated groups compared to the control (Figure A). However, this was corrected by urine volume per day (Table S1); only the high-MSG consumption group had significantly lower PA (Figure B).

4.

4

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Urinary PA excretion. The urinary PA concentration (A) and urinary PA excretion (B) of PA at week 12 from all animal groups. Data are presented as mean ± SD, and p-values were determined through one-way ANOVA (*p < 0.05) and Tukey’s multiple comparisons test.

3.3. Metabolic Profile Changes in Kidney Tissue after MSG Consumption

As a decrease in the active form of vitamin B6 (PLP) was observed in the kidneys of MSG-treated animals, this might suggest that the kidneys experience an insufficiency of B6. We therefore further explored the kidney metabolome, using 1H NMR spectroscopy. Significant changes in kidney metabolites included alanine, aspartate, ethanolamine, fumarate, hypoxanthine, leucine, methionine, niacinamide, phenylalanine, threonine, and tyrosine, which were all higher, and inosine and PN, which were lower in MSG-treated groups compared to control (Figure ).

5.

5

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Kidney metabolic profile changes after MSG consumption. Data are shown as mean ± SD and p-values calculated by Kruskal–Wallis test (*p < 0.05, **p < 0.01, ***p < 0.001).

To provide a clearer visualization of the relationships between metabolites in the kidney tissue, a metabolic pathway map was constructed using KEGG IDs and MetaboAnalyst (Figure ). Metabolites impacted by MSG consumption could be broadly classified into several different pathways, including branched-chain amino acid metabolism, vitamin B6 metabolism, and glycerophospholipid, as well as purine metabolism. The metabolic pathways within the kidney that are impacted by MSG consumption are illustrated in Figure .

6.

6

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MetaboAnalyst results of the metabolic pathways associated with MSG consumption in the rat kidney.

7.

7

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Schematic depiction of the metabolites impacted in the kidney by MSG consumption. Metabolites surrounded by red frames indicate those that are higher, while those surrounded by blue frames are lower in rats consuming MSG. Abbreviations: G-6-P, glucose-6-phosphate; F-6-P, fructose-6-phosphate; F-1,6-BP, fructose-1,6-bisphosphate; PEP, phosphoenolpyruvate; PEPCK, phosphoenolpyruvate carboxykinase; PLP, pyridoxal 5′-phosphate; AMP, adenosine monophosphate; MNA, methylnicotinamide; 2-PY, N′-methyl-2-pyridone-5-carboxamide; 4-PY, N′-methyl-4-pyridone-5-carboxamide.

4. Discussion

This study demonstrates that MSG consumption alters vitamin B6 metabolism in both the liver and the kidney. Based on the plasma PLP levels, MSG does not appear to cause vitamin B6 deficiency systemically but may cause B6 insufficiency in kidney tissue. Our findings provide novel insights into the metabolic consequences of dietary MSG intake, particularly its impact on vitamin B6 homeostasis and kidney metabolism.

The liver is the primary site of vitamin B6 metabolism, where vitamin B6 vitamers, including PL, PN, and PM, are converted to PLP, the active form for many enzymatic reactions, especially amino acid metabolism. In our previous study, a decline of PN in the kidney and a reduction of metabolites related to PLP-dependent enzymes were observed in rats that received 1% MSG in drinking water for 14 days. In the present study, we aimed to determine whether MSG consumption caused vitamin B6 deficiency. Adult male Wistar rats were fed with a standard diet and were grouped based on the amount of MSG intake (zero, low, medium, or high)-MSG consumption via the drinking water for 12 weeks. Our findings suggest that MSG did not induce vitamin B6 deficiency, as plasma PLP concentrations in MSG-treated groups were higher than in the control group. It was previously reported that rats fed a vitamin B6-deficient diet for 4 weeks exhibited significantly lower plasma PLP levels (167 ± 50 nmol/L) compared to control rats (340 ± 157 nmol/L). In our case, both control and MSG-treated rats received the same amount of vitamin B6, which equated to 0.45–0.5 g/day for 12 weeks, depending on the food intake (Table ). This suggests that if dietary B6 is sufficient, depletion of vitamin B6 by MSG may not occur. To our surprise, MSG consumption induced the elevation of plasma PL and PLP levels. This might be the consequence of either high PLP production or low PLP degradation in the liver, supported by an elevated PLP and a reduced PA/(PL + PLP) ratio in MSG-treated groups compared to controls. How MSG increases PLP production and/or lowers PLP degradation in the liver will need to be further explored.

In contrast to the liver, kidney PLP levels were significantly lower in MSG-treated animals compared with controls (Figure B), suggesting that MSG consumption may contribute to vitamin B6 deficiency in the kidney. This finding is consistent with our previous study, where rats that received 1 g % MSG for 2 weeks had significantly lower PN levels in their kidneys compared to controls. The present study also confirmed a reduction of PN levels in the kidneys of MSG-treated rats, though it was only observed with medium or high intakes of MSG (0.83–2 g/day) (Figure K). Since the renal PL levels in the control and MSG-treated groups are comparable, altered transport from the plasma pool to the kidney may not be the case. Possible reasons for this may be renal utilization or localized degradation supported by global metabolite changes in kidney tissue after MSG consumption. We observed that many metabolites involved with branched-chain amino acid, vitamin B6, glycerolipid, and purine metabolism were impacted by MSG consumption (Figure ). PLP serves as a coenzyme in many reactions related to amino acid biosynthesis and degradation, including transamination, aldol cleavages, α-decarboxylations, racemizations, β- and γ-eliminations, as well as replacement reactions. Therefore, in the case of vitamin B6 deficiency, PLP-dependent enzymatic activities could be impaired, potentially leading to the accumulation of their precursors, i.e., alanine, aspartate, methionine, threonine, leucine, phenylalanine, and tyrosine (Figure ). Elevated levels of branched-chain amino acids (BCAAs: leucine), aromatic amino acids (phenylalanine and tyrosine), and threonine were found in vitamin B6-deficient Drosophila larvae and PLP-depleted cells.

Ethanolamine is a product from sphingolipid degradation and plays an essential role in cellular viability. Mammals cannot synthesize ethanolamine, and thus it is obtained from the diet as free ethanolamine or in the form of phosphatidylethanolamine (PE), which is degraded by phosphodiesterases to yield glycerol and ethanolamine. The increased ethanolamine found in this study (Figure C) may relate to PE degradation as part of cell membrane degradation. MSG may induce oxidative stress, potentially leading to renal tubular cell injury or death, as previously demonstrated in rats receiving MSG in drinking water for 9 months, as well as in rats that received MSG followed by its withdrawal and in hamsters receiving 2 g % MSG in drinking water for 8 months. As a consequence of membrane damage, several intracellular components, such as nucleic acids and proteins, are released. The elevated levels of hypoxanthine found in MSG-treated rats (Figure E) may be due to purine degradation, as it has previously been reported that mice receiving a dose of 30 mM (0.5 g %) MSG in drinking water for 30 weeks had higher nucleotide turnover and purine degradation, resulting in elevated uric acid in plasma and intrahepatic tissue.

In this study, the amounts of MSG given to animals (Table ) were converted to human equivalent doses (HED), which correspond to 5 g/day (low), 18 g/day (medium), and 35 g/day (high) for a person weighing 60 kg. To put this into perspective, a tablespoon of MSG is about 15 g, and a half rice ladle is about 30 g. These are quantities that are commonly added to a single dish or meal, such as a papaya salad, by some street food vendors and restaurants in Thailand. Therefore, the MSG doses used in this study are relevant to the levels of MSG that humans realistically consume on a daily basis.

We acknowledge that restricting the study to male rats is a limitation, and future research should include females to provide a more comprehensive understanding.

5. Conclusions

This study demonstrates that MSG alters vitamin B6 distribution in Wistar rats, characterized by elevated plasma and hepatic PLP but reduced renal PLP. Associated changes in amino acid, purine, and glycerophospholipid metabolites indicate that MSG impacts kidney metabolic pathways. These results suggest localized alterations in vitamin B6 homeostasis in the kidneys, warranting further mechanistic investigation.

Supplementary Material

ao5c08154_si_001.pdf (400.2KB, pdf)

Acknowledgments

We would like to thank the Northeast Laboratory Animal Center (NELAC) at Khon Kaen University for its animal husbandry facilities. Thanks to the Research Assistantship Program, Faculty of Medicine, for providing financial support to U.C. and K.N. Thanks to Central Laboratory (Thailand) Co., Ltd., Songkhla branch, for vitamin B6 determination. Thanks to Vidyasirimedhi Institute of Science and Technology (VISTEC), Wangchan Valley, Rayong, Thailand, for NMR technical support.

Glossary

Abbreviations

MSG

monosodium glutamate

PL

pyridoxal

PLP

pyridoxal 5′-phosphate

PA

pyridoxic acid

G-6-P

glucose-6-phosphate

F-6-P

fructose-6-phosphate

F-1,6-BP

fructose-1,6-bisphosphate

PEP

phosphoenolpyruvate

PEPCK

phosphoenolpyruvate carboxykinase

AMP

adenosine monophosphate

MNA

methylnicotinamide

2-PY

N′-methyl-2-pyridone-5-carboxamide

4-PY

N′-methyl-4-pyridone-5-carboxamide.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08154.

  • Growth curves of animals during the experimental period; urine volume of rats in the control and MSG-treated groups during the experimental period (PDF)

¶.

M.S. and K.N. contributed equally to this work. Conceptualization, U.C.; methodology, M.S. and U.C.; software, C.S.; validation, U.C., K.N., and M.S.; formal analysis, M.S., K.N., and U.C.; investigation, K.N., M.S., and C.S.; resources, A.S., W.L., C.C., S.P., P.P., A.C., and S.A.; data curation, U.C. and C.S.; writingoriginal draft preparation, K.N. and M.S.; writingreview and editing, U.C. and C.S.; visualization, K.N. and M.S.; supervision, A.S., W.L., S.P., and S.A.; project administration, U.C.; funding acquisition, U.C., C.C., P.P., S.P., and S.A. All authors have read and agreed to the published version of the manuscript.

This research was funded by the Fundamental Fund of Khon Kaen University (FF67), which received funding from the National Science Research and Innovation Fund (NSRF), Thailand.

Institutional Review Board Statement: the animal study protocol was approved by the Institutional Animal Care and Use Committee of Khon Kaen University under the Ethics of Animal Experimentation of the National Research Council of Thailand (IACUC-KKU-6/63).

The authors declare no competing financial interest.

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