The tricarboxylic acid cycle
and related metabolites

What is the tricarboxylic acid cycle?

The tricarboxylic acid (TCA) cycle (also known as the citric acid cycle or Krebs cycle) is a series of reactions forming a loop that ultimately generate energy, mainly by oxidation of acetyl-CoA generated from glycolysis, and catabolism of fatty acids and amino acids. The energy is stored in guanosine triphosphate (GTP), NADH and FADH2 (Figure 1). The citric acid cycle also provides precursors for many compounds, including certain amino acids.
Recent basic research have demonstrated roles of Krebs cycle intermediates in intra- and intercellular signalling affecting physiology and pathogenesis (1). Krebs cycle intermediates have been linked to DNA and histone methylation (α-ketoglutarate and α-hydroxyglutarate), immune modulation (acetyl-CoA, itaconate, fumarate and succinate), thermogenesis (succinate), inhibition of hypoxia state and HIF-α stabilization (succinate, fumarate and α-hydroxyglutarate), and some are considered as “oncometabolites” defined as conventional metabolites that, when aberrantly accumulated, have pro-oncogenic functions (α-hydroxyglutarate, succinate (1), fumarate (2) and lactate (3)).
Studies on metabolomics involving Krebs cycle intermediates in relation to human health and disease usually include few patients and have been performed only recently. These metabolites have been related to BMI, cardiovascular disease (pyruvate, citrate, succinate), diabetes (pyruvate, isocitrate, succinate), NAFLD (isocitrate and citrate), longevity (isocitrate), asthma (succinate), disease activity in rheumatoid arthritis patients (itaconate), and worsening of clinical outcome in cancer patients (succinate, fumarate and α-hydroxyglutarate).

What does BEVITAL measure?

Citrate (Cit), cis-aconitate (cAco), isocitrate (iCit), α-ketoglutarate (aKG), fumarate (Fum), malate (Mal), oxaloacetate (Oaa), oxalate (Oxa), pyruvate (Pyr), lactate (Lac), itaconate (Ita).
Method(s): GC-MS/MS.


To investigate the metabolomic signature of human diseases.

Specimen, collection and processing

Matrix: EDTA plasma (preferred) or serum.
Volume: Minimum volume is 60 µL, but 200 µL is optimal and allows reanalysis.
Preparation and stability: Krebs cycle metabolites are probably stable in serum/plasma, but there are concern over oxaloacetate. Samples should be put on ice immediately after collection and stored at -80 °C.


Frozen, on dry ice. (for general instruction on transportation, click here)

Reported values, interpretation

Reported values: Cit: 90-200 µmol/L; cAco: 0.9-1.6 µmol/L; iCit: 2-22 µmol/L; aKG: 2-40 µmol/L; Suc: 2-12 µmol/L; Fum: 0.5-4 µmol/L; Mal: 2-21 µmol/L; Oaa: – ; Oxa: 6-12 µmol/L; Pyr: 80-160 µmol/L; Lac: 500-1600 µmol/L; Ita: 0.2-2.3 µmol/L.
Intraclass correlation coefficient (ICC): na.

June 16, 2022
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Amino acids, amino acid catabolites, acylcarnitines, TCA metabolites, ketone bodies, AGEs

Kynurenines, ratio-derived metabolites, proteins

SCFAs, indoles, choline metabolites, amino acid derived

B-vitamins, functional markers, and methyl donors, fat-soluble vitamins, essential amino acids, meat & fish intake, tobacco use & coffee intake
Biomarkers (n)

Amino acids: Alanine, Arginine, Asparagine, Aspartic acid, Glutamic acid, Glutamine, Glycine, Histidine, Isoleucine, Kynurenine, Leucine, Lysine, Methionine, Ornithine, Phenylalanine, Proline, Sarcosine, Serine, Threonine, Total cysteine, Tryptophan, Tyrosine, Valine

Amino acid catabolites: 2-Aminoadipic acid, 2-Hydroxybutyrate, 3-Hydroxysiobutyrate, α-Hydroxyglutaric acid, β-Alanine, β-Aminoisobutyrate, β-Hydroxy B-methylbutyric acid, Phenylacetylglutamine

Acylcarnities: BB, C0, C2, C3, C3-DC, C4, C4-OH, C4-DC, iC5, C5-DC, C5:1, C6, C8, C10, C12, C14, C14-OH, C16, C16-OH, C18, C18-OH, C18:1, C18:2

TCA metabolites: α-Ketoglutarate, Citrate, Fumarate, Isocitrate, Lactate, Malate, Pyruvate

Ketone bodies: Acetoacetate, 3-Hydroxybutyrate

AGEs: Carboxyethyllysine, Carboxymethyllysine


Kynurenines: 3-hydroxykynurenine, 3-hydroxyanthranilic acid, Anthranilic acid, Kynurenine, Kynurenic acid, Nicotinic acid, Nicotinamide, N1-methylnicotinamide, Picolinic acid, Quinaldic acid, Quinolinic acid, Xanthurenic acid


Proteins: C-Reactive protein, Calprotectin, Serum Amyloid A

Ratio-derived: Kynurenine/tryptophan ratio, PAr index (PLP, PL, PA)


SCFAs: Acetate, Butyrate, Formate, Isobutyrate, Isovalerate, Propionate, Valerate, α-Methylbutyrate

Indoles: 3-Indoxyl sulfate, Imidazole propionate, Indole-3-acetamide, Indole-3-acetate, Indole-3-aldehyde, Indole-3-lactate, Indole-3-propionate

Choline metabolites: Choline, Betaine, DMG, TMAO

Amino acid derived: Phenylacetylglutamine


B-vitamins, functional markers, and methyl donors: 4-Pyridoxic acid, Betaine, Choline, Cobalamin, Flavin mononucleotide, Folate, Methylmalonic acid, N1-methylnicotinamide, Nicotinamide, Nicotinic acid, Pyridoxal, Pyridoxal 5-phosphate, Pyridoxine, Riboflavin, Thiamine, Thiamine monophosphate, Total homocysteine

Fat-soluble vitamins: 25-hydroxy vitamin D2, 25-hydroxy vitamin D3, α-Tocopherol (Vit. E), All-trans retinol (Vit. A), Phylloquinone (Vit. K1), y-Tocopherol (Vit. E)

Essential amino acids: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine

Meat and fish intake: 1-Methylhistidine, 3-Methylhistidine, B-Alanine, Creatine, Creatinine, TMAO

Tobacco use and coffee intake: Cotinine, Trans-3-hydroxycotinine, Trigonelline

Volume (μl)
Analytical techniques
GC- and LC-MS/MS
GC- and LC-MS/MS
GC- and LC-MS/MS
Turnaround time (weeks)

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