Targeted Metabolomics Services

Carnitine and acylcarnitines

Updated 15/05/2024

What is measured?

Butyrobetaine (BB), carnitine (C0), total carnitine (tC0), acetylcarnitine (C2), propionylcarnitine (C3), malonylcarnitine (C3-DC), butyrylcarnitine (C4), hydroxybutyrylcarnitine (C4-OH), succinylcarnitine (C4-DC), isovalerylcarnitine (iC5), glutarylcarnitine (C5-DC), tiglylcarnitine (C5:1), hexanoylcarnitine (C6), octanoylcarnitine (C8), decanoylcarnitine (C10), dodecanoylcarnitine (C12), myristoylcarnitine (C14), hydroxytetradecanoylcarnitine (C14-OH), palmitoylcarnitine (C16), hydroxyhexadecanoylcarnitine (C16-OH), stearoylcarnitine (C18), hydroxyoctadecanoylcarnitine (C18-OH), oleoylcarnitine (C18:1), linoleylcarnitine (C18:2).
Methods: LC-MS/MS using mixed-mode (multi-modal) chromatography

Carnitine, acylcarnitines and related metabolites

Acylcarnitines (ACs) are esters formed by conjugation of L-carnitine (LC; L-3-hydroxy-4-aminobutyrobetaine) with coenzyme A-activated acyl groups (acyl-CoA), such as acetate, propionate, butyrate, medium-chain, long-chain, and very-long-chain fatty acyl-CoA, and these fatty acid metabolites are necessary for cellular energy homeostasis.

In humans, 75% of the LC requirement is obtained from dietary sources (eventually including supplementation), while the remaining 25% is supplied by de novo synthesis from trimethyllysine [1], which takes place in the liver and kidneys. The final step in the synthesis is catalyzed by the α-ketoglutarate-dependent enzyme, gamma-butyrobetaine dioxygenase (BBOX) that converts gamma-butyrobetaine into LC.

LC and its acyl esters are essential for the oxidative catabolism of fatty acids in mitochondria and peroxisomes, involved in the metabolism of branched-chain amino acids and related metabolic pathways, and serve as a detoxification pathway [2, 3].

The best-known biological function of LC is to enable the transport of medium- to long-chain fatty acids (>C10) and their CoA derivates from the cells’ cytosol into the mitochondrial matrix, where the beta-oxidation of fatty acids takes place, and various AC intermediates are formed in this process. Accordingly, the AC profile in blood has historically been used in the diagnosis of inborn errors of fatty acid oxidation.

A number of other physiological roles of the LC/AC system have been discovered more recently. This includes the removal of excess acyl groups, modulation of the intracellular level of CoA, and thus the regulation of the ratio between free CoA and acyl-CoA in the mitochondria [4]. This ratio regulates the activity of many mitochondrial enzymes involved in the TCA cycle, gluconeogenesis, urea cycle, and fatty acid oxidation [5, 6].

The intramitochondrial free CoA/acyl-CoA ratio is reflected in the ACs/LC ratio outside the mitochondria, which under normal conditions should be around 0.25. Ratios larger than 0.4 are considered abnormal and indicate disturbed mitochondrial (and/or peroxisomal) metabolism [4, 7]. Thus, the AC profile in blood and other tissues is used as a marker for various conditions associated with mitochondrial (and/or peroxisomal) dysfunction, including imbalances between glucose and fatty acid oxidation as well as disturbances in the metabolism of (branched-chain) amino acids.

Over the past 30 years, ACs have been identified as important biomarkers for a growing number of diseases and conditions, including cardiovascular diseases, insulin resistance, metabolic syndrome, diabetes, NAFLD/MAFLD, depression, neurologic disorders, certain cancers, inflammatory conditions, psoriasis, and chronic fatigue syndrome [2, 8]


To investigate the metabolomic signature of human diseases.

Specimen, collection and processing

Matrix: EDTA plasma or serum.
Volume: Minimum volume is 60 µL, but 200 µL is optimal and allows reanalysis.
Preparation and stability: 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 (µmol/L):BB, 0.2–2; C0, 7–90; tC0, 30–90; C2:0, 1–35; C3:0, 0.1–1.5; C3:0-DC, 0–0.5; C4:0, 0.04–1.2; C4:0-OH, 0.01–0.5; C4:0-DC, 0.01–1.0; iC5:0, 0.04–0.5; C5:0-DC, 0.01–0.3; C5:1, 0.005–0.1; C6:0, 0.005–2.1; C8:0, 0.03–1.1; C10:0, 0.03–2.1; C12:0, 0.01–0.5; C14:0, 0.015–0.3; C14:0-OH, 0.005–0.08; C16:0, 0.04–1.3; C16:0-OH, 0–0.05; C18:0, 0.01–0.5; C18:1, 0.02–1.0; C18:2, 0.02–0.3.

Intraclass correlation coefficients (ICCs): C0, 0.69; C2:0, 0.63; C3:0, 0.62; iC5:0, 0.68; C8:0, 0.41; C14:0, 0.38; C16:0, 0.49; C18:0, 0.40; C18:1, 0.66.


1. Longo N, Frigeni M, Pasquali M. Carnitine transport and fatty acid oxidation. Biochim Biophys Acta 2016; 1863: 2422-35.
2. Dambrova M, Makrecka-Kuka M, Kuka J et al. Acylcarnitines: nomenclature, biomarkers, therapeutic potential, drug targets, and clinical trials. Pharmacol Rev 2022; 74: 506-51.
3. Schooneman MG, Vaz FM, Houten SM, Soeters MR. Acylcarnitines: reflecting or inflicting insulin resistance? Diabetes 2013; 62: 1-8.
4. Reuter SE, Evans AM. Carnitine and acylcarnitines: pharmacokinetic, pharmacological and clinical aspects. Clin Pharmacokinet 2012; 51: 553-72.
5. Steiber A, Kerner J, Hoppel CL. Carnitine: a nutritional, biosynthetic, and functional perspective. Mol Aspects Med 2004; 25: 455-73.
6. Calabrese V, Giuffrida Stella AM, Calvani M, Butterfield DA. Acetylcarnitine and cellular stress response: roles in nutritional redox homeostasis and regulation of longevity genes. J Nutr Biochem 2006; 17: 73-88.
7. Pons R, De Vivo DC. Primary and secondary carnitine deficiency syndromes. J Child Neurol 1995; 10: 2S8-2S24.
8. Bene J, Szabo A, Komlósi K, Melegh B. Mass spectrometric analysis of L-carnitine and its esters: potential biomarkers of disturbances in carnitine homeostasis. Curr Mol Med 2020; 20: 336-54.

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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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