β-Hydroxybutyrat (BHB): the pathway in the body

β-Hydroxybutyrat (BHB) is part of the pathway “Ketone bodies”. This page shows the whole pathway; the station of β-Hydroxybutyrat (BHB) is highlighted.

Where this laboratory value sits: β-hydroxybutyrate — BHB, measurable in blood. The dehydrogenase BDH1 transfers hydrogen from NADH to acetoacetate, producing β-hydroxybutyrate. By quantity it is the largest ketone body in the blood. Source 1, 2

In brief

Ketone bodies are small molecules that the liver builds from fatty acids; the largest share is β-hydroxybutyrate. Muscle, heart, kidney and brain turn them back into acetyl-CoA and feed it into the citrate cycle.

The pathway step by step

1. Fatty acids → Acetyl-CoA β-oxidation · carnitine (CPT1), NAD⁺, FAD In β-oxidation, the fatty acid is shortened step by step by two carbon atoms at a time; this yields acetyl-CoA. Source 4
2. Acetyl-CoA → Acetoacetyl-CoA Thiolase A thiolase joins two acetyl-CoA molecules. This step is reversible and depends on how much acetyl-CoA is present. Source 2
3. Acetoacetyl-CoA → HMG-CoA HMG-CoA synthase 2 · acetyl-CoA Mitochondrial HMG-CoA synthase adds a further acetyl-CoA. This enzyme sets the pace of the whole pathway and is found mainly in the liver. Source 3, 2
4. HMG-CoA → Acetoacetate HMG-CoA lyase HMG-CoA lyase splits off acetoacetate. A small part breaks down spontaneously into acetone, which is breathed out through the lungs. Source 2, 1
5. Acetoacetate → β-hydroxybutyrate BDH1 · NADH The dehydrogenase BDH1 transfers hydrogen from NADH to acetoacetate, producing β-hydroxybutyrate. By quantity it is the largest ketone body in the blood. Source 1, 2
6. BHB in the cell → Acetoacetate BDH1 · NAD⁺ The same dehydrogenase BDH1 runs in reverse in the tissues: it removes hydrogen and passes it on to NAD⁺. Source 2
7. Acetoacetate → Acetoacetyl-CoA SCOT · succinyl-CoA The enzyme SCOT transfers a CoA group from succinyl-CoA to acetoacetate. This step occurs in almost all tissues, but not in the liver. Source 2, 1
8. Acetoacetyl-CoA → Acetyl-CoA Thiolase · CoA A thiolase splits acetoacetyl-CoA into two acetyl-CoA. Source 2
9. Acetyl-CoA → Citric acid cycle Acetyl-CoA enters the citric acid cycle. The electrons gained there drive ATP production in the respiratory chain. Source 1

Cofactors in this pathway

• Carnitine — Carrier with which CPT1 brings the fatty acids into the mitochondrion Source 4In the ORY catalogue as a laboratory value: Carnitin
• Coenzyme A — Carries the acetyl and acyl groups all the way from fatty acid to ketone body Source 1, 2
• NAD⁺ — Hydrogen carrier in β-oxidation and at BDH1, which interconverts acetoacetate and BHB Source 4, 1In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Vitamin B2 (FAD) — Flavin cofactor of the acyl-CoA dehydrogenases in β-oxidation Source 4
• Succinyl-CoA — In tissues it hands its CoA group to acetoacetate; the enzyme SCOT performs this step Source 2
• Leucine — Amino acid whose breakdown reaches acetoacetate through the same HMG-CoA lyase Source 5, 2In the ORY catalogue as a laboratory value: Leucin

Sources

1. Puchalska P, Crawford PA. Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics. Cell Metab 2017 · PubMed 28178565
2. Fukao T, Mitchell G, Sass JO et al. Ketone body metabolism and its defects. J Inherit Metab Dis 2014 · PubMed 24706027
3. Hegardt FG. Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase: a control enzyme in ketogenesis. Biochem J 1999 · PubMed 10051425
4. Adeva-Andany MM, Carneiro-Freire N et al. Mitochondrial β-oxidation of saturated fatty acids in humans. Mitochondrion 2019 · PubMed 29551309
5. Arnedo M, Latorre-Pellicer A, Lucia-Campos C et al. More Than One HMG-CoA Lyase: The Classical Mitochondrial Enzyme Plus the Peroxisomal and the Cytosolic Ones. Int J Mol Sci 2019 · PubMed 31817290

Whole pathway: Ketone bodies

Related pathways

• Cholesterol and lipoproteins — hmg-coa
• Alanine — NAD⁺ (Nicotinamidadenindinukleotid), Leucin
• Trans fatty acids — Carnitin, NAD⁺ (Nicotinamidadenindinukleotid)
• Triglycerides — Carnitin, NAD⁺ (Nicotinamidadenindinukleotid)

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert approval pending
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