Osteocalcin (undercarboxylated)
The bone-secreted hormone that turns out to talk to muscle, pancreas, testis and brain — proposed as the signal that lets the skeleton coordinate exercise capacity, glucose handling and fertility.
Also known as ucOC, bone gamma-carboxyglutamate protein, BGLAP, bone-derived hormone
Observational — Human data without randomisation. Suggestive, and easily confounded.
A large and influential rodent literature from one dominant lab, partially contradicted by independent knockout models that failed to replicate the metabolic phenotype. Human evidence is entirely observational — undercarboxylated osteocalcin correlates with insulin sensitivity, testosterone and muscle function, but correlation is all it is. No human administration study exists.
How it works
Osteocalcin is made by osteoblasts and gamma-carboxylated at three glutamate residues in a vitamin-K-dependent reaction, which binds it to bone mineral. Osteoclastic bone resorption decarboxylates it, releasing the undercarboxylated form into circulation as an endocrine signal. Through GPRC6A it enhances glucose-stimulated insulin secretion, increases beta-cell proliferation, improves insulin sensitivity via adiponectin, drives testosterone synthesis in Leydig cells independently of LH, and promotes nutrient uptake and catabolism in exercising muscle — the Karsenty group showed osteocalcin rises acutely with exercise and that supplying it restores exercise capacity in aged mice. It also crosses into the brain to influence hippocampal neurotransmitter synthesis, memory and anxiety, and maternal osteocalcin is required for fetal brain development. The whole model has been seriously challenged: independent osteocalcin-knockout mouse lines generated with CRISPR failed to reproduce the metabolic phenotype, and the field has not resolved that contradiction. Human data are correlational.
Targets: GPRC6A, Pancreatic beta cells, Leydig cells, Skeletal muscle, Hippocampus
Dosing
| Protocol | Dose | Frequency | Route |
|---|---|---|---|
| No human protocol existsNot applicable. | — | not established | subcutaneous |
- · Animal work uses osmotic minipump infusion or body-weight-scaled daily injection of recombinant undercarboxylated osteocalcin. No human administration study has been published. Note that vitamin K2 supplementation moves circulating osteocalcin in the opposite direction — toward the carboxylated, bone-binding, hormonally inactive form.
Cycling
Not applicable.
Pharmacology
- Half-life
- Circulating osteocalcin is cleared within minutes to a few hours; renal clearance dominates.
- Onset
- Acute effects on muscle glucose uptake within hours in animal models; endocrine effects over weeks.
- Routes
- subcutaneous, intravenous
- Molecule
- Endogenous 49-amino-acid bone-derived peptide hormone; the undercarboxylated form is the hormonally active species
- Sequence length
- 49 amino acids
Handling
- Diluent
- Bacteriostatic water
- Lyophilised
- Freezer at -20 C.
- Reconstituted
- Refrigerated, short term.
Mixing
The distinction between carboxylated and undercarboxylated material is the entire point; generic 'osteocalcin' reagents are frequently the wrong species for the claimed effect.
Side effects
- commonUnknown in humans— Never administered to people in a published study.
Do not use if
- Human use outside research.
Combining it
- conflictvitamin K2 — K2 promotes gamma-carboxylation, shifting osteocalcin toward the bone-bound inactive form and lowering the undercarboxylated fraction. Good for bone, directionally opposite for the endocrine hypothesis.
What to monitor
- · Serum total and undercarboxylated osteocalcin are established clinical bone-turnover markers with real assays behind them.
Legal status
Research reagent. Not approved anywhere as a therapeutic.
References
- Lee et al. 2007, Cell — endocrine regulation of energy metabolism by the skeleton (preclinical)
- Mera et al. 2016, Cell Metabolism — osteocalcin signalling in myofibres is necessary for exercise capacity (preclinical)
- Diegel et al. 2020, PLOS Genetics — independent osteocalcin-null mice lacking the reported metabolic phenotype (preclinical)
Mechanism in depth
The hypothesis is that undercarboxylated osteocalcin, released from bone during resorption, acts as a hormone on GPRC6A to improve insulin secretion and sensitivity, raise testosterone and support muscle - making the skeleton an endocrine organ. It was one of the most influential ideas in bone biology of the last two decades.
What usually goes wrong
This is the entry in this class that most needs its caveat stated first. Independent groups generating new osteocalcin-knockout mouse lines around 2020 failed to reproduce the metabolic and reproductive phenotypes that the original work reported, and the field has been in genuine dispute since. Citing the endocrine-bone hypothesis as established overstates it considerably.
Receptor targets
- GPRC6A — Disputed
Proposed effects on insulin secretion, testosterone and muscle
Genuinely uncertain
- The core metabolic phenotype failed independent replication in newly generated knockout models; the hypothesis is contested rather than settled.
- GPRC6A as the receptor is itself disputed.
- No human interventional data exists.