Vitamin K comes in two main forms: K1 from green vegetables and K2 from fermented foods and animal products. Both help activate proteins involved in bone, which is why K2 supplements, often combined with vitamin D, are popular for bone health. The idea is plausible, but trial evidence that K2 supplements prevent fractures is limited and inconsistent. Anyone on warfarin must not change vitamin K intake without advice.
Vitamin K directly counteracts warfarin. Do not start, stop or change vitamin K supplements, or make big changes to green vegetable intake, without talking to the clinician who manages your anticoagulation.
How much you need
The adequate intake for adults is 120 micrograms a day for men and 90 micrograms for women, from both forms combined. No upper limit has been set because no toxicity has been seen from food or supplements.1
| Form | Main sources |
|---|---|
| K1 (phylloquinone) | Leafy greens such as kale, spinach, broccoli; some vegetable oils |
| K2 (menaquinones, such as MK-4 and MK-7) | Natto (fermented soybeans), cheese, egg yolk, some meats; also made by gut bacteria |
What it does for bone
Vitamin K is needed to activate osteocalcin and other proteins that bind calcium in bone. Low vitamin K intake has been linked with lower bone density and more fractures in observational studies, but trials of supplements have given mixed results.1
A 2026 review of supplements for bone health concluded that, although vitamin K matters for bone physiology, the clinical evidence for supplementing it is limited, heterogeneous or context dependent, whereas calcium and vitamin D have better support in selected groups.2
Practical advice
- Leafy greens and fermented foods cover most needs.
- K2 with D3 products are not proven to prevent fractures; if you have osteoporosis, follow the treatment your doctor recommends. See osteoporosis of the spine.
- K2 has no known effect on back pain.
Related: vitamin D, calcium, supplement safety.
Footnotes
General information, not medical advice. Every claim carries its source; where the evidence is thin, the page says so.Editorial policy · How we grade evidence · All sources · Disclaimer