What it's actually good for
Vitamin K2 activates two calcium-handling proteins: osteocalcin, which pulls calcium into bone, and matrix Gla-protein (MGP), which keeps calcium out of arterial walls. The working theory is that low K2 status lets calcium end up in the wrong places — soft arteries instead of hard bone. The MK-7 form (menaquinone-7) is what most supplements use, because it has a much longer half-life than MK-4 and keeps blood levels steady on a single daily dose.
The evidence isn't as deep as vitamin D's, but it's more than theory. Two RCTs in the same cohort of 244 healthy postmenopausal women (Knapen 2013, Knapen 2015) found that 180 mcg/day of MK-7 for three years slowed bone density decline and improved arterial stiffness versus placebo. And in 2026, a JAMA Cardiology trial went a step further, testing MK-7 in people who already have coronary artery disease and finding it modestly slowed calcium buildup in their arteries over two years.
What the research says
Arterial calcification (Grade B). The Knapen 2015 trial found MK-7 improved a measure of arterial stiffness in healthy postmenopausal women, with the biggest benefit in those who started stiffest. The 2026 VitaK-CAC trial (Vossen et al., JAMA Cardiology) extended this to actual coronary artery disease: 167 symptomatic CAD patients randomized to 360 mcg/day MK-7 or placebo for two years showed about 29% less progression in coronary artery calcium score than placebo (184 vs. 214 Agatston units by year two). The investigators themselves called the effect modest and said the trial wasn't designed to show whether slower calcification translates into fewer heart attacks — a real, early signal, not proof that K2 prevents cardiovascular events.
Bone density (Grade B). The Knapen 2013 trial found MK-7 slowed the expected decline in spine and hip bone density over three years in postmenopausal women. Japanese trials using much higher doses of MK-4 (menatetrenone, 15-45 mg/day) have shown reduced fracture risk in osteoporosis patients, but those results have not reliably replicated outside Japan, and the doses used are far above what typical MK-7 supplements provide — translate cautiously.
How much, and which form
Most supplements use 100-200 mcg/day of MK-7, matching the dose used in the postmenopausal bone and artery trials. The newer cardiovascular trial used a higher 360 mcg/day — worth knowing if you're specifically targeting arterial calcification, though that dose hasn't been tested for as long or in as many people as the lower range. MK-4 requires much higher, more frequent dosing to have an effect and is less practical for daily use. There's no separate RDA for K2 — total vitamin K intake targets (120 mcg/day for men, 90 for women) are set around K1, the more common dietary form.
Safety & interactions
K2 has no established upper limit and no known toxicity at supplemental doses — one of the more benign supplements on the shelf. The one hard rule: if you're on warfarin or another vitamin K antagonist, don't add K2 without talking to the prescribing physician, since it directly counteracts how those drugs work. It pairs naturally with vitamin D3, since D3 increases production of the calcium-handling proteins K2 activates. This is informational, not medical advice — check with a clinician before starting.
How we picked the brand
A vitamin K2 product earns a spot when it clearly states the form (MK-7, MK-4, or both) and dose, discloses whether the MK-7 is natto-derived or synthetic, and passes independent third-party testing. Thorne's combination product is included primarily for its NSF Certified for Sport status and third-party verification across all three K forms it contains.