Vitamin K2: the co-pilot vitamin D needs
Vitamin D gets a lot of deserved attention. Vitamin K2 gets almost none, despite doing a related job on the same calcium.
The two jobs, and why they differ
Vitamin D's relevant role is getting calcium absorbed from the gut. Vitamin K activates a set of proteins that decide what happens to it next. Two matter here: osteocalcin, which is present in bone and may be involved in bone mineralization and turnover, and matrix Gla protein, present in vascular smooth muscle, which is studied for its potential to reduce abnormal calcification.
One honest note on how that gets sold: the leap from "these proteins are vitamin K-dependent" to "K2 moves calcium out of your arteries and into your bones" runs ahead of the evidence. The proteins are real. The clinical claim is still being worked out.
Why K1 and K2 aren't interchangeable
K1 (phylloquinone) is the main dietary form and comes primarily from green leafy vegetables. K2 (menaquinones) are largely bacterial in origin and appear in modest amounts in animal-based and fermented foods, with MK-7 the most studied of them. The Adequate Intake for adult women is 90 mcg/day, and it doesn't distinguish between the forms.
Eating plenty of spinach covers K1. It does very little for K2 specifically. Natto — fermented soybeans, an acquired taste — is far and away the richest source at roughly 850 mcg per 3-ounce serving; cheese and other fermented foods contribute smaller and less predictable amounts.
What the evidence actually supports
The best evidence here is for bone, and it's modest. A three-year randomized trial of low-dose MK-7 in healthy postmenopausal women found it decreased the age-related decline in bone mineral density and bone strength. Helpful, measured over three years, not dramatic.
The safety note that matters
Vitamin K affects clotting. If you take warfarin or a similar anticoagulant, consistency of intake is the whole game — sudden changes in vitamin K from food or supplements can shift the anticoagulant effect in either direction. That's a conversation with your prescriber before you add K2, not a reason to avoid the subject.
Otherwise: if you already supplement D3, check whether K2 is in it. It usually isn't.
