A fat-soluble vitamin known mainly for blood clotting — the K comes from the German 'koagulation.' But vitamin K's role goes beyond clotting: it directs calcium to the right places, putting it into bones and teeth and keeping it out of arteries and soft tissues. Two main forms exist — K1 from leafy greens, and K2 from fermented foods and animal products, which has different and arguably more important effects.

Recommended daily intake

Adult Men · 120 mcg (adequate intake)
Adult Women · 90 mcg (adequate intake)
K2 Separate Intake · no formal RDA for K2 specifically; clinical research uses 90-180 mcg of MK-7

Upper intake limit

Not formally established

Signs of deficiency

Easy bruising and bleeding
Heavy menstrual bleeding
Blood in urine or stool
Soft, fragile bones
Calcification in arteries (suggested by some research as a sign of K2 inadequacy)

Required for normal blood clotting — the original known role
K2 directs calcium into bones and teeth and away from arteries and soft tissues
May support cardiovascular health by reducing arterial calcification
Supports bone density, particularly in postmenopausal women
K1 is found in leafy greens, broccoli, and Brussels sprouts
K2 (menaquinones) is found in natto (fermented soybeans), aged cheese, egg yolks, and grass-fed meat and dairy
MK-7 (a long-chain K2) has the longest half-life and is the form most-studied for cardiovascular and bone benefit

Major interaction with warfarin (Coumadin) — vitamin K is the antidote to warfarin, so changes in K intake disrupt blood thinning. People on warfarin need consistent K intake, not zero K.

Newer blood thinners (apixaban, rivaroxaban, dabigatran) do not interact with vitamin K the same way

Newborns receive a vitamin K injection at birth to prevent vitamin K deficiency bleeding — refusing this has caused infant deaths and severe brain bleeding

Use caution combining K2 supplements with other supplements that affect calcium balance

Generally well tolerated; toxicity is essentially unknown at dietary doses

Synthetic K3 (menadione) is no longer used in human supplements due to toxicity concerns

Works with

Vitamin D Vitamin K2 activates osteocalcin and matrix Gla protein, directing calcium absorbed via vitamin D into bone rather than soft tissue. Calcium Vitamin K2 carboxylates osteocalcin, enabling it to bind calcium in bone matrix; the two are synergistic for bone mineral density.

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