The Complete Vitamin K Guide

K1 (phylloquinone) is found in green leafy vegetables and is primarily used by the liver for blood clotting. K2 (menaquinone) comes in several forms — MK-4 from animal foods and MK-7 from bacterial fermentation (natto, aged cheese). K2 has a much longer half-life than K1 and reaches extrahepatic tissues like bone and arterial walls, where it activates osteocalcin (bone building) and matrix Gla protein (arterial calcification prevention). K1 handles clotting; K2 handles calcium direction. Most people get enough K1 from diet but are deficient in K2.

Yes — strongly recommended. Vitamin D3 increases calcium absorption from your gut by 30-40%. Without K2 to direct that extra calcium, it can deposit in your arteries, heart valves, and kidneys instead of your bones. D3 without K2 is an incomplete (and potentially counterproductive) protocol. Always pair D3 with at least 100mcg of K2 (MK-7). The standard recommendation is D3 (2,000-5,000 IU) + K2 (100-200mcg MK-7), taken together with a fat-containing meal.

Natto is by far the richest source at approximately 1,100mcg per 100g serving — there is nothing else remotely close. However, aged cheeses like Gouda (75mcg/100g) and Brie (50mcg/100g) provide meaningful amounts of MK-7 through bacterial ripening. Sauerkraut and miso contain small amounts. For MK-4 (the other K2 form), goose liver, egg yolks, and grass-fed butter are good sources. Realistically, most people who do not eat natto regularly will benefit from MK-7 supplementation.

It depends entirely on which blood thinner. If you take warfarin (Coumadin), do NOT start K2 without your doctor's guidance — vitamin K directly opposes warfarin's mechanism, and unsupervised supplementation can be dangerous. If you take a DOAC (apixaban, rivaroxaban, dabigatran), these work through vitamin K-independent pathways, and K2 supplementation is generally safe — but always confirm with your prescribing physician. Never self-prescribe vitamin K while on any anticoagulant.

The evidence-based dose for most adults is 100-200mcg of MK-7 per day. 100mcg is sufficient for general maintenance and osteocalcin activation. 200mcg is recommended for those at higher risk of osteoporosis, those taking high-dose D3 (5,000+ IU), or those with known arterial calcification. There is no established upper limit — studies using up to 800mcg/day showed no adverse effects. Take with a fat-containing meal for optimal absorption.

Observational data (the Rotterdam Study) strongly suggests that high K2 intake prevents arterial calcification, with a 52% reduction in severe aortic calcification in the highest K2 intake group. Whether K2 can reverse existing calcification is less established but promising. A 2015 trial (Knapen et al.) found that 3 years of MK-7 supplementation (180mcg/day) improved arterial stiffness, suggesting functional improvement. However, reversing heavily calcified arteries likely requires years of consistent K2 alongside comprehensive lifestyle changes.

Three key reasons. First, half-life: MK-7 stays active in the body for approximately 72 hours, allowing once-daily dosing. MK-4 has a half-life of only 1-2 hours and must be taken 3 times per day at therapeutic doses. Second, dose efficiency: 100-200mcg of MK-7 effectively activates both osteocalcin and matrix Gla protein, while MK-4 requires doses of 500mcg to 45mg for comparable effects. Third, clinical evidence: most positive bone and cardiovascular trials have used MK-7 specifically.

For K1 (blood clotting), yes — a single serving of kale, spinach, or broccoli easily exceeds the 90-120mcg daily adequate intake. For K2, it is much harder unless you regularly eat natto (uncommon outside Japan), aged cheese, and pasture-raised animal products. The modern Western diet provides minimal K2 because factory-farmed animals produce less K2 than pastured animals, and fermented foods have largely disappeared from the standard diet. Most people benefit from MK-7 supplementation.

Vitamin K is relatively heat-stable compared to other vitamins. Cooking leafy greens actually improves K1 bioavailability by breaking down cell walls and releasing the vitamin. Boiling causes some loss into cooking water (use that liquid in soups). Fermented K2 sources like natto are typically eaten without additional cooking. K2 supplements are stable at normal cooking temperatures. The main concern is not heat but fat: always consume K-rich foods with dietary fat, as vitamin K absorption is dramatically reduced without it.

Clinical K1 deficiency manifests as easy bruising, excessive bleeding from cuts, nosebleeds, heavy menstrual periods, and blood in stool or urine. This is rare in healthy adults who eat vegetables. Subclinical K2 insufficiency is far more common and insidious: it shows up as accelerated bone loss (osteopenia/osteoporosis), dental decay, and arterial calcification — conditions that develop silently over years. By the time these are detected, significant damage has occurred. Proactive K2 supplementation is preventive medicine.