Vitamin K

Vitamin K

Vitamin K is a fat-soluble vitamin whose best-known role — blood clotting — is only part of the story. Its name comes from the German word Koagulation, but research over the past two decades has focused on its second role: routing calcium to the right place in the body — to the bones rather than the arteries.

K1 vs. K2 — not really the same vitamin

This is the key distinction, and most of the confusion around vitamin K comes from ignoring it.

The practical difference comes from the half-life: K1 is cleared from the blood within hours and used almost entirely in the liver to make clotting factors. K2, especially the MK-7 form, stays in circulation for days — which is why it is the one that reaches tissues outside the liver and activates the K-dependent proteins there.

What vitamin K actually does

It enables one chemical reaction: carboxylation of glutamate. Proteins that undergo this process gain the ability to bind calcium; without vitamin K they stay inactive.

• Clotting factors II, VII, IX, X — in the liver. This is the mechanism discovered first, and the one Coumadin targets.
• Osteocalcin — in bone. It binds calcium and fixes it into the bone matrix.
• MGP (Matrix Gla Protein) — in blood vessel walls. Its job is to prevent calcium from depositing in the arteries. Uncarboxylated MGP is an accepted marker of K2 deficiency in the tissues.

Hence the concept of the “calcium paradox”: a situation where there is enough calcium in the diet but it migrates to the arteries instead of the bone. K2 is the link that directs it.

Food sources and amounts

Vitamin K1 (mcg per 100 grams):

Vitamin K2 (mcg per 100 grams):

Natto is the strongest source by a huge margin — one tablespoon (about 15 grams) provides more MK-7 than any other source does in a whole day. Gut bacteria produce some K2, but their contribution to absorption is considered limited.

How much you need

The Adequate Intake (AI) that has been set is based on clotting needs only, not on the needs of bones and arteries:

• Men: 120 mcg per day. Women: 90 mcg per day.
• A serving of spinach or broccoli easily covers this. True K1 deficiency is very rare in a healthy adult.
• No separate recommendation has been set for K2. Intervention studies on bone density and arterial flexibility mostly used 180 mcg of MK-7 per day for 3 years.
• Absorption requires fat. Taking it with a meal containing fat significantly increases bioavailability — this is true of all fat-soluble vitamins.

The shot babies get at birth

This is the context in which vitamin K deficiency is a real, not theoretical, danger to life.

Babies are born with very low vitamin K stores: transfer across the placenta is limited, breast milk is low in vitamin K, and the newborn’s gut is not yet populated by the bacteria that make it. Without it, about 1 in 10,000-25,000 babies develop vitamin K deficiency bleeding (VKDB) — which in its late form usually shows up as bleeding in the brain, with high rates of death and disability.

A single 1 mg intramuscular injection at birth prevents almost all forms of the syndrome. It is one of the most effective preventive treatments in pediatrics, and it is the accepted standard in most of the world.

Downsides, risks and interactions

• Coumadin (warfarin) — the most important interaction. The drug works precisely by blocking the vitamin K cycle. The common mistake is thinking you need to avoid green vegetables — that’s not true. Cardiologists’ guidance is to keep vitamin K intake steady and consistent, so that the drug dose can be calibrated to it. Sharp swings — a week of salads and then a week without — are what’s dangerous, not the vegetables themselves. Don’t take a K2 supplement while on Coumadin without your treating doctor knowing.
• DOAC-type drugs (Eliquis, Xarelto) are not affected by vitamin K.
• Toxicity: no toxicity has been documented for natural K1 and K2, even at high doses. An important exception is menadione (vitamin K3), a synthetic form that has been banned from human supplements because of liver toxicity and hemolysis.
• Drugs that reduce absorption: orlistat, cholestyramine, and broad-spectrum antibiotics used long term (which damage the gut flora).
• Fat malabsorption conditions: celiac disease, Crohn’s, cystic fibrosis, cholestatic liver disease and bariatric surgery — here true K deficiency is possible and warrants testing.
• The evidence for K2 is still not clear-cut. Observational studies (including the Rotterdam Study) found a consistent link between high K2 intake and less arterial calcification and cardiac mortality, and several intervention studies showed improvements in arterial flexibility and bone density. But other large studies did not reproduce the finding, and there is no consensus yet. It is a promising direction, not an established fact.

Practical use

• Most people don’t need a supplement. A daily serving of green leafy vegetables covers K1, and aged cheese, egg yolks and fermented foods contribute K2.
• If you’re considering a K2 supplement: choose MK-7 (long half-life) rather than MK-4, at a dose of 100-200 mcg, and take it with a fatty meal.
• Combining with vitamin D: vitamin D increases calcium absorption from the gut, and K2 directs it to the bone. For people taking high doses of vitamin D over a long period, this is why many recommend adding K2, although direct evidence for it is still limited.
• Cook green vegetables with olive oil — simple and straightforward, and it doubles absorption.

Summary

Vitamin K1 from green vegetables is entirely sufficient for clotting needs, and deficiency in a healthy adult is rare. The open question is K2: it is the form that reaches bones and arteries, it is scarce in the Western diet, and the evidence on its role in preventing arterial calcification is promising but not yet settled. The cautious recommendation is to diversify — green vegetables alongside aged cheeses, egg yolks and fermented foods — and not rush to a supplement. People on Coumadin need to remember the one rule that is truly critical: steady intake, not low intake.