Vitamin K2 supplements are primarily used to support bone strength and cardiovascular health. The vitamin works by activating proteins that direct calcium to the right places in your body, specifically into bones and teeth, and away from places it can cause harm, like your arteries. While vitamin K1 (found in leafy greens) handles blood clotting in the liver, K2 plays a distinct role in tissues throughout the body.
How K2 Works in Your Body
All forms of vitamin K activate specific proteins, but K1 and K2 behave differently once absorbed. K1 is quickly taken up by the liver, where it supports blood clotting. K2 has a longer side chain that allows it to circulate in the blood for days rather than hours, giving it time to reach bones, blood vessels, and other tissues far from the liver.
Once there, K2 switches on two key proteins. The first, osteocalcin, binds calcium into bone. The second, called matrix Gla protein (MGP), prevents calcium from accumulating in artery walls and soft tissues. Without enough K2, these proteins remain inactive, and calcium can end up where it doesn’t belong.
Bone Health and Fracture Risk
K2’s effect on bones centers on osteocalcin. When K2 activates this protein, osteocalcin binds calcium into the bone matrix, which helps regulate how bone mineral matures and strengthens. A review of controlled studies found that K2 supplementation (specifically the MK-4 form) significantly reduced fracture risk. In one clinical trial, postmenopausal women who took 180 mcg of MK-7 daily for three years showed improved bone strength and less height loss in the vertebrae compared to those who didn’t supplement.
That said, the picture isn’t entirely clear-cut. While observational studies consistently link low vitamin K status to age-related bone loss, clinical trials haven’t uniformly supported supplementation for the general population. The strongest evidence applies to postmenopausal women and others already at elevated risk for bone loss.
Cardiovascular Protection
Calcium buildup in the arteries is a well-established risk factor for heart disease. K2 helps prevent this by activating matrix Gla protein, the body’s primary defense against arterial calcification. When vitamin K levels are too low, MGP stays in its inactive form, and the arteries lose that protection. Inactive MGP in the blood is independently associated with higher cardiovascular risk.
Several large observational studies have suggested that K2 is more effective than K1 at reducing calcium deposits in blood vessels and lowering heart disease risk. This makes sense given K2’s longer circulation time and its preference for tissues outside the liver. Clinical trials are ongoing to confirm these findings, particularly in people with kidney disease, who face extremely high rates of vascular calcification.
Why K2 and Vitamin D Work Together
Vitamin D increases how much calcium your body absorbs from food. Vitamin K2 then directs where that calcium goes. Without enough K2, the extra calcium that vitamin D helps absorb may deposit in soft tissues and arteries rather than in bone. This is why many supplements pair the two together.
Research supports this pairing. An adequate supply of vitamin K, on top of optimal vitamin D levels, appears to add measurable benefit for maintaining bone health. The two vitamins co-regulate calcium balance alongside parathyroid hormone, forming a system where each nutrient depends on the others to function properly. If you’re already taking vitamin D for bone health, K2 is a logical complement.
Teeth and Enamel Support
The same osteocalcin that strengthens bones also supports teeth. When K2 activates osteocalcin in dental tissue, it helps bind calcium into the tooth structure, contributing to enamel maintenance and repair. K2 works alongside vitamins D and A to regulate calcium absorption into teeth, keeping them mineralized and resilient. This benefit is less studied than bone or heart effects, but it follows directly from the same biological mechanism.
MK-4 vs. MK-7: Choosing a Form
K2 supplements come in two main forms. MK-4 has a short half-life and clears the body quickly, which means it requires higher doses and multiple servings per day, typically taken with fat for absorption. MK-7 stays in circulation much longer, so a single daily dose is effective. Most clinical trials studying bone and cardiovascular outcomes now use MK-7 for this reason, and it’s the more common form on supplement shelves.
The clinical trial dose that showed bone benefits in postmenopausal women was 180 mcg of MK-7 per day. Many supplements fall in the 100 to 200 mcg range, which aligns with the available evidence.
Food Sources of K2
Your gut bacteria produce some K2, but not enough to rely on. Dietary K2 comes almost exclusively from fermented foods and animal products. The richest source by far is natto, a Japanese fermented soybean dish, with 150 mcg per tablespoon. Most Western diets don’t include natto, which is one reason K2 intake tends to be low in North America and Europe.
Other notable sources per serving:
- Egg yolk: 67 to 192 mcg (varies widely by the hen’s diet)
- Eel: 63 mcg per 100-gram serving
- Munster cheese: 50 mcg per 50 grams
- Camembert: 34 mcg per 50 grams
- Aged Gouda or Edam: 32 mcg per 50 grams
- Cheddar: 12 mcg per 50 grams
- Beef liver: 11 mcg per 100 grams
- Chicken: 10 mcg per 100 grams
- Sauerkraut: 2.75 mcg per half-cup
- Butter: 2.1 mcg per tablespoon
Aged and fermented cheeses are the most practical everyday sources for people eating a typical Western diet. Egg yolks can be surprisingly rich in K2, but the amount depends heavily on what the chickens were fed, with pasture-raised eggs generally containing more.
Who Should Be Cautious
If you take warfarin or another blood thinner that works by blocking vitamin K, adding a K2 supplement can interfere with your medication. Warfarin’s effectiveness depends on keeping vitamin K levels consistent, and a sudden increase from supplementation can reduce the drug’s ability to prevent clotting. UC San Diego Health guidelines specifically flag vitamin K supplements as requiring consistency in patients on warfarin. If you’re on anticoagulant therapy, this is a conversation to have with whoever manages your medication before starting K2.
For most other people, K2 supplements at standard doses (100 to 200 mcg of MK-7) have not shown significant side effects in clinical trials. Unlike some fat-soluble vitamins, K2 does not appear to accumulate to toxic levels at these amounts.