Vitamin K has spent decades in the shadow of more famous nutrients like vitamin D and calcium. But a growing body of research suggests that one form in particular—vitamin K2, especially the long-acting subtype menaquinone-7 (MK-7)—may play a distinct role in bone strength and cardiovascular health. Here is what current science says about how K2 works, where to find it, and who might benefit.
Vitamin K1 vs. K2: Not the Same Nutrient
Dietary vitamin K comes in two main forms. Vitamin K1 (phylloquinone) is found in leafy greens like kale, spinach, and broccoli, and is the form most people consume daily. Vitamin K2 (menaquinones) is produced by bacteria and found primarily in fermented foods and some animal products.
The two forms share a chemical backbone but behave differently in the body. According to research published in Nutrients, K1 is preferentially transported to the liver and used for blood clotting, while K2 circulates longer and reaches extra-hepatic tissues such as bone and arterial walls, where it activates specialized proteins.
What Makes MK-7 Unique
Menaquinones are classified by the length of their side chain, from MK-4 to MK-13. MK-7, which comes largely from fermented soybeans (natto) and certain cheeses, has attracted the most research attention. Pharmacokinetic studies indicate that MK-7 has a substantially longer half-life in blood (about 3 days) than MK-4 (around 1–2 hours), which allows even modest daily doses to maintain steady tissue levels.
How Vitamin K2 Works in the Body
K2 acts as a cofactor for enzymes that activate vitamin K-dependent proteins through a process called carboxylation. Two of these proteins are especially important:
- Osteocalcin, produced by bone-building cells, helps bind calcium to the bone matrix.
- Matrix Gla protein (MGP) inhibits calcium from depositing in arteries and soft tissues.
Without adequate K2, these proteins remain in an inactive (undercarboxylated) form. Studies indicate that higher levels of undercarboxylated osteocalcin are associated with reduced bone mineral density, while undercarboxylated MGP is linked to arterial calcification.
Vitamin K2 and Bone Health
The most notable clinical evidence for MK-7 and bone comes from a 3-year randomized controlled trial by Knapen and colleagues, published in Osteoporosis International in 2013. In 244 postmenopausal women, 180 micrograms of MK-7 per day slowed the age-related decline in bone mineral density at the lumbar spine and femoral neck compared with placebo. Bone strength indices also improved.
A 2019 meta-analysis in the Journal of Orthopaedic Surgery and Research pooled results from multiple randomized trials and concluded that vitamin K2 supplementation was associated with improved lumbar bone mineral density and a lower fracture risk in postmenopausal women, although the authors noted variability in study quality.
Not every trial has found a benefit. A large Canadian study published in the New England Journal of Medicine in 2008 found no fracture reduction from vitamin K1. This underscores that K1 and K2 appear to behave differently, and results from one cannot be generalized to the other.
Vitamin K2 and Cardiovascular Health
The Rotterdam Study, published in the Journal of Nutrition in 2004, followed more than 4,800 adults for about 7 years and found that participants in the highest tertile of K2 intake had significantly lower rates of severe aortic calcification and cardiovascular mortality compared with those in the lowest tertile. Vitamin K1 intake showed no similar association.
A later analysis from the Prospect-EPIC cohort, published in Nutrition, Metabolism & Cardiovascular Diseases in 2009, reported that every 10 micrograms per day of dietary K2 was associated with a 9 percent lower risk of coronary heart disease.
Trial evidence remains more limited. A 3-year MK-7 study in healthy postmenopausal women, published in Thrombosis and Haemostasis in 2015, found that daily supplementation reduced arterial stiffness measures compared with placebo, particularly in participants with elevated baseline stiffness. Larger long-term trials with cardiovascular endpoints are still needed.
Food Sources of Vitamin K2
Because K2 is produced by bacteria, the richest food sources are fermented. Approximate menaquinone content per 100 grams, based on values reported in the British Journal of Nutrition:
- Natto (fermented soybeans): about 1,000 micrograms, almost entirely as MK-7
- Hard cheeses such as Gouda and Edam: 50–75 micrograms, mainly MK-8 and MK-9
- Soft cheeses such as Brie and Camembert: 30–60 micrograms
- Egg yolks: about 15–30 micrograms, mainly MK-4
- Chicken liver and dark chicken meat: 5–10 micrograms
Sauerkraut, kefir, and other fermented foods provide smaller amounts. A gut microbiome capable of producing menaquinones may contribute additional K2, although the extent of absorption from bacterial synthesis remains uncertain.
Who Might Benefit from Extra Vitamin K2
Overt vitamin K deficiency is uncommon in healthy adults, but low or suboptimal K2 status may be more widespread, especially in populations that rarely eat fermented foods. Research suggests certain groups may have higher needs:
- Postmenopausal women at risk of accelerated bone loss
- Older adults with concerns about vascular calcification
- People on long-term corticosteroids or medications that affect bone turnover
- Individuals with fat malabsorption, such as those with cystic fibrosis or inflammatory bowel disease
Safety and Interactions
Vitamin K2 has a favorable safety profile at doses studied in trials, typically 45 to 180 micrograms per day of MK-7. No upper limit has been established, and unlike fat-soluble vitamins A and D, no clear toxicity threshold has been identified. Independent reviews by the European Food Safety Authority have not raised safety concerns at supplemental doses used in research.
The important exception is anyone taking warfarin or similar vitamin K antagonists. These medications work by blocking vitamin K, so any change in K1 or K2 intake—from food or supplements—can affect blood clotting parameters and requires medical supervision. Newer anticoagulants such as apixaban and rivaroxaban do not interact with vitamin K.
Choosing a Supplement
Most K2 supplements marketed for bone and heart health contain MK-7, often derived from natto fermentation. Doses in randomized trials typically range from 45 to 180 micrograms per day. Because K2 is fat-soluble, absorption improves when taken with a meal containing fat. Some formulations combine K2 with vitamin D3, based on the biological rationale that both vitamins are needed for calcium to be properly directed into bone rather than soft tissue.
The Bottom Line
Vitamin K2, and MK-7 in particular, is emerging as a nutrient with distinct roles in bone and cardiovascular biology. Observational studies consistently link higher K2 intake with better skeletal and vascular outcomes, and randomized trials support modest benefits for bone density and arterial stiffness. Dietary intake of K2 is low in many Western populations, and fermented foods offer the richest natural sources.
K2 is not a substitute for calcium, vitamin D, regular weight-bearing exercise, or established cardiovascular prevention. Yet it is one of the more promising micronutrients to watch as research continues. Before starting a K2 supplement—especially if you take blood thinners or have chronic conditions—talk with a qualified healthcare provider who can review your individual risks and interactions.
Disclosure: This content is for informational purposes only and is not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen.