Vitamin K

What vitamin K actually does

Vitamin K is a fat-soluble vitamin that activates a family of proteins involved in blood clotting, bone metabolism, and calcium regulation. Without it, your blood won't clot properly and calcium drifts into your arteries instead of your bones. There are two main forms: vitamin K1 (phylloquinone), found in green leafy vegetables, and vitamin K2 (menaquinone), found in fermented foods and animal products. While K1 handles coagulation, K2 has a broader reach, activating osteocalcin (which builds bone) and matrix GLA protein (which keeps calcium out of your blood vessels) ^[1].

Vitamin K and aging

Research published in Antioxidants found that vitamin K acts as a cofactor for proteins that protect against several age-related diseases, including cardiovascular disease, osteoarthritis, dementia, and frailty ^[1]. A 2023 review in Nutrients documented connections between vitamin K status, gut microbiome health, and multiple hallmarks of aging, including inflammation, mitochondrial dysfunction, and cellular senescence ^[2]. The data from the Rotterdam Study is particularly striking: people with the highest menaquinone (K2) intake had a 57% lower risk of dying from coronary heart disease compared to those with the lowest intake ^[3].

Subclinical vitamin K deficiency is far more common than most people realize. One study using undercarboxylated MGP measurements found insufficiency in up to 97% of older adults tested ^[4]. This doesn't cause obvious bleeding problems, but it means calcium-regulating proteins aren't fully activated, silently contributing to arterial stiffness and bone loss over decades.

Bone health: beyond calcium and vitamin D

Most people know bones need calcium and vitamin D. Fewer know that vitamin K2 is the third piece of this puzzle. Osteocalcin, the protein that binds calcium into bone matrix, only works when carboxylated by vitamin K. A meta-analysis of randomized controlled trials showed that vitamin K supplementation increased lumbar spine bone mineral density and significantly raised carboxylated osteocalcin levels ^[5]. In women with higher K1 intake (above 99 mcg per day), hip fractures were 49% less common than in those consuming below 61 mcg daily.

Cardiovascular protection

Matrix GLA protein (MGP) is the body's most potent natural inhibitor of arterial calcification, and vitamin K is required to activate it. Observational studies consistently link higher K2 intake to less aortic calcification and lower cardiovascular mortality ^[3]. Clinical trial results have been mixed: some trials show slowed progression of vascular calcification with K2 supplementation, while others show no clear benefit in patients who already have advanced calcification ^[6]. The evidence suggests K2 works best as prevention, not reversal.

K1 versus K2: which form matters more

Vitamin K1 is abundant in kale, spinach, broccoli, and other greens. It handles coagulation efficiently but doesn't reach bones and arteries as well. Vitamin K2 comes in several subtypes. MK-4 is found in meat, eggs, and dairy; your body also converts some K1 to MK-4. MK-7, found almost exclusively in the fermented soybean product natto, has a much longer half-life (about 72 hours versus 1-2 hours for MK-4), meaning it stays in circulation long enough to reach bone and vascular tissue ^[7]. For supplementation, MK-7 at doses of 100-200 mcg daily is the most practical choice. MK-4 requires milligram-level doses (typically 45 mg) because of its rapid clearance.

Food sources and daily intake

The adequate intake for vitamin K is 120 mcg per day for men and 90 mcg for women, but these recommendations were set primarily for coagulation, not for optimal bone and cardiovascular health. Longevity-focused researchers argue higher intakes are likely needed. Good K1 sources include kale (817 mcg per cup cooked), spinach (888 mcg per cup cooked), and broccoli (220 mcg per cup). For K2, natto is the richest source at roughly 1,000 mcg MK-7 per 100 grams. Hard cheeses like Gouda and Emmental contain moderate amounts of MK-4 and MK-9. Egg yolks and organ meats provide smaller amounts.

Who should be cautious

People taking warfarin (Coumarin) or other vitamin K antagonist anticoagulants need to keep their vitamin K intake consistent, not necessarily low. Sudden changes in K intake can destabilize INR values. If you're on blood thinners, talk to your doctor before supplementing. For everyone else, vitamin K has no known toxicity at any tested dose ^[1].