How Vitamin D Increases Calcium Levels and Why That Requires K2
One of vitamin D's most well-established functions is the regulation of calcium absorption in the gut and calcium metabolism throughout the body. Active vitamin D (calcitriol) stimulates the production of calcium-binding proteins in the intestine, substantially increasing the fraction of dietary calcium that enters the bloodstream. It also affects calcium reabsorption in the kidneys and influences calcium flux from bone.
This calcium-mobilizing effect is one of vitamin D's primary benefits — particularly for bone mineralization, where increased calcium availability supports the deposition of calcium hydroxyapatite in the bone matrix. But the calcium that vitamin D makes available needs to be directed appropriately. Specifically, it needs to go to bone and teeth, and not to soft tissues, blood vessels, or other locations where calcium deposition is problematic.
This routing function is performed primarily by vitamin K2 — specifically by K2-dependent proteins called carboxylation-dependent calcium-binding proteins, the most studied of which are osteocalcin (which directs calcium into bone mineral) and matrix Gla protein (MGP), which prevents calcium deposition in arterial walls and other soft tissues.
The Rotterdam Study and Cardiovascular Implications
The most compelling population data on vitamin K2 comes from the Rotterdam Study — a large prospective cohort study of Dutch residents over 55 years of age, which has followed thousands of participants for decades. A dietary intake analysis by Geleijnse and colleagues, published in the Journal of Nutrition in 2004, examined the relationship between intake of various vitamin K forms and cardiovascular outcomes over a ten-year follow-up period.
The findings were striking. While vitamin K1 (phylloquinone, the form found in leafy greens) showed no significant association with cardiovascular outcomes, vitamin K2 (menaquinone, particularly the MK-7 form found in fermented foods) was significantly associated with a reduced risk of coronary heart disease mortality. Participants in the highest tertile of K2 intake had a 57% lower risk of dying from coronary heart disease compared to those in the lowest tertile.
The mechanistic interpretation offered by the researchers centered on K2's role in carboxylating MGP, the vitamin K-dependent protein that prevents arterial calcification. An extensive body of research has demonstrated that undercarboxylated MGP — the inactive form that accumulates when K2 is insufficient — is associated with measurable increases in arterial stiffness and vascular calcification in human populations.
The Vermeer Research and K2 Characterization
Dr. Cees Vermeer of Maastricht University has dedicated much of his career to characterizing the biology of vitamin K2. In a comprehensive 2012 review published in Food and Nutrition Research, Vermeer summarized the evidence for K2's role in bone, cardiovascular, and other tissues.
Of particular relevance is Vermeer's documentation of the K2 forms and their respective tissue distribution. Vitamin K2 exists in multiple forms depending on the length of the side chain — MK-4 (found in organ meats and egg yolks) and MK-7 (found in fermented foods, particularly natto) are the most studied. MK-7 has a substantially longer half-life in circulation than MK-4 or vitamin K1, making it more effective at maintaining K2-dependent protein function in tissues including bone and vasculature.
Vermeer and colleagues have consistently recommended that vitamin D supplementation be paired with K2 supplementation — particularly in individuals supplementing at higher vitamin D3 doses — to ensure that the calcium mobilized by vitamin D is appropriately directed to bone rather than to soft tissues.
Practical Considerations for D3 + K2 Pairing
The clinical relevance of pairing D3 and K2 has gained increasing acceptance among functional medicine practitioners. The rationale is mechanistically sound: vitamin D mobilizes calcium, K2 ensures that calcium is properly deposited in bone rather than soft tissues, and adequate K2 status is required for this protective function.
Dietary sources of K2 include fermented foods (natto has exceptional MK-7 content), hard cheeses, soft cheeses, egg yolks, and organ meats. The Western diet is generally low in fermented foods and organ meats, suggesting that K2 inadequacy may be widespread in populations consuming a typical modern diet.
For individuals supplementing with vitamin D3, including a K2 supplement — particularly MK-7 form, which has superior bioavailability and a longer half-life — is supported by the mechanistic and epidemiological evidence. As with all supplementation, consultation with a qualified health professional is recommended, particularly for individuals on anticoagulant medications for which vitamin K interactions are relevant.