Vitamin D is widely supplemented. Vitamin K2 is rarely taken seriously by people who are not already deeply invested in health optimisation. Taking them separately misses the point — the evidence for their synergistic relationship is strong enough that taking vitamin D without K2 is, for many people, an incomplete intervention.
The Calcium Transport Problem
Vitamin D increases calcium absorption from the gut. This is its primary mechanism — without adequate vitamin D, the intestines absorb only 10 to 15% of dietary calcium, regardless of intake levels. With sufficient vitamin D, calcium absorption increases to 30 to 40% of intake. This is why vitamin D deficiency produces the bone demineralisation of rickets in children and osteomalacia in adults.
The problem is that vitamin D does not direct where the calcium goes after absorption. It simply increases circulating calcium. If calcium is being deposited in arteries instead of bone, increased vitamin D means increased arterial calcification. This is not a theoretical concern — several large observational studies have found associations between high vitamin D supplementation and increased cardiovascular events in people who were also low in vitamin K2.
The mechanism is well-characterised: vitamin K2 activates osteocalcin, a protein produced by osteoblasts that binds calcium to the bone matrix. Without K2-dependent activation, osteocalcin is inactive and calcium floats in the bloodstream. Vitamin K2 also activates matrix Gla protein, which prevents calcium from depositing in soft tissue and arterial walls. Without these K2-dependent proteins, calcium from the diet and supplementation has nowhere to go except soft tissues.
The Clinical Evidence
The Rotterdam Study, a large prospective cohort study in the Netherlands, found that people with the highest vitamin K2 intake had 57% lower risk of cardiovascular disease and 26% lower all-cause mortality compared to those with the lowest intake. This observational association has been supported by several intervention studies showing that combined vitamin D and K2 supplementation improves bone mineral density more than vitamin D alone, and does so without increasing arterial calcification.
A 2023 meta-analysis of RCTs examining the combination of vitamin D and K2 found that the combination was superior to vitamin D alone for bone health markers, with the strongest effects in postmenopausal women — the highest-risk population for osteoporosis. The evidence for cardiovascular protection from K2 specifically is still accumulating, but the mechanistic rationale is strong enough that most functional medicine practitioners consider K2 non-negotiable when supplementing vitamin D.
Forms of Vitamin K2
Vitamin K2 exists in several forms, of which the most clinically relevant are MK-4 and MK-7. MK-4 is shorter-acting and requires higher doses to achieve adequate plasma levels. MK-7 has a longer half-life — approximately 72 hours versus MK-4’s 1 to 2 hours — meaning once-daily dosing produces more stable blood levels. Most clinical research uses MK-7 at doses of 100 to 200 micrograms daily.
MK-7 is found in natto — a fermented soy dish popular in Japan — and in smaller amounts in cheese and egg yolks. The conversion from dietary K2 to clinical effect is variable, which is why supplementation rather than dietary reliance is the more reliable approach for people using D+K2 therapeutically.
Dosing Practicalities
For most adults, 2000 to 4000 IU of vitamin D3 daily is a reasonable maintenance dose, adjusted based on testing to maintain serum 25-hydroxyvitamin D in the optimal range of 50 to 80 ng/mL. The corresponding K2 dose is 100 to 200 micrograms of MK-7 daily. Some formulations combine these in a single capsule, which is a practical approach for compliance.
If using higher-dose vitamin D — above 5000 IU daily — K2 should be considered mandatory rather than optional. The higher the vitamin D dose, the more important the K2 cofactor becomes for directing calcium to the right destination.
Who Most Needs This Combination
Postmenopausal women are the highest-priority group for combined D+K2 supplementation, given their elevated risk for both osteoporosis and cardiovascular disease. People with existing cardiovascular disease or significant family history should also prioritise this combination. Anyone taking vitamin D supplements at doses above 2000 IU daily without K2 is accumulating calcium in the wrong places and should add K2 immediately.
Testing Vitamin D Status
The single most useful test for vitamin D status is serum 25-hydroxyvitamin D — written as 25(OH)D on lab reports. This reflects the total vitamin D coming from both diet and sunlight exposure, with a half-life of approximately 2 to 3 weeks. The optimal range for general health is 50 to 80 ng/mL (125 to 200 nmol/L), though functional medicine often targets the upper end of this range for therapeutic applications.
Below 30 ng/mL is considered deficient. Below 20 ng/mL is associated with frank deficiency symptoms including bone demineralisation and muscle weakness. Most people in northern latitudes are below the optimal range, particularly in winter months when UVB exposure is insufficient for vitamin D synthesis. Testing is the only reliable way to know your status — guessing based on symptoms or supplementation history is unreliable.
Getting K2 From Food vs Supplements
Dietary K2 is difficult to obtain in meaningful quantities from food alone for most people. The primary food source is natto — fermented soybeans — which is an acquired taste and not a common part of Western diets. Cheese and egg yolks contain smaller amounts of K2, but the quantities are insufficient for therapeutic purposes without impractical quantities of those foods.
This is why supplementation is the practical route for K2. MK-7 supplements at 100 to 200 micrograms daily are affordable, stable, and well-studied. Combining K2 with vitamin D in a single capsule is the most convenient approach and ensures both nutrients are present at meaningful doses. When choosing a combined D+K2 supplement, verify the K2 dose is in the 100 to 200 microgram range — some formulations include K2 at trace levels that are insufficient for the intended cofactor effect.
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