Should you take vitamin D together with vitamin K2?

Vitamin K2 has its own effect on bone health, independent of vitamin D. In an RCT involving 311 postmenopausal women, 90 micrograms of K2 per day significantly slowed bone loss at the hip compared with a placebo. Notably, adding calcium and vitamin D to K2 provided no additional benefit over K2 alone. That is no reason to leave out vitamin D, but it does undermine the idea that the two vitamins necessarily need each other in order to work.

For the heart and blood vessels, the story is often told as follows: high-dose vitamin D increases calcium absorption, and K2 ensures that calcium ends up in bones rather than in blood vessels. Large randomised studies, however, yield contradictory results for this claim. The effect on vascular calcification remains unclear in terms of both direction and magnitude, and depends strongly on whether a person already has a deficiency or carries other risk factors.

There are specific situations in which the combination scores better than vitamin D alone. After spinal surgery in people with osteoporosis, 92% of patients taking K2+D3+calcium achieved complete bone fusion after six months, compared with 74% in those not taking K2. And in a small pilot study in children with a blood disorder that causes bone problems, the combination improved bone density. These are, however, small and specific groups; generalising to healthy people goes too far.

Official guidelines do not yet recommend K2 supplementation for bone health. The available evidence is too variable and too inconsistent. Calcium and vitamin D are recommended. If you do want to buy a combination supplement, pay attention to quality: vitamin D3 is usually present in products at the stated dose, but the amount of K2 frequently deviates from what is listed on the label. You therefore cannot always be certain of what you are actually getting.