November 13, 2025, 6:11 pm | Read time: 3 minutes
Vitamin D and Vitamin K2 are two essential nutrients that play a crucial role in bone health. Therefore, many supplement manufacturers recommend taking both vitamins as a combination product, arguing that Vitamin D alone is less effective. But is that really true? FITBOOK nutrition expert Sophie Brünke clarifies and warns that the combination of both vitamins can also be risky for some people.
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The Idea Behind the Vitamin Combo
Vitamin D ensures that calcium from food is absorbed and transported into the blood. For it to reach the bones from there and not deposit in the arteries, Vitamin K2 is needed. This theoretical interaction led to the assumption that Vitamin D is less effective without K2 or could even increase the risk of vascular calcification when a lot of calcium circulates in the blood, such as due to high doses of Vitamin D. This theoretical interplay is marketed as a “synergistic effect” for bones and heart and is the main reason why combination products are so popular.
Vitamin D Deficiency is Common, K2 Deficiency is Not
About 30 percent of adults in Germany have a Vitamin D deficiency, according to the Robert Koch Institute.1 A look at the rest of the population shows that the situation is not particularly rosy: The German Nutrition Society emphasizes that almost 60 percent of the population does not reach the desirable Vitamin D concentration in the blood. As a result, most people do not fully utilize the preventive potential of Vitamin D for bone health.2 Supplementation can therefore be quite sensible–in consultation with a doctor or nutritionist.
The situation is different for Vitamin K2. In healthy people who eat a varied diet, a deficiency is considered unlikely. The vitamin is mainly found in green vegetables, dairy products, and eggs.3
Fat-soluble vitamins like D and K can be easily overdosed because the body stores them in fat tissue. According to current knowledge, there is no evidence of harmful effects for Vitamin K. However, an excess of Vitamin D can lead to kidney stones or kidney calcification.
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For Whom the Combination Can Be Dangerous
The joint intake of Vitamin D and K2 can pose risks for certain groups of people–especially for those with osteoporosis or those taking blood-thinning medications. According to the Consumer Center and the Federal Institute for Risk Assessment (BfR), there is still no scientific evidence that Vitamin K2 improves the effect of Vitamin D in osteoporosis or protects against vascular calcification. The current osteoporosis guideline from 2023 even explicitly advises against using Vitamin K2 specifically for therapy.4,5
Interactions with medications can also be problematic. Vitamin K antagonists are often used to prevent thrombosis–these are drugs that specifically inhibit the effect of Vitamin K to slow blood clotting. Those taking such medications should not use additional Vitamin K supplements without first consulting their doctor. According to the BfR, supplements containing Vitamin K should therefore carry the following warning: “People taking anticoagulant medications should seek medical advice before consuming supplements containing Vitamin K.”
Conclusion
Vitamin D and K2 are often offered together because both are involved in calcium metabolism. In theory, K2 ensures that the calcium absorbed by Vitamin D reaches the bones and does not deposit in the vessels. However, there is still no scientific proof for this. On the contrary, high-dose preparations can pose risks–for example, for people with osteoporosis or when taking blood-thinning medications. For healthy individuals, additional K2 intake is generally unnecessary.