Should you take vitamin D together with vitamin K2?
For osteoporosis, there is reasonable evidence that vitamin K2 and D3 together do more for bone density than either one alone; for healthy people without bone problems, the necessity of the combination has not been demonstrated. Do not use vitamin K with blood thinners, and always discuss high vitamin D doses with your doctor.
No, you do not always need to take vitamin D and K2 together. Whether it makes sense depends on what you want to achieve.
For osteoporosis, the combination has the strongest evidence. A study in 92 postmenopausal women showed that vitamin D3 and K2 taken together improved lumbar spine bone density significantly more than either one alone. An analysis of eight studies involving 971 participants confirms this: the combination increases total bone density and lowers a blood marker that indicates vitamin K is insufficiently active. If you have osteoporosis, discuss it with your doctor. In research, postmenopausal women received 180 mcg of K2 per day, as menaquinone-7 (MK-7, the most studied form), and less than 500 mcg per day in combination studies, for a minimum of 1 to 3 years. On the label, look for 'Vitamin K2 as MK-7 (menaquinone-7)' with the dose in mcg. Whether it reliably prevents actual bone fractures is not yet certain enough to rely on fully. Whether K2 is listed as MK-7 or MK-4 does matter for bone density: MK-7 raises blood levels more effectively at normal dietary doses, whereas MK-4 did not demonstrably do so in older studies.
For vascular protection, the combination sounds promising: vitamin D at high doses may potentially promote calcium deposits in blood vessels, and K2 activates a protein that inhibits this. However, completed studies with real cardiac outcomes are still entirely lacking. This is too under-researched to recommend.
For healthy adults without bone problems, there is no conclusive evidence that adding K2 is necessary. Review authors conclude that the data on K2 as a supplement are not yet convincing enough for clear advice. The combination is not worth the money if you have no established bone loss and are using a normal vitamin D dose. Do not use vitamin K if you take blood thinners such as warfarin: this can be dangerous and should only be done in consultation with your doctor. If you are already taking more than 2000 IU of vitamin D per day, discuss that with your doctor as well.
Evidence is based on one meta-analysis (8 RCTs, 971 participants), one RCT (92 participants), one prospective study (71 participants), multiple reviews, and animal and laboratory research. No large independent RCTs exist for blood vessels; that section rests on mechanistic and observational evidence.
Does vitamin K2 help your bones and blood vessels?
For bones in postmenopausal women the evidence is reasonably strong; the combination with vitamin D gives the greatest effect. For blood vessels the indications are interesting, but hard evidence from completed studies is still lacking.
How do I know whether I have enough vitamin D, and should I be supplementing?
A blood test (25-hydroxyvitamin D) tells you whether you have a deficiency. At a value below 20 ng/mL, supplementing is worthwhile, particularly for your bones. For your heart, vitamin D has no proven effect.
What is the optimal vitamin D level?
For bone health, 50 nmol/l is the most widely accepted target level, achievable with 800 IU of vitamin D per day. Whether a higher level also provides benefit beyond the skeleton has not been demonstrated.
Which form of vitamin K2 is better: MK-4 or MK-7?
MK-7 is actually absorbed into the blood at normal supplementation doses, whereas MK-4 does not rise measurably at that dose. If you take vitamin K2, choose a supplement that lists MK-7 (menaquinone-7) on the label.
How much vitamin D should you take per day?
For healthy adults under 75, extra vitamin D is probably not necessary. For children, adults aged 75 and over, pregnant women, and people at high risk of diabetes, there are indications that supplementation helps, but discuss this with your doctor.
Can you get too much vitamin D?
Too much vitamin D is genuinely harmful, but that risk only arises at blood levels far above the recommended values. Stick to the dose on the packaging, and if you are unsure about your level, have it measured by your GP.