Health Reference LibraryWhich form of vitamin D should you buy: D3, D2, and does K2 matter?
This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.
Summary
Between the two vitamin D forms the evidence is unusually clear: D3 (cholecalciferol) raises and holds blood 25-hydroxyvitamin D better than D2 (ergocalciferol) in the systematic review of head-to-head trials, so D3 is the default buy, with lichen-derived D3 covering vegans. The K2 pairing is a different kind of claim: mechanistically reasonable for directing calcium, popular, and supported by far weaker outcome evidence than the D3 case, so buy D3 with K2 as a plausible extra rather than a proven requirement. UK guidance is 10 micrograms (400 IU) daily through autumn and winter, and testing beats guessing at higher doses.
How it works
D3 and D2 are both prohormones converted in the liver to 25-hydroxyvitamin D, the measured storage form, then activated in the kidney. Their side chains differ, and the practical consequence is that D3 binds the transport protein and sustains circulating 25-hydroxyvitamin D better. Vitamin K2 activates proteins involved in placing calcium in bone and keeping it out of soft tissue, which is the mechanistic story behind the pairing.
Effective dose
UK government advice: everyone should consider 10 micrograms (400 IU) daily through autumn and winter, and year-round for people with little sun exposure or darker skin. Higher corrective dosing belongs alongside a tested 25-hydroxyvitamin D level rather than guesswork, because vitamin D accumulates. The UK upper guidance for adults is 100 micrograms (4000 IU) daily.
Forms compared
D3 (cholecalciferol) comes from lanolin conventionally and from lichen for vegan products; D2 (ergocalciferol) comes from irradiated fungi. The Tripkovic systematic review and meta-analysis of randomised trials found D3 more effective than D2 at raising serum 25-hydroxyvitamin D. Formats (capsule, oil drop, spray) matter less than taking it with some dietary fat, since vitamin D is fat-soluble.
Timing
With a meal containing fat, any time of day. Daily dosing keeps levels steadier than infrequent large doses, and consistency again beats clock time.
Safety profile
Vitamin D excess raises blood calcium, with nausea, thirst and kidney effects, essentially always from sustained high supplement doses rather than sun or food. Staying at or under the UK 100 microgram ceiling without a tested reason to exceed it removes the risk. K2 at supplement doses has a benign record, with one hard exception below.
Special populations
People on warfarin must not add vitamin K2 without their anticoagulation team, because K antagonises the drug. Darker-skinned, housebound, and fully covered people are the groups UK guidance flags for year-round vitamin D. Pregnancy vitamin D follows the same 10 microgram advice.
Interactions
Vitamin K2 antagonises warfarin, the one interaction in this pairing that is absolute. Some weight-loss drugs and fat-malabsorption conditions reduce vitamin D absorption. High-dose vitamin D raises calcium absorption, relevant alongside calcium supplements.
Guideline positions
NHS and UK government advice: 10 micrograms daily considered for everyone in autumn and winter, year-round for at-risk groups, upper level 100 micrograms daily for adults. Tripkovic 2012 systematic review and meta-analysis: D3 more effective than D2 at raising serum 25-hydroxyvitamin D. NIH ODS vitamin D fact sheet: metabolism, hypercalcaemia risk profile and drug interactions.
Practical framework
Decide in three steps. First, default to D3 over D2 on the trial evidence, choosing lichen-derived D3 if vegan. Second, dose at the UK 10 microgram baseline, and only run corrective doses against a measured blood level, which our Blood Result Interpreter on the vitamin D entry can help you read. Third, regard K2 as an optional companion with promising but unproven outcome evidence, never alongside warfarin. The combined worked example on our shelf is Vitamin D3 and K2 at nutritailor.co.uk/products/vitamin-d3-k2, with a vegan version alongside.
Common misconceptions
That D2 and D3 are interchangeable because both are vitamin D: the head-to-head trials say otherwise for maintaining blood levels. That K2 is required for vitamin D to be safe or effective: the outcome evidence for the pairing is far thinner than the marketing, and vitamin D functions without supplemental K2. That more vitamin D is better: above the ceiling, accumulating vitamin D raises calcium with real consequences.
Sources
- NHS (UK government health service) 2024. Vitamin D - Vitamins and minerals. NHS.uk.
- NIH Office of Dietary Supplements 2025. Vitamin D: Fact Sheet for Health Professionals. National Institutes of Health, Office of Dietary Supplements.
- Tripkovic L, Lambert H, Hart K, Smith CP, Bucca G, Penson S, Chope G, Hyppönen E, Berry J, Vieth R, Lanham-New S 2012. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition 95(6):1357-64, DOI 10.3945/ajcn.111.031070, PMID 22552031.