The Short Answer
Vitamin D3 (cholecalciferol) is the better form. It raises serum 25(OH)D levels more effectively than D2 (ergocalciferol), sustains those levels longer, and is the form your body produces naturally from sunlight. D2 is not worthless, but the evidence against treating the two as equivalent is now strong enough that several major medical bodies have revised their guidance. If you have any choice in the matter, choose D3.
The Biological Difference
D2 and D3 are both converted by the liver into 25-hydroxyvitamin D, the circulating form measured in blood tests. But the conversion is not equally efficient. D3 is the form produced in human skin when UVB radiation hits 7-dehydrocholesterol. D2 is produced by fungi and yeast in response to UV radiation. Both are "vitamin D," but the structural difference - a methyl group and double bond on D2's side chain - changes how the body handles them.
The liver enzyme CYP2R1 hydroxylates both forms, but it has a higher affinity for D3. D3 also binds more effectively to vitamin D-binding protein in the bloodstream, which is the carrier molecule that determines how long the vitamin circulates before being cleared. The result: D3 produces a higher, longer-lasting rise in blood levels per unit dose.
What the Clinical Evidence Shows
Head-to-head trials favor D3
A 2012 meta-analysis published in the American Journal of Clinical Nutrition, covering seven randomized controlled trials with direct D2-vs-D3 comparisons, found that D3 was significantly more effective at raising serum 25(OH)D levels. The pooled analysis showed D3 produced 25(OH)D concentrations approximately 50% higher than equivalent doses of D2. This was not a small or ambiguous effect.
A particularly well-designed 2011 study by Heaney and colleagues gave 50 healthy adults either 50,000 IU of D2 or 50,000 IU of D3 weekly for 12 weeks. The D3 group achieved serum levels 70% higher than the D2 group at study end. More importantly, the D2 group showed a decline in 25(OH)D3 levels (the D3-specific metabolite), suggesting that D2 supplementation may actually interfere with the body's ability to maintain D3 status.
The sustainability gap
The difference is most pronounced in how long levels stay elevated after a dose. D3 produces a rise in 25(OH)D that persists for weeks, while D2's effect decays more rapidly. A 2004 study in the Journal of Clinical Endocrinology and Metabolism found that a single dose of D3 maintained elevated blood levels for approximately 14 days longer than an equivalent dose of D2. For people taking vitamin D intermittently (weekly or monthly dosing, which some physicians prescribe), this matters significantly.
Does D2 work at all?
Yes. D2 does raise blood vitamin D levels, and it has been used successfully to treat deficiency in clinical settings for decades. The issue is not that D2 is inert - it is that it requires higher or more frequent doses to achieve the same blood levels as D3. A 2013 Cochrane-style systematic review estimated that D2 may need to be dosed approximately 3x higher than D3 to achieve equivalent 25(OH)D levels when given as a bolus dose. For daily supplementation at standard consumer doses (1,000-5,000 IU), the gap narrows but D3 still wins consistently.
Why D2 Still Exists
Two reasons. First, history: D2 was the first form synthesized commercially, and early vitamin D research did not distinguish between the two. The Institute of Medicine's 2011 guidelines treated D2 and D3 as interchangeable, and many prescription vitamin D formulations (particularly the 50,000 IU capsules used to treat severe deficiency) still use D2 because that is what has been available in prescription form.
Second, D2 is plant-derived (from UV-irradiated mushrooms or yeast), making it the only traditional option for strict vegans. This has changed recently - D3 sourced from lichen (a symbiotic organism of algae and fungi) is now widely available and is vegan-friendly. Lichen-derived D3 has the same chemical structure as animal-derived D3 and the same efficacy. If you are vegan, lichen-sourced D3 eliminates the last practical reason to choose D2.
The Dosing Implications
If you are taking D3, the standard supplemental range of 1,000-5,000 IU per day is well-supported. Most adults with limited sun exposure benefit from 2,000-4,000 IU/day to maintain blood levels in the 40-60 ng/mL range that most current research associates with optimal health outcomes. The Endocrine Society guidelines suggest up to 4,000 IU/day for adults at risk of deficiency.
If for some reason you must use D2, consider dosing approximately 2-3x higher than you would with D3, taken daily rather than weekly. But this raises cost and pill burden without any compensating advantage. There is no scenario where D2 is the superior choice for a consumer who has access to D3.
What About D3 + K2 Combinations?
Vitamin K2 (specifically the MK-7 form) plays a role in directing calcium into bones and teeth rather than soft tissues and arteries. The theoretical rationale for combining D3 with K2 is that vitamin D increases calcium absorption, and K2 ensures that calcium goes where you want it. This is biologically plausible and supported by mechanistic evidence, but the clinical trial data specifically linking D3+K2 supplementation to better outcomes than D3 alone is still limited.
A 2017 study in Osteoporosis International found that D3+K2 improved bone mineral density more than D3 alone in postmenopausal women, but the study was small. A 2019 randomized trial of healthy adults found no significant effect of K2 supplementation on arterial calcification over three years.
The combination is not unreasonable, particularly for people supplementing vitamin D at higher doses (4,000+ IU/day) or for those at risk of arterial calcification. But it should not be presented as essential. If you are choosing between a high-quality D3 supplement and a lower-quality D3+K2 combo, the D3 alone is the better bet. See our vitamin D3 scorecard for specific product recommendations.
Who Needs Vitamin D Supplementation?
More people than you would expect. Research estimates that approximately 42% of US adults have insufficient vitamin D levels (below 20 ng/mL by conservative standards, below 30 ng/mL by Endocrine Society standards). Risk factors for deficiency include:
- Living above 37 degrees latitude (north of San Francisco, roughly) from October through March
- Dark skin pigmentation (melanin reduces UVB-driven D3 synthesis)
- Office-based work with limited outdoor time
- Sunscreen use (SPF 30 reduces D3 synthesis by approximately 95%)
- Obesity (vitamin D is sequestered in adipose tissue)
- Age over 65 (skin synthesis declines with age)
If two or more of these apply to you, supplementation with D3 is almost certainly beneficial. A blood test measuring 25(OH)D is the only way to know your actual status. Ask your doctor to include it in routine bloodwork.
FAQ
Can you get enough vitamin D from food?
Unlikely for most people. Fatty fish (salmon, mackerel, sardines) and fortified milk are the primary dietary sources, but a serving of salmon provides roughly 400-600 IU. Reaching 2,000-4,000 IU/day from food alone would require eating salmon at nearly every meal. Supplementation is the practical solution.
Is it possible to take too much vitamin D?
Yes, though toxicity is rare at standard supplemental doses. The tolerable upper intake level set by the Institute of Medicine is 4,000 IU/day, though many researchers argue this is conservative. Toxicity symptoms (hypercalcemia, nausea, kidney issues) typically appear at sustained doses above 10,000-40,000 IU/day over extended periods. Blood levels above 100 ng/mL are considered potentially toxic. At 1,000-5,000 IU/day, toxicity is virtually unheard of in people with normal kidney function.
Do mushrooms provide the same vitamin D as sunlight?
Mushrooms exposed to UV light produce vitamin D2, not D3. While UV-treated mushrooms can contain meaningful amounts (some commercial varieties list 400+ IU per serving), the vitamin D they provide is the less effective D2 form. They are a useful dietary source but not a substitute for D3 supplementation.
Should I take vitamin D with fat?
Yes. Vitamin D is fat-soluble, and absorption improves significantly when taken with a meal containing dietary fat. A 2015 study found that taking vitamin D with the largest meal of the day (which typically contains the most fat) increased absorption by approximately 50% compared to taking it on an empty stomach.
These statements have not been evaluated by the FDA. Dietary supplements are not intended to diagnose, treat, cure, or prevent any disease.