Vitamin D
Vitamin D is a prohormone. The liver converts it to 25-hydroxyvitamin D, the circulating form that labs measure, and the kidney converts that to calcitriol, which acts through the vitamin D receptor to drive intestinal calcium and phosphate absorption. How much anyone needs depends on where the sufficiency line is drawn, and the two authorities that drew it in 2011 put it in different places.
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What It Does
Calcitriol switches on the transport proteins that move calcium across the intestinal wall. Without enough of it, the gut absorbs a small fraction of dietary calcium, parathyroid hormone rises, and bone is resorbed to hold serum calcium steady. Severe deficiency causes rickets in children and osteomalacia in adults. Those outcomes are settled.
The threshold is not. The Institute of Medicine set the RDA at 600-800 IU on the finding that a 25(OH)D of 20 ng/mL (50 nmol/L) covers the bone needs of nearly everyone (Ross 2011). The Endocrine Society guideline published the same year called anything below 30 ng/mL insufficient and suggested 1,500-2,000 IU daily to get there (Holick 2011). Its 2024 replacement dropped those cutoffs, recommended against routine 25(OH)D testing in healthy adults, and limited empiric supplementation to children, adults over 75, pregnancy, and high-risk prediabetes (Demay 2024).
The large trials explain the retreat. VITAL gave 2,000 IU daily to nearly 26,000 adults for about five years and found no reduction in invasive cancer or major cardiovascular events (Manson 2019), and no reduction in fractures (LeBoff 2022). In D2d, 4,000 IU daily in prediabetes produced a 12% lower risk of diabetes that did not reach significance (Pittas 2019). Most participants in these trials were not deficient at baseline, which is both the main criticism of them and the main lesson: supplementation helps people who are low.
People likely to be low include those with little sun exposure, darker skin (melanin reduces cutaneous synthesis), older adults, and people with obesity (vitamin D is sequestered in adipose tissue). Fat malabsorption from Crohn's disease, celiac disease, cystic fibrosis, or bariatric surgery reduces absorption of oral doses.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Absorption | Typical dose | Cost | Labels |
|---|---|---|---|---|
| Vitamin D3 (Cholecalciferol)cholecalciferol, D3 | HighFat-soluble; incorporated into mixed micelles and absorbed in the small intestine, best when taken with a meal containing fat. Hydroxylated in the liver to 25(OH)D, the circulating marker of status. | 1,000-2,000 IU (25-50 mcg) daily for maintenance | Low | 4,891 |
| Vitamin D2 (Ergocalciferol)ergocalciferol, D2 | ModerateAbsorbed like D3 via micelles, but 25-hydroxylated less efficiently and cleared faster, so it raises total 25(OH)D less per IU than D3 (Tripkovic 2012 meta-analysis). | 1,000-2,000 IU daily; 50,000 IU weekly as a prescription repletion regimen | Low | 96 |
Labels column: number of labels in the Vitamin D category that list this form. A product can list more than one form, and some labels do not state a form at all.
D3 (cholecalciferol) is the form the skin makes. D2 (ergocalciferol) comes from UV-irradiated yeast or mushrooms. A meta-analysis of head-to-head trials found D3 raised 25(OH)D more effectively, with the gap widest when the dose was given weekly or monthly in large boluses (Tripkovic 2012). With daily dosing the meta-analysis found no significant difference. The 50,000 IU prescription capsules in the US are usually D2, which is a reason to recheck levels after a course of them. Some 25(OH)D immunoassays under-detect the D2 metabolite, so a level measured on D2 can read falsely low.
Vegan D3 is extracted from lichen and is the same molecule as lanolin-derived D3. A rough rule: each additional 100 IU per day raises 25(OH)D by about 1 ng/mL over two to three months, less in people with obesity or a high starting level.
Typical Dosing
| Group | RDA |
|---|---|
| Adults 19-70 | 600 IU (15 mcg) |
| Adults 71+ | 800 IU (20 mcg) |
| Pregnancy and lactation | 600 IU (15 mcg) |
| Upper limit (UL) | 4,000 IU (100 mcg) per day from all sources |
Maintenance supplements commonly provide 1,000-2,000 IU. Repletion doses depend on the starting 25(OH)D level and body weight.
Food sources: cod liver oil, trout, salmon, mushrooms exposed to UV light, fortified milk, fortified cereals, egg yolk.
What to Look For
Since 2020, US labels state vitamin D in micrograms, with IU often in parentheses: 25 mcg equals 1,000 IU. Check that the panel names cholecalciferol or D3. Oil-filled softgels and tablets taken with a meal that contains fat are both reasonable; the meal matters more than the format.
Strengths of 5,000 or 10,000 IU are sold over the counter for daily use. They have a place in supervised repletion with a measured starting level and a recheck at about three months. As an indefinite daily dose for someone who has never been tested, 10,000 IU is above the UL and there is no reason for it. Reported cases of vitamin D toxicity have involved manufacturing errors that put many times the labeled amount in each dose, which is the strongest argument for third-party certification (USP Verified, NSF) on this product in particular.
Red flags: drops or gummies without a stated mcg per serving, and products that claim D3 is unsafe without K2. Green flags: a single form, mcg and IU both stated, and third-party testing.
Vitamin D3 (Cholecalciferol)
The default. Raises 25(OH)D more per IU than D2 and costs very little.
What to look for: 1,000-2,000 IU (25-50 mcg) per dose for maintenance. Higher strengths only with a measured 25(OH)D and a plan to recheck. USP Verified or NSF Certified.
Find on iHerb Affiliate linkVitamin D2 (Ergocalciferol)
Works, but less durably than D3, especially in large weekly or monthly doses. Lichen-derived D3 has removed the main reason vegans chose it.
What to look for: Daily rather than intermittent dosing if D2 is what you have. Recheck 25(OH)D with an assay that measures D2 metabolites.
Find on iHerb Affiliate linkProducts on Titrate
The Vitamin D category in Titrate's index holds 5,153 labels from the NIH Dietary Supplement Label Database, 2,595 of them currently on market, out of 214,745 labels indexed in total.
Top on-market labels, ranked by label completeness and simplicity:
- D3 5000 IU K2 200 mcg
- Magnesium with Zinc and Vitamin D3
- Vitamin K2 + D3
- Vitamin D + K2
- Vitamin D-3 with Vitamin K-2 MenaQ7
Supplement Search loads one category at a time. Choose the Vitamin D tab, then type a form such as "d3" under Must contain, or a form to avoid under Must not contain.
Related Tools
References
NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals.
Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930.
Ross AC, et al. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. J Clin Endocrinol Metab. 2011;96(1):53-58.
Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364.
Manson JE, et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. N Engl J Med. 2019;380(1):33-44.
Pittas AG, et al. Vitamin D Supplementation and Prevention of Type 2 Diabetes. N Engl J Med. 2019;381(6):520-530.
LeBoff MS, et al. Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults. N Engl J Med. 2022;387(4):299-309.
Demay MB, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947.
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