Vitamin D Overview
Vitamin D is a fat-soluble vitamin that supports key metabolic functions, including calcium homeostasis and bone health, immune function and cell turnover.1 Evidence highlights the health benefits linked to adequate vitamin D levels, including for osteomalacia and rickets, oral health, cardiovascular health, cancer and autoimmune diseases, as well as some conditions related to allergies.1,2
Explore the latest evidence on osteoporosis prevention and bone health management in adults
Click hereVitamin D is made up of two different forms:
- Vitamin D3 (cholecalciferol) – Produced in human and animal skin when exposed to sunlight (UVB). It is also found in some animal-based foods, such as fatty fish, egg yolks, and certain cuts of meat (e.g., liver), typically in modest but variable amounts.
- Vitamin D2 (ergocalciferol) – Found in certain fungi, such as mushrooms, and in some algae, typically in modest but variable amounts.
Because natural food sources provide limited vitamin D and most Canadians do not get enough from their diet, Canada maintains a vitamin D fortification strategy. Under this strategy, fortification of fluid milk is mandatory, and the vitamin D content has recently been increased, while some yogurts and kefirs may now also be fortified.3 For example, a 250 mL serving of 2% milk provides about 5 µg of vitamin D, roughly 25% of the daily value of 20 µg.3 In Canada, fluid milk and dairy are typically fortified with vitamin D3, as indicated on product labels.
Currently, the main measure of vitamin D status is serum concentrations of 25-hydroxyvitamin D [25(OH)D], which includes both 25(OH)D2 and 25(OH)D3, and represents both endogenously synthesized vitamin D as well as that obtained from dietary and supplemental sources.2 More specifically, vitamin D2 supplementation increases 25(OH)D2 levels and vitamin D3 supplementation increases 25(OH)D3 levels.2 There has been ongoing research in the field of vitamin D to better understand functional differences between vitamin D2 and D3, including its ability to increase serum 25(OH)D levels.
The Evidence
In a 2025 systematic review and meta-analysis of randomized controlled trials, a statistically significant reduction in serum 25(OH)D3 concentrations was observed after vitamin D2 supplementation compared to control:1
- Meta-analysis found vitamin D₂ supplementation significantly reduced serum 25(OH)D₃ by about 18 nmol/L compared to controls.
- Absolute change analysis found a 9 nmol/L decrease in 25(OH)D₃ levels in the vitamin D2-supplemented group.
Considering the detrimental effects of vitamin D2 supplementation on vitamin D status, the study authors recommend prioritizing vitamin D3.1
A 2024 systematic review and meta-analysis found that, while both vitamin D2 and D3 supplementation increase their respective 25(OH)D hydroxylated forms, vitamin D3 produces a greater overall increase in total serum 25(OH)D concentrations, by about 10 nmol/L.4 In fact, vitamin D3 consistently produced greater increases in serum 25(OH)D compared to vitamin D2, even with standard daily dosing, indicating its effects extend beyond high-dose or long-term supplementation.
In line with this, a 2012 meta-analysis of randomized controlled trials comparing the effects of vitamin D3 and D2 supplementation found that vitamin D3 is more efficacious than vitamin D2 at raising concentrations of serum 25(OH)D.5
Similar findings have been obtained from more recent randomized, placebo-controlled trials. These studies have also found that vitamin D3 is more effective than vitamin D2 at increasing and maintaining higher serum concentrations of 25(OH)D.6,7 Furthermore, it has been demonstrated that vitamin D3 supplements may be more potent than vitamin D2 supplements in raising serum 25(OH)D levels in older adults as well as in maintaining these levels during the wintertime.8,9
Some studies have shown that daily administration or a loading dose of vitamin D2 may be comparably effective as vitamin D3 in raising and maintaining serum 25(OH)D concentrations in the short term; however, in the long term, vitamin D3 appears to better sustain these levels. 9-11
In Canada, fluid milk is typically fortified with vitamin D3, as indicated on product labels. This form of vitamin D has been shown to be more effective than vitamin D2 for increasing serum 25(OH)D levels.
Potential Mechanisms
Several mechanisms could explain the superior biological activity of vitamin D3 to that of vitamin D2. A plausible explanation is that vitamin D3 has a higher affinity for vitamin D binding proteins as well as for the vitamin D receptor. It has also been suggested that vitamin D3 is the preferred substrate for the hydroxylase enzyme in the liver, which converts vitamin D to 25(OH) D.1,12 These are critical factors in the activation of vitamin D in the body. Moreover, vitamin D2 may have a shorter half-life than vitamin D3.4,13
It should be noted that, in addition to its lower bioefficacy, vitamin D2 is considerably less stable than vitamin D3, particularly when in a crystalline powder form that is subjected to varying temperatures, humidity levels and even storage methods. In addition, some evidence suggests vitamin D2 may have a slightly higher potential for toxicity, possibly related to its decreased binding ability to plasma vitamin D binding protein, resulting in higher serum concentrations of free, biologically active forms of vitamin D2.12
Overall, evidence highlights that vitamin D3 more effectively raises serum 25(OH)D concentrations than vitamin D2, and fortified foods (e.g., Canadian milk and some yogurts and kefirs) can support meeting dietary requirements and maintaining adequate vitamin D status.
References
- Brown EIG et al. Effect of vitamin D2 supplementation on 25-hydroxyvitamin D3 status: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev 2025;nuaf166.
- National Institutes of Health. 2025. Vitamin D. www.ods.od.nih.gov. Accessed February 23, 2026.
- Health Canada. 2025. Fortified foods: Canada’s approach to fortification. www.canada.ca. Accessed March 2, 2026.
- van den Heuvel EGHM et al. Comparison of the effect of daily vitamin D2 and vitamin D3 supplementation on serum 25-Hydroxyvitamin D concentration (total 25(OH)D, 25(OH)D2, and 25(OH)D3) and importance of body mass index: a systematic review and meta-analysis. Adv Nutr 2024;15:100133.
- 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;95:1357-1364.
- Lehmann U et al. Bioavailability of vitamin D2 and D3 in healthy volunteers, a randomized placebo-controlled trial. J Clin Endocrinol Metab 2013;98:4339-4345.
- Itkonen ST et al. Effects of vitamin D2-fortified bread v. supplementation with vitamin D2 or D3 on serum 25-hydroxyvitamin D metabolites: an 8-week randomised-controlled trial in young adult Finnish women. Br J Nutr 2016;115:1232-1239.
- Cashman KD et al. Relative effectiveness of oral 25-hydroxyvitamin D3 and vitamin D3 in raising wintertime serum 25-hydroxyvitamin D in older adults. Am J Clin Nutr 2012;95:1350-1356.
- Logan VF et al. Long-term vitamin D3 supplementation is more effective than vitamin D2 in maintaining serum 25-hydroxyvitamin D status over the winter months. Br J Nutr 2013;109:1082-1088.
- Oliveri B et al. Vitamin D3 seems more appropriate than D2 to sustain adequate levels of 25OHD: a pharmacokinetic approach. Eur J Clin Nutr 2015;69:697-702.
- Holick MF et al. Vitamin D2 is as effective as vitamin D3 in maintaining circulating concentrations of 25-hydroxyvitamin D. J Clin Endocrinol Metab 2008;93:677-681.
- Houghton LA and Vieth R. The case against ergocalciferol (vitamin D2) as a vitamin supplement. Am J Clin Nutr 2006;84:694-697.
- Jones KS et al. 25(OH)D2 half-life is shorter than 25(OH)D3 half-life and is influenced by DBP concentration and genotype. J Clin Endocrinol Metab 2014;99:3373-3381.
