Vitamin D is important for calcium absorption and regulation of calcium and phosphate levels in blood. Bone health depends on this vitamin. Yet, vitamin D is also involved in other physiological roles: functioning immunological system1, growth and differentiation of cells2 and is also implicated in glucose metabolism3.
This fat-soluble vitamin is unique because it can be produced by the body with sufficient UVB radiation from sun exposure. Vitamin D is also naturally occurring in some foods, but mostly of animal origin; but vitamin D2 is found in mushrooms which have been exposed to UV light. Vitamin D can also be found in fortified animal-free foods such as plant milks and yogurts.
There are two forms of vitamin D: D2 (ergocalciferol) and D3 (cholecalciferol). D2 is synthesised by fungi; D3 by animals (including humans). D3 seems to increase active forms of vitamin D in the blood more than D2 4.
The following groups are at increased risk of vitamin D deficiency:
- Pregnant and lactating women
- Breastfed infants
- Older adults
- Individuals that get low sun exposure
- Dark skinned individuals
- Obese individuals
- Individuals with certain chronic diseases
- Certain medications
I also think that vegans are at greater risk because it's currently fairly impractical to reliably obtain sufficient vitamin D from food.
The National Institutes of Health's RDA for vitamin D is displayed in the table below:
*Adequate Intake (AI)
However, the guidelines-based recommendations vary considerably (200 to 2000IU! 5). The need for vitamin D depends on your country of residence, season of the year, smog levels, age, skin tone, sun exposure among other factors. It's therefore important that individuals use local guidelines when estimating their vitamin D requirements.
If you are taking medications but consider supplementation please speak to your doctor as vitamin D can interfere with certain medications.
High doses of vitamin D are associated with negative health outcomes; under usual circumstances it is not advisable to take higher than the recommended doses, unless advised to do so by your doctor.
Higher doses may be necessary at times, but you would have to be monitored by a doctor.
1. Deluca, H.F. and Cantorna, M.T., 2001. Vitamin D: Its role and uses in immunology 1. The FASEB journal, 15(14), pp.2579-2585. (https://faseb.onlinelibrary.wiley.com)
2. Reichel, H., Koeffler, H.P. and Norman, A.W., 1989. The role of the vitamin D endocrine system in health and disease. New England Journal of Medicine, 320(15), pp.980-991
3. Sung, C.C., Liao, M.T., Lu, K.C. and Wu, C.C., 2012. Role of vitamin D in insulin resistance. Journal of biomedicine and biotechnology, 2012.
4. Tripkovic, L., Lambert, H., Hart, K., Smith, C.P., Bucca, G., Penson, S., Chope, G., Hyppönen, E., Berry, J., Vieth, R. and 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. The American journal of clinical nutrition, 95(6), pp.1357-1364. (https://academic.oup.com)
5. Bouillon, R., 2017. Comparative analysis of nutritional guidelines for vitamin D. Nature Reviews Endocrinology, 13(8), pp.466-479.