Vitamins · 5 min read ·

Vitamin D: the sunshine vitamin in winter

In winter your skin hardly makes any, and supplements are everywhere. Who they help, which form is better and why it matters how often and with what you take it.

D

In a nutshell

1Where it comes from

The body makes most of its vitamin D in the skin from the sun’s UVB rays. From food it comes mainly from oily fish, eggs and fortified foods. In the body it helps absorb calcium and phosphorus and keep bones and muscles healthy.1

Strong evidence October–March An experiment with human skin showed that in Edmonton (52° north, similar to Czechia) the winter sun makes no vitamin D at all from October to March.4
Data 16 µg A hundred grams of mackerel contain 16 µg of vitamin D, rainbow trout 16 µg, baked farmed salmon 13 µg and egg yolk 5.4 µg.5

2How much you need

EFSA recommendation 15 µg a day EFSA set an adequate intake of 15 µg (600 IU) a day for adults — at this intake most people have at least 50 nmol/L in their blood even when the skin is making none. When the sun shines, less is needed from food.1
Upper level 100 µg The tolerable upper intake level for adults is 100 µg (4,000 IU) a day from all sources. Higher doses over the long term can raise calcium levels in the blood.2

3Forms and how to take it

Strong evidence D3 > D2 A summary of 24 studies with 1,277 people: vitamin D3 (cholecalciferol) raised blood vitamin D on average by 16 nmol/L more than D2 (ergocalciferol), regardless of the dose and the type of product.6
Weak evidence — one study +32% People who took vitamin D3 with a meal containing fat had 32% higher blood levels than after a fat-free meal.7
Weak evidence — 4 studies oil A review of 4 studies: vitamin D in oil raised blood levels more than in powder or in alcohol.8
Moderate evidence — a randomised trial 35% vs 8% Calcifediol is a form of vitamin D already processed by the liver. In a study of 298 post-menopausal women with a deficiency, after 4 months 35% of women on calcifediol and 8% on ordinary D3 reached levels above 75 nmol/L.9

In the EU, supplements may contain vitamin D as cholecalciferol (D3), ergocalciferol (D2) or calcidiol monohydrate.10 EFSA counts 1 µg of calcidiol (at doses up to 10 µg a day) as 2.5 µg of vitamin D.3

4Daily, not in big doses

Moderate evidence −8% A summary of 43 trials with 48,488 people: vitamin D slightly lowered the risk of acute respiratory infections. But it protected only when taken daily, most at doses of 400–1,000 IU; doses given weekly to every three months did not.11
Moderate evidence — a randomised trial +26% Women over 70 were given one large dose of 500,000 IU of vitamin D or placebo every autumn. With vitamin D they had 15% more falls and 26% more fractures, most in the first 3 months after the dose.12

5Do supplements help?

Strong evidence 25,871 people, 5 years In the large VITAL trial, people over 50 took 50 µg (2,000 IU) of vitamin D a day or placebo. The supplement did not reduce cancer or heart events, and in people without vitamin D deficiency or osteoporosis it did not reduce fractures either.13,14

This does not mean vitamin D is useless — it helps where there is a deficiency. The trial only showed that in healthy people who have enough, an extra supplement adds no health.14

What to take from this

Sources

Written on , based on studies available at that date. Newer research may change the conclusions.

  1. EFSA NDA Panel (2016). Dietary reference values for vitamin D. EFSA Journal 14:4547. doi.org/10.2903/j.efsa.2016.4547
  2. EFSA NDA Panel (2023). Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate. EFSA Journal 21:e08145. doi.org/10.2903/j.efsa.2023.8145
  3. EFSA (2025). Overview on Tolerable Upper Intake Levels as derived by the Scientific Committee on Food (SCF) and the EFSA Panel on Nutrition (version 11). efsa.europa.eu
  4. Webb A. R., Kline L., Holick M. F. (1988). Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab 67:373–378. pubmed.ncbi.nlm.nih.gov/2839537
  5. USDA ARS (2018). FoodData Central — SR Legacy, vitamin D per 100 g: mackerel, raw 16.1 µg (FDC 175119), rainbow trout, raw 15.9 µg (FDC 173717), farmed salmon, baked 13.1 µg (FDC 175168), egg yolk 5.4 µg (FDC 172184). fdc.nal.usda.gov
  6. Balachandar R. et al. (2021). Relative efficacy of vitamin D2 and vitamin D3 in improving vitamin D status: systematic review and meta-analysis. Nutrients 13:3328. doi.org/10.3390/nu13103328
  7. Dawson-Hughes B. et al. (2015). Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet 115:225–230. doi.org/10.1016/j.jand.2014.09.014
  8. Grossmann R. E., Tangpricha V. (2010). Evaluation of vehicle substances on vitamin D bioavailability: a systematic review. Mol Nutr Food Res 54:1055–1061. doi.org/10.1002/mnfr.200900578
  9. Pérez-Castrillón J. L. et al. (2021). Calcifediol is superior to cholecalciferol in improving vitamin D status in postmenopausal women: a randomized trial. J Bone Miner Res 36:1967–1978. doi.org/10.1002/jbmr.4387
  10. Directive 2002/46/EC on food supplements, Annex II — forms of vitamins and minerals permitted in supplements (consolidated version 26 Nov 2025). eur-lex.europa.eu
  11. Jolliffe D. A. et al. (2021). Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol 9:276–292. doi.org/10.1016/S2213-8587(21)00051-6
  12. Sanders K. M. et al. (2010). Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial. JAMA 303:1815–1822. doi.org/10.1001/jama.2010.594
  13. Manson J. E. et al. (2019). Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med 380:33–44. doi.org/10.1056/NEJMoa1809944
  14. LeBoff M. S. et al. (2022). Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med 387:299–309. doi.org/10.1056/NEJMoa2202106

This is general information and does not replace a doctor. If you have kidney or parathyroid disease or take medicines, talk to a doctor about vitamin D doses.