If you type "Vitamin D" into PubMed, you get more than 100,000 results. So this is a difficult question to answer, isn't it? Well, first of all, there's no way to live without Vitamin D. Vitamin D can be obtained from the diet and from exposure of the skin to sunlight. When the skin is exposed to the UV rays of sunlight, photolytic conversion of 7-dehydrocholesterol to Previtamin D3 occurs, followed by thermal isomerisation to Vitamin D3 (Adams & Hewison, 2010; for a more detailed insight into biochemistry, see Dusso et al., 2005). But before we talk about Vitamin D and Vitamin D3 in particular, let's take a look at how Vitamin D was discovered.

If you don't want to read the whole text, I will give you my recommendation right away: Yes, in most cases you should take vitamin D because many people have insufficient serum levels. A daily dose of 800–1,200 IU is a sensible, year-round default. This is simple, safe within current upper limits, and aligned with the dosing that most consistently shows benefit in trials and meta-analyses (see below).

The discovery of Vitamin D
Rickets, a disease with disturbed mineralisation of bone tissue and growth plates, can lead to weak bones in infants and children. The word "rickets" was first used in 1634. Reports from the Royal Infirmary in Manchester indicate that cod liver oil can cure rickets (O'Riordan & Bijvoet, 2014). In 1822, Sniadecki pointed out the connection between sunlight and rickets. At the end of the First World War, when rickets was an untreatable major problem in Vienna, Harriet Chick led a British Medical Research Council group to study the disease. They concluded that both cod liver oil and sun exposure could cure rickets. In 1922, McCollum and his colleagues coined the term "Vitamin D" in work that pointed to the existence of a vitamin that promoted calcium retention (Chang & Lee, 2019).

How common is low Vitamin D today?
Population surveys continue to show that suboptimal 25-hydroxyvitamin-D [25(OH)D] status is common in higher latitudes and during winter. In Germany’s DEGS1 survey (2008–2011), serum 25(OH)D levels <50 nmol/L remained frequent among adults, particularly in winter and spring; modifiable correlates included higher BMI and lower physical activity (Rabenberg et al., 2015). On a global scale, a recent pooled meta-analysis of ~7.9 million participants estimated a substantial prevalence of deficiency, with notable regional and seasonal variation (Cui et al., 2023). Against this background, a small, steady supplemental dose is a pragmatic hedge for many.

What outcomes actually improve with supplementation?
A broad 2022 synthesis concluded that vitamin D’s most reliable benefits remain musculoskeletal (Bouillon et al., 2022). Two more focused, recent analyses are especially useful for practical dosing:

Fractures and falls: A dose-by-regimen meta-analysis (32 RCTs) found that daily 800–1,000 IU was the most probable strategy to reduce osteoporotic fractures (RR≈0.87) and falls (RR≈0.91); lower or higher daily doses and intermittent (e.g., monthly) boluses did not show benefit (Kong et al., 2022). In contrast, the large VITAL trial (25,871 generally healthy adults, 2,000 IU/d) showed no fracture reduction overall—unsurprising because participants were not selected for deficiency and event rates were low (LeBoff et al., 2022). Read together, the message is not that vitamin D “does nothing”, but that context and regimen matter: modest daily dosing, particularly in people at risk of low status, is where signal appears most consistently.

Respiratory infections: An updated stratified meta-analysis combining 61,589 participants found that, overall, vitamin D did not significantly reduce the risk of acute respiratory infections (OR 0.94; 95% CI 0.88–1.00), and pre-specified subgroup analyses did not show clear effect modification by baseline status or dosing frequency (Jolliffe et al., 2025). Earlier positive signals attenuated as newer trials accumulated. Bottom line: don’t take vitamin D primarily for colds/flu—take it for skeletal health and as an insurance against low status.

Daily vs. bolus dosing: why “little and often” wins
Physiology suggests that stable, daily cholecalciferol better mirrors endogenous production (Adams & Hewison, 2010; Dusso et al., 2005). Trials agree: daily dosing tracks with benefit (Kong et al., 2022), whereas high bolus regimens can be counterproductive. In older women, annual 500,000 IU increased falls and fractures vs. placebo (Sanders et al., 2010). A monthly high-dose trial likewise found more falls at 60,000 IU/month than at 24,000 IU/month (Bischoff-Ferrari et al., 2016). For everyday prevention, skip megadoses.

So—should you take Vitamin D?
A reasonable, evidence-conform recommendation for most adults is a dose of 800–1,200 IU (20–30 μg) vitamin D₃ daily, taken consistently year-round. This pragmatic baseline is particularly relevant where deficiency is common, while specific groups—children and adolescents (for rickets prevention), adults aged ≥75 years (mortality signal), pregnant people (reduced obstetric risks), and adults with high-risk prediabetes—are explicitly supported for routine supplementation without routine testing by the 2024 Endocrine Society guideline (Demay et al., 2024). For generally healthy adults under 75, routine measurement of 25(OH)D is not recommended; instead, supplementation may proceed empirically unless there is a clear clinical indication to test (Demay et al., 2024). As a formulation, cholecalciferol (vitamin D₃) remains standard and is best taken with a meal containing fat to optimise absorption. Parallel calcium intake should come primarily from diet; high-dose calcium tablets should not be added automatically, but reserved for cases with a demonstrable need.

Safety, upper limits, and when to see a clinician
Vitamin D has a wide safety margin at physiological doses. The European Food Safety Authority sets the tolerable upper intake level (UL) for adolescents and adults (including pregnancy) at 100 μg/day (4,000 IU) (EFSA NDA Panel, 2023). Toxicity (hypercalcaemia, nephrolithiasis) is rare and typically linked to sustained intakes far above the UL or to erroneous manufacturing/dosing. Seek medical advice and targeted testing if you have malabsorption, chronic kidney or liver disease, granulomatous disease, are on certain medications (e.g., anticonvulsants, glucocorticoids), or if symptoms/signs suggest deranged calcium metabolism.

The balanced takeaway

1. Evidence is strongest for small, daily doses supporting musculoskeletal health, particularly where deficiency risk is non-trivial.
2. Intermittent high-dose strategies are not superior and can be harmful in older adults.
3. With sunlight, diet, season, latitude and lifestyle all pushing 25(OH)D up and down, a daily 800–1,200 IU D₃ is a simple, science-aligned default for most people—no drama, no megadoses, no routine lab tests required.

References:

• Adams, J. S., & Hewison, M. (2010). Update in vitamin D. The Journal of Clinical Endocrinology & Metabolism, 95(2), 471–478. https://doi.org/10.1210/jc.2009-1773
• Bischoff-Ferrari, H. A., Dawson-Hughes, B., Orav, E. J., Staehelin, H. B., Meyer, O. W., Theiler, R., Dick, W., Willett, W. C., & Egli, A. (2016). Monthly high-dose vitamin D treatment for the prevention of functional decline: A randomized clinical trial. JAMA Internal Medicine, 176(2), 175–183. https://doi.org/10.1001/jamainternmed.2015.7148
• Bouillon, R., Manousaki, D., Rosen, C., Trajanoska, K., Rivadeneira, F., & Richards, J. B. (2022). The health effects of vitamin D supplementation: evidence from human studies. Nature Reviews Endocrinology, 18(2), 96–110. https://doi.org/10.1038/s41574-021-00593-z
• Chang, S. W., & Lee, H. C. (2019). Vitamin D and health – The missing vitamin in humans. Pediatrics and Neonatology, 60(3), 237–244. https://doi.org/10.1016/j.pedneo.2019.04.007
• Cui, A., Zhang, T., Xiao, P., Fan, Z., Wang, H., & Zhuang, Y. (2023). Global and regional prevalence of vitamin D deficiency in population-based studies from 2000 to 2022: A pooled analysis of 7.9 million participants. Frontiers in Nutrition, 10, 1070808. https://doi.org/10.3389/fnut.2023.1070808
• Demay, M. B., et al. (2024). Vitamin D for the prevention of disease: An Endocrine Society Clinical Practice Guideline. The Journal of Clinical Endocrinology & Metabolism, 109(8), 1907–1947. https://doi.org/10.1210/clinem/dgae290
• Dusso, A. S., Brown, A. J., & Slatopolsky, E. (2005). Vitamin D. American Journal of Physiology – Renal Physiology, 289(1), F8–F28. https://doi.org/10.1152/ajprenal.00336.2004
• EFSA NDA Panel (EFSA Panel on Nutrition, Novel Foods and Food Allergens). (2023). Scientific opinion on the tolerable upper intake level for vitamin D, including a conversion factor for calcidiol monohydrate. EFSA Journal, 21(8), e08145. https://doi.org/10.2903/j.efsa.2023.8145
• Jolliffe, D. A., et al. (2025). Vitamin D supplementation to prevent acute respiratory infections: Systematic review and meta-analysis of stratified aggregate data. The Lancet Diabetes & Endocrinology, 13(4), 307–320. https://doi.org/10.1016/S2213-8587(24)00348-6
• Kong, S. H., Jang, H. N., Kim, J. H., Kim, S. W., & Shin, C. S. (2022). Effect of vitamin D supplementation on risk of fractures and falls according to dosage and interval: A meta-analysis. Endocrinology and Metabolism (Seoul), 37(2), 344–358. https://doi.org/10.3803/EnM.2021.1374
• LeBoff, M. S., Chou, S. H., Ratliff, K. A., Cook, N. R., Khurana, B., Kim, E., Cawthon, P. M., Bauer, D. C., Black, D., Gallagher, J. C., Lee, I.-M., Buring, J. E., & Manson, J. E. (2022). Supplemental vitamin D and incident fractures in midlife and older adults. The New England Journal of Medicine, 387(4), 299–309. https://doi.org/10.1056/NEJMoa2202106
• O'Riordan, J. L., & Bijvoet, O. L. (2014). Rickets before the discovery of vitamin D. BoneKEy Reports, 3, 478. https://doi.org/10.1038/bonekey.2013.212
• Rabenberg, M., Scheidt-Nave, C., Busch, M. A., Rieckmann, N., Hintzpeter, B., & Mensink, G. B. M. (2015). Vitamin D status among adults in Germany – results from the German Health Interview and Examination Survey for Adults (DEGS1). BMC Public Health, 15, 641. https://doi.org/10.1186/s12889-015-2016-7
• Sanders, K. M., Stuart, A. L., Williamson, E. J., Simpson, J. A., Kotowicz, M. A., Young, D., & Nicholson, G. C. (2010). Annual high-dose oral vitamin D and falls and fractures in older women: A randomized controlled trial. JAMA, 303(18), 1815–1822. https://doi.org/10.1001/jama.2010.594