There’s been so much hype around vitamin D—and, unfortunately, most studies have been disappointing when it comes to real-world benefits.
But a brand-new study points to a place where vitamin D seems genuinely helpful. Researchers found it cut sick days by nearly 20 %. They also discovered that we have to get the dose right. So in this article I’ll explain the study and the current guidelines for the proper dose.
Table of Contents
Background
Most people know vitamin D is linked to bone health, but it’s also a key regulator of immune function, muscle activity, cell growth, and sugar metabolism. In fact, most tissues in the body respond to vitamin D in some way.
Because of this, researchers started exploring how low vitamin D might relate to a host of health problems. Once we looked, we found associations everywhere. Early observational studies—where researchers simply watch what happens in the real world—showed that low vitamin D levels correlated with cancer, infectious diseases, autoimmune conditions, diabetes, and heart disease [1].
At the same time, experts were warning that many people were deficient in this crucial nutrient. A 2006 paper reported that a third of healthy young adults and more than half of general-medicine in-patients had inadequate vitamin D [2].
That evidence prompted the Endocrine Society to release influential guidelines in 2011. They defined deficiency (serum 25-hydroxy-vitamin D < 20 ng/mL), recommended broad testing for at-risk groups, and suggested daily intakes up to 2,000 IU for some people [3].
Demand for supplements exploded. The global vitamin D market is projected to reach USD 1.5 billion by 2034 [4].
1. When randomized trials arrived, enthusiasm cooled
Once randomized controlled trials started testing vitamin D directly, the benefits looked far smaller than hoped.
Cardiovascular disease and cancer. The five-year VITAL trial of 25,000 older adults found that 2,000 IU/day did not reduce heart-attack rates or cancer incidence [5]. An Australian trial of 20,000 adults came to the same conclusion about cancer [6].
All-cause mortality. A 2014 Cochrane review hinted at a modest (≈3 %) mortality reduction [7], but a 2020 meta-analysis covering 21 trials and more than 83,000 participants found no mortality benefit at all [8].
As evidence accumulated, the Endocrine Society issued far more cautious guidelines in 2024 [9].
2. Kids may be an exception
Despite the mixed adult data, two paediatric trials suggested vitamin D could reduce respiratory infections.
- China: In 400 infants, a high dose (1,200 IU/day) cut influenza A cases almost in half compared with 400 IU/day [10].
- Canada: In ~700 children aged 1–5 years, 2,000 IU/day did not outperform 400 IU/day in preventing upper-respiratory infections [11].
Why the contradictory outcomes? A new Danish study helps answer that question.
The new study
Researchers in Denmark enrolled 189 healthy children (6–8 years old) just before winter. Half received a placebo, and half received 25 µg (1,000 IU) of cholecalciferol daily for six months [12].
What happened?
- 17 % fewer sick days. Supplemented children missed fewer school days due to respiratory infections (risk ratio 0.83).
- 43 % fewer fever-days. Days with both infection and fever were almost halved (risk ratio 0.57).
3. Why did 1,000 IU help when 2,000 IU did not in Canada?
The Danish authors point to baseline vitamin D status. In northern countries, serum levels tend to drop during winter. At study start (late summer), all Danish children had adequate vitamin D; by late winter almost half of the placebo group fell below 50 nmol/L, but none of the supplemented kids did [12].
In the Canadian study, even the 400 IU group maintained levels around 92 nmol/L throughout winter [11]—high enough that extra vitamin D added nothing.
Supporting this view, the same Danish team had earlier shown that 800 IU/day (but not 400 IU) maintained innate-immune markers during winter [13]. Knowing children would occasionally skip doses, they chose 1,000 IU to give an effective average close to 800 IU.
Endocrine Society guidelines (2024 update)
After reviewing hundreds of trials, the panel now recommends targeted supplementation rather than population-wide megadosing [9, 14].
1. Children and adolescents (1–18 years). Empiric vitamin D helps prevent nutritional rickets and may lower respiratory-infection risk.
2. Pregnancy. About 3,000 IU/day can reduce pre-eclampsia, preterm birth, and neonatal mortality.
3. High-risk prediabetes. Roughly 3,500 IU/day, alongside lifestyle changes, lowers progression to diabetes.
4. Adults ≥ 75 years. Regular supplementation has the potential to reduce overall mortality.
For most younger and middle-aged adults, the guideline recommends the Institute of Medicine’s RDA—600 IU/day (rising to 800 IU after age 50). That is far lower than the 1,500–2,000 IU figure promoted in 2011 [3].
Should we supplement?
4. More is not always better
A Canadian three-year trial randomized healthy adults to 400 IU, 4,000 IU, or 10,000 IU daily. Wrist bone-mineral density declined by 2.4 % in the 4,000 IU group and 3.5 % in the 10,000 IU group, compared with just 1.2 % in the 400 IU group [15].
5. Practical takeaways
- Kids in dark winters: 800–1,000 IU/day helps maintain immunity and cut sick days.
- Pregnancy, prediabetes: Higher doses (1,000–3,500 IU) are recommended by the Endocrine Society
- Typical healthy adults: 600–800 IU/day is usually enough.
- Watch co-nutrients: Magnesium and vitamin K modulate vitamin D metabolism and calcium balance—hence their inclusion, along with 1,000 IU of vitamin D, in my MicroVitamin formula.
That’s why I reduced the vitamin D in MicroVitamin from 2,000 IU to 1,000 IU when the new guidelines appeared. The 600-to-800 IU range safeguards against deficiency while avoiding the downsides of excess. And just because I take a supplement doesn’t mean you have to; use these data—plus context such as diet, latitude, and sun exposure—to decide.
Reference List
1. https://academic.oup.com/jcem/article/109/8/1961/7686350
2. https://pubmed.ncbi.nlm.nih.gov/16529140/
3. https://academic.oup.com/jcem/article/96/7/1911/2833671
4. https://www.precedenceresearch.com/vitamin-d-market
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC7089819
6. https://pubmed.ncbi.nlm.nih.gov/40096917/
7. https://pubmed.ncbi.nlm.nih.gov/24414552
8. https://pubmed.ncbi.nlm.nih.gov/31215980/
9. https://www.endocrine.org/clinical-practice-guidelines/vitamin-d-for-prevention-of-disease
10. https://journals.lww.com/pidj/fulltext/2018/08000/preventive_effects_of_vitamin_d_on_seasonal.5.aspx
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC5817430/
12. https://link.springer.com/article/10.1007/s00394-025-03674-1
13. https://link.springer.com/article/10.1007/s00394-018-1671-8
14. https://academic.oup.com/jcem/article/109/8/1907/7685305
