Written by Alexa Heathorn, MS, CNS-c. Daily supplementation of 20 μg of vitamin D3 during extended winter effectively reduced the number of sick days from ARTI and fever in children aged 6-8 years old.

Acute respiratory tract infections (ARTI) are among the leading causes of illness worldwide¹. Children are estimated to experience this viral infection 6-10 times a year¹. While this ARTI is typically mild in manner, presenting as runny nose, cough, sneezing, headache, sore throat, fever, lethargy, myalgia, headache, and malaise, it poses a substantial economic burden as caretakers have to stay home with the child, resulting in loss of work hours and the use of healthcare services¹. A new study published in the European Journal of Nutrition (2025) sheds light on a promising strategy to reduce the impact of these infections through supplementation of Vitamin D¹.

Vitamin D, a fat-soluble vitamin synthesized in the skin upon sun exposure and found in limited foods, has been shown to exert immunoprotective effects on ARTI among young individuals^1,2.  Additionally, insufficient levels of vitamin D have been associated with increased rates of ARTI³. These recent findings indicate the importance of sufficient vitamin D status for a healthy immune defense³.

Vitamin D deficiency and insufficiency are common among white individuals in Europe and the US, with areas in high latitude and long winters experiencing an increased risk of insufficient vitamin D status¹. Despite this risk, there is no recommendation for vitamin D supplementation in white skin children residing in higher latitude areas with extended winters¹. Therefore, the impact of vitamin D supplementation on ARTI among children residing in higher latitude areas during extended winters was studied¹.

The double blinded, placebo-controlled randomized clinical trial was conducted in Copenhagen, Denmark, a northern latitude (55°N) where sunlight-driven vitamin D synthesis is minimal during extended winter months¹. The study included 200 participants (6-8 years of age) randomized to receive either 20 μg of vitamin D3 (n=99) or a placebo (n=101) over a 24-week period¹. Parents tracked respiratory symptoms using a validated questionnaire every 4 weeks.

The results were as follows after 24 weeks of supplementation:

• Children receiving 20 μg of vitamin D3 daily experienced 17% fewer sick days due to ARTI compared to the placebo group (95% CI, 10–24%; P < 0.001)
• Children receiving 20 μg of vitamin D3 daily experienced 43% fewer sick days due to ARTI with fever compared to the placebo group (95% CI, 0.48–0.67; P < 0.001)
• No other significant differences were observed in the total number of children who experienced ARTI, the use of antibiotics or painkillers, or the severity of symptoms.
• Serum 25-hydroxyvitamin D  increased by 9.4nmol/L  in the vitamin D group and decreased by 32.7 nmol/L in the placebo group.

Potential limitations of the study include:

• The study population was limited to healthy white Danish children, therefore results may not generalize to other ethnicities or children with underlying health conditions.
• The trial was powered primarily for bone health outcome, ARTI outcomes were secondary in this study.

In conclusion, daily supplementation of 20 μg of vitamin D3 during extended winter effectively reduced the number of sick days from ARTI and fever in children aged 6-8 years old¹. While more research is warranted for more diverse populations and in cases of vitamin D deficiency, this study provides compelling evidence supporting wintertime Vitamin D supplementation as a safe and accessible intervention for reducing respiratory illness in otherwise healthy children living at northern latitudes.

Source: Clerico, Julia Weeke, Line Thams, Nanna Groth Stounbjerg, Hanne Hauger, Camilla Trab Damsgaard, and Christian Mølgaard. “Effects of vitamin D supplementation on acute respiratory tract infections in 6–8-year-old children: a randomized clinical trial.” European Journal of Nutrition 64, no. 4 (2025): 1-9.

© Springer-Verlag GmbH Germany, part of Springer Nature 2025

Click here to read the full text study.

Posted June 16, 2025.

Alexa Heathorn, MS, CNS-c, is a clinical nutritionist specializing in metabolic health, hormonal balance, and gastrointestinal restoration through root-cause functional nutrition. She earned her master’s degree in Nutrition from Bastyr University and is currently a Certified Nutrition Specialist (CNS) candidate. Alexa also works as a research writer and functional health consultant, translating complex science into actionable strategies for practitioners and wellness companies. Learn more at www.bloomedwellness.com.

References:

1. Clerico JW, Thams L, Stounbjerg NG, Hauger H, Damsgaard CT, Mølgaard C. Effects of vitamin D supplementation on acute respiratory tract infections in 6-8-year-old children: a randomized clinical trial. Eur J Nutr. May 1 2025;64(4):170. doi:10.1007/s00394-025-03674-1
2. Health NIo. Vitamin D. 2025. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
3. Jolliffe DA, Camargo CA, Jr., Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. The lancet Diabetes & endocrinology. May 2021;9(5):276-292. doi:10.1016/s2213-8587(21)00051-6