The study was part of the D-Pro trial, which assessed 20 micrograms (μg) vitamin D3, along with dairy protein, on children’s health and growth. The actual amount in the Minisun-branded vitamin D supplement was analyzed as 25μg, and the protein aspect was reported as a fixed factor with no effects on outcomes.
“In conclusion, a daily supplement of 25 μg vitamin D3 (target dose: 20 μg) maintained s-25(OH)D sufficiency during extended winter and showed fewer sick days with ARTI in otherwise healthy, Danish 6–8-year-old children,” wrote researchers from the University of Copenhagen and Aarhus University in Denmark.
“The results support the importance of maintaining vitamin D sufficiency, especially during winter at northern latitudes, where respiratory illnesses are common, as fewer sick days may have a large impact on both children, caregivers, and societal economy,” they added.
Vitamin D’s immunomodulation in children
Vitamin D may help reduce symptoms of ARTI due to its immunomodulatory effects. It plays a key role in innate immunity, reducing the production and expression of pro-inflammatory cytokines and inducing antimicrobial activity.
The study noted that circulating vitamin D may stimulate the production of antimicrobial peptides such as cathelicidin.
Previous research indicates that cathelicidins in the respiratory tissue linings are expressed in early life and help protect against inflammation and respiratory infections in children. Other studies explain that cathelicidin destabilizes the microbial membranes of invasive bacteria and fungi, effectively destroying them.
Study details
As part of the larger D-Pro trial, the researchers included the intervention of 260 g of daily dairy as a “fixed factor”.
The vitamin D part of the trial randomized 200 healthy, six- to eight-year-old children to receive 20 micrograms a day (μg/d) of vitamin D3 or placebo for 24 weeks. The researchers noted that this dosage was previously found to produce peptides involved in defense against infections.
However, when analyzing the Minisun intervention tablets provided by Oy Verman AB, Finland, using liquid chromatography-tandem mass spectrometry, they found “an actual dose of 25 μg vitamin D per tablet.
Only children with white skin were eligible for the study, as children with darker skin had already been recommended to supplement vitamin D.
The researchers collected data using blood samples, anthropometric measurements and ARTI questionnaires completed by caregivers. Baseline visits were conducted from August to October and endpoint visits between February and April of the following year.
Results revealed that children taking the vitamin D supplement had 17% fewer sick days with at least one respiratory symptom compared to placebo, and 43% risk reduction of cases, with fever in the extended winter.
“In contrast, there were no effects of vitamin D on number of episodes, symptom severity or use of medicine,” the researchers wrote, calling for further research to resolve the limitations in their study.
Source: European Journal of Nutrition 64, 170 (2025). doi: 10.1007/s00394-025-03674-1. “Effects of vitamin D supplementation on acute respiratory tract infections in 6–8-year-old children: a randomized clinical trial”. Authors: J.W. Clerico et al.