The review of seven randomised controlled clinical trials weighed up "inconsistent" findings on the effect of vitamin D supplementation on the prevention of childhood acute respiratory infections (ARI).
Published in the British Journal of Nutrition, it found overall there was not a statistically significant reduction in the risk of ARI, all-cause mortality or the rate of hospital admission due to respiratory infection in healthy children.
However, in children previously diagnosed with asthma, vitamin D supplementation resulted in a 74% reduction in the risk of asthma exacerbation.
"Our findings indicate a lack of evidence supporting the routine use of vitamin D supplementation for the prevention of ARI in healthy children; however, they suggest that such supplementation may benefit children previously diagnosed with asthma."
According the World Health Organisation (WHO) about 235 million people suffer from asthma, which is particularly common in children. It cited asthma as one of
the major non-communicable diseases facing the world today.
The condition means air passages of the lungs become inflamed and narrowed.
The causes of asthma are not completely understood but it is thought to be a combination of genetic predisposition and environmental exposure to irritants like allergens, tobacco smoke and air pollution.
ARI refers to the infection of the sinuses, throat, airways or lungs usually caused by viruses or bacteria. They can be particularly dangerous for people with asthma. According to a 2013 paper, an estimated 11.9 million episodes of severe ARI and three million episodes of very severe ARI in young children resulted in hospital admissions in 2010 globally.
Meanwhile a separate paper from the Child Health Epidemiology Reference Group of WHO and UNICEF found almost two-thirds of the 7.6 million children worldwide who died within the first five years of life died of infectious diseases. Within these two-thirds, pneumonia was the leading cause for a total of 1.396 million deaths.
WHO has said in the past that further research on vitamin D supplementation and the possible decrease in frequency and severity of respiratory infections in children was needed before specific recommendations could be made.
The Chinese researchers wrote in their review: "Although vitamin D is widely recognised for its importance in calcium metabolism and bone health, researchers have spent several years focusing on its growing number of possible non-calcaemic health effects.
"One of the more promising areas of study is the relationship between vitamin D status and respiratory infection. Recent research has indicated that vitamin D may play a role in protecting against ARI by increasing the body’s production of naturally acting antibiotics."
The review was conducted by researchers at the Anhui Medical University, Shaoxing Center for Disease Control and Prevention, Huzhou Center for Disease Control and Prevention, Anhui Institute of Schistosomiasis Control and the Anhui Provincial Family Planning Institute of Science and Technology in China.
It included trials in Japan, Afghanistan, India, Poland and Mongolia that compared vitamin D supplementation with either placebo or no intervention in children younger than 18 years of age.
Source: British Journal of Nutrition
Published online ahead of print, doi:10.1017/S000711451500207X
"Vitamin D supplementation for the prevention of childhood acute respiratory infections: a systematic review of randomised controlled trials"
Authors: L. Xiao, C. Xing, Z. Yang, S. Xu, M. Wang, H. Du, K. Liu and Z. Huang