The largest clinical trial investigating vitamin D supplementation in children found that long-term treatment didn’t increase bone strength or prevent bone fractures in cases of deficiency. The study, published in Lancet Diabetes and Endocrinology, was conducted by researchers from Queen Mary University of London and the Harvard T.H. Chan School of Public Health.
 
Vitamin D has attracted attention for its potential bone health benefits for many years due to its role in bone mineralization. Most human observational and clinical studies have found that supplementation with vitamin D, alone or in combination with calcium, significantly reduces bone fracture risk in middle-aged and older subjects. Little if any research investigating these risk factors in children has been completed. 

According to the authors, around one in three children have at least one fracture before the age of 18, which can lead to disability and quality of life effects down the line.
 
The researchers worked with partners in Mongolia to recruit children for the study, due to the region having a high fracture burden and high prevalence of vitamin D deficiency. Over the course of three years, 8,851 schoolchildren between the ages of six and 13 living in Mongolia received a weekly oral dose of 14,000 IU of vitamin D or a placebo, making it the largest clinical study on vitamin D supplements ever conducted in children.
 
Nearly all (95.5%) of the participants were vitamin D deficient at baseline, and the supplements were highly effective in raising vitamin D levels above deficiency.
 
Compared to placebo, vitamin D supplementation had no effect on fracture risk, and quantitative ultrasound tests in a subset of 1,438 participants revealed no differences in bone strength between placebo and treatment groups.
 
Null Findings Emphasize Importance of Calcium
 
The authors stated that findings should prompt health professionals to reconsider the effects of vitamin D supplements on bone health, particularly in children. But the fact that calcium intake wasn’t tracked, and that calcium supplements weren’t provided, underscores the need to account for both nutrients in research, the authors and the Council for Responsible Nutrition, noted.
 
“The absence of any effect of sustained, generous vitamin D supplementation on fracture risk or bone strength in vitamin D deficient children is striking,” said Ganmaa Davaasambuu, MD, PhD, associate professor at the Harvard T.H. Chan School of Public Health. “In adults, vitamin D supplementation works best for fracture prevention when calcium is given at the same time, so the fact that we did not offer calcium alongside vitamin D to trial participants may explain the null findings from this study.”
 
“It is also important to note that children who were found to have rickets during screening for the trial were excluded from participation, as it would not have been ethical to offer them placebo. Thus, our findings only have relevance for children with low vitamin D status who have not developed bone complications,” said Professor Adrian Martineau, lead of the center for immunobiology at Queen Mary University of London. “The importance of adequate vitamin D intake for prevention of rickets should not be ignored, and UK government guidance recommending a daily intake of 400 IU vitamin D remains important and should still be followed.” 
 
“CRN agrees with comments made by the lead and senior author of the study,” the association said in a statement. “Calcium is a key nutrient for bone health that works together with vitamin D to strengthen bones. The lack of calcium supplementation in this study is a significant limitation. Additionally, vitamin D is critical in the prevention of rickets in children and the importance of vitamin D in pediatric bone health should not be dismissed.”