medwireNews: Randomised trial findings suggest that the target level of dietary vitamin D intake may not be sufficient to maintain children’s circulating 25-hydroxyvitamin D (25[OH]D) concentration at a desired level.
Current advice on the best 25(OH)D level for optimal bone health is split between 20 and 30 ng/mL. But the researchers found that 39% of 42 children with baseline levels below 20 ng/mL did not achieve even this threshold after 6 months of taking daily vitamin D3 1000 IU.
“These findings suggest that currently recommended daily dietary allowances of vitamin D of 600 IU may be inadequate for preventing vitamin D deficiency in children”, say Kumaravel Rajakumar (University of Pittsburgh School of Medicine, Pennsylvania, USA) and study co-authors.
In the overall study population of 157 children, those taking vitamin D achieved a significant rise in 25(OH)D levels after 2 months of supplementation, from 19.8 to 26.4 ng/mL. By month 6, the average level was 26.7 ng/mL, but it never rose to the 30 ng/mL threshold.
After accounting for confounders including pubertal status and summertime sunlight exposure, post-treatment 25(OH)D levels were significantly higher than in children taking placebo, which were 18.9 and 22.4 ng/mL at 2 and 6 months, respectively. But on closer inspection, the significant differences were limited to the 84 Black children in the study, which the researchers attribute to their markedly lower baseline levels relative to the 73 White participants.
Vitamin D supplementation had no effect on markers of bone turnover, but the team notes that this “may not reflect effects on skeletal health as single measurements of bone turnover markers do not correlate with bone density in children and young adults.”
The plateauing of parathyroid hormone (PTH) concentrations has been used as a marker of vitamin D sufficiency in adults, but the researchers found no evidence of this in the children taking vitamin D, “thereby calling into question the appropriateness of using 25(OH)D-PTH dynamics as a surrogate indicator for defining optimal vitamin D status in children.”
They say that lack of a plateauing effect could have been due to a limited range of 25(OH)D concentrations among the children, but also observe that the association between 25(OH)D and PTH was weaker within individual children during follow-up than in the whole cohort at a single time point.
This suggests that cross-sectional associations are partly caused by unmeasured confounders and “are overly biased estimates of the causal effects of vitamin D on PTH”, they write in The Journal of Clinical Endocrinology & Metabolism.
The team concludes: “Well-designed clinical trials with longer duration of follow-up are warranted to examine the skeletal health benefits of enhancing the vitamin D status of otherwise healthy vitamin D-deficient children.”
By Eleanor McDermid, Senior medwireNews Reporter
J Clin Endocrinol Metab 2015; Advance online publication
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