Cutoff levels of serum 25-OH-D for defining vitamin D deficiency in early infancy.

Background: Various organizations use different thresholds to define vitamin D deficiency (VDD), which complicates its diagnosis, particularly in infants, as these thresholds are often extrapolated from adult data. Routine vitamin D3 supplementation in infants based on these criteria may impose unnecessary economic costs without clear benefits. Recent evidence suggests that the Institute of Medicine's (IOM) current threshold of <12 ng/ml might overestimate VDD in infants.

Method: This study is a secondary analysis of data derived from a randomized controlled trial that compared the effects of 800 IU/day versus 400 IU/day of oral vitamin D3 supplementation in healthy, term, breastfed infants from 48 hours after birth to 14 weeks of age. The primary goal was to determine the serum 25-hydroxyvitamin D (25-OH-D) inflection point at which parathyroid hormone (PTH) levels begin to rise, in order to establish a more accurate cutoff for VDD in early infancy.

Result: Among the 99 infants analyzed, 16 (16.2%) exhibited both elevated PTH levels and VDD at 14 weeks. The optimal inflection point for serum 25-OH-D was identified as 6.83 ng/ml (95% CI, 5.02-8.64), with an adjusted R² of 39.4% (p < 0.001). Using a revised cutoff of 8.64 ng/ml based on the 95% CI, only 10 (10.1%) infants would be classified as VDD, compared to 16 (16.2%) (p = 0.18) under the current threshold.

Conclusion: The lower inflection point of 6.83 ng/ml and a revised cutoff of 8.64 ng/ml, supported by limited existing literature, suggest the need to reassess the IOM's current threshold for defining VDD in infants.