Optimal vitamin D status for Chinese infants in Hong Kong: insights from the relationship between serum 25-hydroxyvitamin D and parathyroid hormone levels.

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This study aimed to identify the 25-hydroxyvitamin D(25OHD) threshold that maximally suppressed parathyroid hormone (PTH) in a group of healthy Chinese Infants in Hong Kong. Healthy infants detected to have low serum 25OHD less than 25 nmol/L in a population study on vitamin D status were referred to Hong Kong Children's Hospital (HKCH) for further management. Their total 25OHD was repeated with serum calcium, phosphate, alkaline phosphatase and PTH. Three-phase segmented regression was used to identify the optimal breakpoint between 25OHD and PTH. Two hundred and twelve infants were included (59 % male). They were reassessed at a median age of 156days (IQR: 111-247days). Using unadjusted three-phase segmented regression, the estimated breakpoint of 25OHD on PTH suppression, after adjusting for factors including age, gender, history of vitamin D supplement and mode of feeding, was 20.0 nmol/L (95 % CI: 13.1 to 26.9). The threshold of 25OHD that triggered the inflection point for PTH in our Hong Kong Chinese infants was lower than that reported in the Western literature. This might imply the cutoff for vitamin D deficiency is lower for Chinese infants. This could be explained by younger age and different ethnicity. Further study with larger sample size is needed to validate the observation.