Threshold Effects of Vitamin D Status on Bone Health in Chinese Adolescents With Low Calcium Intake

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Abstract

Context:

There is no consensus on the definition of vitamin D deficiency for bone health based on serum 25-hydroxyvitamin D (25OHD) levels.

Objective:

Our objective was to determine whether thresholds exist for associations between 25OHD levels and bone outcomes and if low 25OHD levels have adverse effects on bone health.

Design:

This is a cross-sectional study.

Participants:

This study included secondary school students in Beijing, China, aged 12–15 years.

Measures:

We measured serum 25OHD; bone mineral density (BMD) of total body, hip, and lumbar spine (LS); serum PTH; bone alkaline phosphatase (BAP); and tartrate-resistant acid phosphatase 5b (TRAP5b) in 222 healthy adolescents (111 girls, 111 boys).

Results:

The prevalence of low 25OHD was 61% (<30 nmol/liter) and 97% (<50 nmol/liter) (mean 25OHD, 30 nmol/liter). Dietary calcium intake was low (294 and 307 mg/d for boys and girls, respectively). In girls, break-points for 25OHD (nmol/liter) were: total body BMD 20 (95% confidence interval [CI], 14–27), hip BMD 25 (17–34), LS BMD 22 (14–30), TRAP5b 37 (22–52), and PTH 31 (23–38). In boys, break-points were: total body BMD 39 (24–55), TRAP5b 33 (20–45), and PTH 35 (27–43); no break-points were identified for hip and LS BMD. No break-points were identified for BAP in either gender. Below these break-points, higher 25OHD is associated with increased total body BMD, reduced PTH, and TRAP5b, whereas above these break-points, no such relationship exists.

Conclusions:

Vitamin D deficiency and insufficiency is common in healthy Chinese adolescents. Attaining serum 25OHD levels of more than 20–37 nmol/liter in girls and 33–39 nmol/liter in boys had positive influences on BMD and bone remodelling markers. However, estimates may be affected by low calcium intake and low serum 25OHD levels, with 97% of adolescents having levels below 50 nmol/liter.

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