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

Objective: To determine whether thresholds exist for associations between25OHDlevels and bone outcomes, below which low 25OHD levels have adverse effects on bone health.

Design: Cross-sectional study.

Setting: Secondary school students in Beijing, China, aged 12-15 years.

Outcome: Measures and Participants: Serum 25OHD, bone mineral density (BMD) of total body, hipand lumbar spine (LS), serum parathyroid hormone (PTH), bone alkaline phosphatase (BAP) andtartrate-resistant acid phosphatase 5b (TRAP5b) in 222 healthy adolescents, (111 girls, 111 boys).

Results: Prevalence of low 25OHD was 61% (< 30 nmol/L) and 97% (< 50 nmol/L) (mean 25OHD = 30 nmol/L). Dietary calcium intake was low (294 and 307 mg/d for boys and girls, respectively). In girls,break-points for 25OHD (nmol/L) were: total body BMD 20 (95%CI: 14-27), hip BMD 25 (17-34), LS BMD 22 (14-0), TRAP5b 37 (22-52), and PTH 31 (23-38). In boys: total body BMD 39 (24-55), TRAP5b 33 (20-45), 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, greater 25OHD is associated with increased total body BMD, reduced PTH and TRAP5b, while above them, no such relationship exists.

Conclusions: Vitamin D deficiency and insufficiency is common in healthy Chinese adolescents. Attaining serum 25OHD levels of > 20-37 nmol/L in girls and 33-39 nmol/L 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% < 50 nmol/L.