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Table 4 Threshold effect analysis of serum calcium and 25(OH)D on total bone mineral density by using two-piecewise linear regression

From: Associations between serum calcium, 25(OH)D level and bone mineral density in adolescents

  Adjusted ß (95% CI), p-value
Serum calcium
 Total
  Fitting by standard linear model 0.0084 (0.0007, 0.0160) 0.0317
  Fitting by two-piecewise linear model  
  Inflection point 9.6
  Serum calcium < 9.6 (mg/dL) 0.0348 (0.0165, 0.0530) 0.0002
  Serum calcium > 9.6 (mg/dL) −0.0051 (− 0.0164, 0.0063) 0.3824
  Log likelihood ratio 0.002
 Boy
  Fitting by standard linear model 0.0034 (−0.0076, 0.0143) 0.5446
  Fitting by two-piecewise linear model  
  Inflection point 9.6
  Serum calcium < 9.6 (mg/dL) 0.0447 (0.0128, 0.0765) 0.0060
  Serum calcium > 9.6 (mg/dL) −0.0109 (− 0.0260, 0.0042) 0.1564
  Log likelihood ratio 0.007
 White
  Fitting by standard linear model 0.0129 (−0.0017, 0.0276) 0.0843
  Fitting by two-piecewise linear model  
  Inflection point 9.6
  Serum calcium < 9.6 (mg/dL) 0.0465 (0.0091, 0.0839) 0.0148
  Serum calcium > 9.6 (mg/dL) −0.0025 (− 0.0240, 0.0191) 0.8218
  Log likelihood ratio 0.055
 Black
  Fitting by standard linear model 0.0053 (−0.0080, 0.0187) 0.4353
  Fitting by two-piecewise linear model  
  Inflection point 9.2
  Serum calcium < 9.2 (mg/dL) 0.1074 (0.0200, 0.1949) 0.0161
  Serum calcium > 9.2 (mg/dL) −0.0011 (− 0.0155, 0.0133) 0.8780
  Log likelihood ratio 0.020
Serum 25(OH)D
 Boy
  Fitting by standard linear model 0.0013 (0.0009, 0.0018) < 0.0001
  Fitting by two-piecewise linear model  
  Inflection point 24
  Serum 25(OH)D < 24(ng/mL) 0.0024 (0.0015, 0.0034) < 0.0001
  Serum 25(OH)D > 24(ng/mL) 0.0007 (0.0000, 0.0014) 0.0400
  Log likelihood ratio 0.008
 Girl
  Fitting by standard linear model 0.0003 (−0.0000, 0.0007) 0.0565
  Fitting by two-piecewise linear model  
  Inflection point 26
  Serum 25(OH)D < 26(ng/mL) 0.0011 (0.0005, 0.0018) 0.0006
  Serum 25(OH)D > 26(ng/mL) −0.0003 (− 0.0008, 0.0003) 0.3218
  Log likelihood ratio 0.004
 White
  Fitting by standard linear model 0.0003 (−0.0002, 0.0009) 0.1945
  Fitting by two-piecewise linear model  
  Inflection point 23
  Serum 25(OH)D < 23(ng/mL) 0.0035 (0.0018, 0.0052) < 0.0001
  Serum 25(OH)D > 23(ng/mL) −0.0005 (− 0.0011, 0.0002) 0.1629
  Log likelihood ratio < 0.001
  1. age, gender, race/ethnicity, body mass index, income to poverty ratio, physical activity, and calcium supplementation use were adjusted
  2. In the analysis for boy/girl, white/black, the model is not adjusted for gender or race/ethnicity, respectively