Table 1. Distribution of anthropometric and behavioral characteristics at 13 and 17 years of age according to bone mineral density (BMD) z-scores at 13 and 17 years of age.
| BMD at 13 years of age | BMD at 17 years of age | |||||
| z-score≥−1† | z-score<−1 | p | z-score≥−1† | z-score<−1 | p | |
| N (%) | 606 (82.9) | 125 (17.1) | 599 (81.9) | 132 (18.1) | ||
| Mean (SD) menarche age (years) | 12.1 (1.3) | 13.2 (1.1) | <0.001 | 12.2 (1.3) | 12.8 (1.3) | <0.001 |
| Mean (SD) height at 13 (cm) | 168.6 (6.2) | 155.2 (7.8) | <0.001 | 158.4(6.4) | 156.3 (7.0) | <0.001 |
| Mean (SD) height at 17 (cm) | 161.1 (6.1) | 160.6 (7.0) | 0.392 | 161.1 (6.3) | 160.6 (6.0) | 0.440 |
| Mean (SD) weight at 13 (kg) | 54.0 (9.4) | 44.5 (7.5) | <0.001 | 53.5 (9.5) | 47.0 (9.2) | <0.001 |
| Mean (SD) weight at 17 (kg) | 58.0 (9.2) | 51.7 (7.3) | <0.001 | 58.1 (9.1) | 51.9 (7.9) | <0.001 |
| Mean (SD) body mass index at 13 (kg/m2) | 21.4 (3.3) | 18.4 (2.4) | <0.001 | 21.3 (3.3) | 19.1 (3.1) | <0.001 |
| Mean (SD) body mass index at 17 (kg/m2) | 22.2 (3.7) | 20.0 (2.3) | <0.001 | 22.2 (3.5) | 19.9 (3.2) | <0.001 |
| Mean (SD) fat-free mass at 13 (kg) | 38.6 (3.9) | 35.4 (3.9) | <0.001 | 38.6 (3.9) | 35.8 (4.1) | <0.001 |
| Mean (SD) fat-free mass at 17 (kg) | 42.8 (3.7) | 40.4 (3.7) | <0.001 | 42.9 (3.7) | 40.3 (3.6) | <0.001 |
| Mean (SD) calcium intake at 13 (mg/day) | 1146.6 (464.0) | 1127.5 (444.6) | 0.707 | 1157.6 (467.4) | 1073.7 (420.6) | 0.095 |
| Mean (SD) vitamin D intake at 13 (µg/day) | 4.5 (2.6) | 4.5 (2.4) | 0.881 | 4.6 (2.6) | 4.2 (2.4) | 0.183 |
| Number (%) using calcium/vit D supplements at 13 | 3 (0.5) | 1 (0.8) | 0.528 | 4 (0.7) | 0 (0.0) | >0.999 |
| Number (%) using calcium/vit D supplements at 17 | ‡ | ‡ | ‡ | 3 (0.5) | 1 (0.8) | 0.550 |
| Number (%) reporting oral contraceptive at 17 | ‡ | ‡ | ‡ | 241 (40.2) | 47 (35.6) | 0.376 |
| Number (%) practicing sports ≥2 times/week at 13 | 265 (44.5) | 56 (47.1) | 0.603 | 269 (45.8) | 52 (40.6) | 0.284 |
| Number (%) practicing sports ≥2 times/week at 17 | ‡ | ‡ | ‡ | 252 (43.7) | 45 (35.2) | 0.077 |
| Mean (SD) parental education (schooling years) | 10.7 (4.4) | 10.5 (5.0) | 0.644 | 10.6 (4.4) | 11.1 (4.9) | 0.234 |
| Number (%) in behavioral clusters* | 0.797 | 0.651 | ||||
| Cluster 1 | 210 (39.9) | 43 (43.5) | 215 (41.5) | 40 (36.6) | ||
| Cluster 2 | 301 (57.1) | 53 (53.5) | 288 (55.7) | 66 (60.6) | ||
| Cluster 3 | 16 (3.0) | 3 (3.0) | 16 (3.1) | 3 (2.8) | ||
P-values were calculated using chi-square of Fisher's exact tests, as appropriate (for the comparisons between proportions), or using Student's t-test (for the comparisons of means).
Behavioral clusters defined using the following variables: sports activities, fruit intake, sleeping hours, time spent in sedentary activities, tobacco and alcohol use [23]. Adolescents were classified in one of three following groups according to behavioral aggregation: healthiest (cluster 1), intermediate (cluster 2) and least healthy (cluster 3).
Z-score −1.0 cutoff corresponded to 0.303 g/cm2 at 13 and 0.386 g/cm2 at 17 years of age.
Estimates are not presented because exposures are behaviors which took place in late adolescence while the outcome (BMD) refers to early adolescence.