Fat mass (FM) and fat free mass (FFM) may influence bone health differentially. However, existing evidences on associations between FM, FFM and bone health are inconsistent and vary according to sex and maturity. The present study aims to evaluate longitudinal associations between FM, FFM and bone stiffness index (SI) among European children and adolescents with 6 years follow-up. A sample of 2468 children from the IDEFICS/I.Family was included, with repeated measurements of SI using calcaneal quantitative ultrasound, body composition using skinfold thickness, sedentary behaviors and physical activity using self-administrated questionnaires. Regression coefficients (β) and 99%-confidence intervals (99% CI) were calculated by sex-specified generalized linear mixed effects models to analyze the longitudinal associations between FM and FFM z-scores (zFM and zFFM) and SI percentiles, and to explore the possible interactions between zFM, zFFM and maturity. Baseline zFFM was observed to predict the change in SI percentiles in both boys (β = 4.57, 99% CI: 1.36, 7.78) and girls (β = 3.42, 99% CI: 0.05, 6.79) after 2 years. Moreover, baseline zFFM (β = 8.72, 99% CI: 3.18, 14.27 in boys and β = 5.89, 99% CI: 0.34, 11.44 in girls) and the change in zFFM (β = 6.58, 99% CI: 0.83, 12.34 in boys and β = 4.81, 99% CI: -0.41, 10.02 in girls) were positively associated with the change in SI percentiles after 6 years. In contrast, a negative association was observed between the change in zFM and SI percentiles in boys after 6 years (β = -3.70, 99% CI: -6.99, -0.42). Besides, an interaction was observed between the change in zFM and menarche on the change in SI percentiles in girls at 6 years follow-up (p = .009), suggesting a negative association before menarche while a positive association after menarche. Our findings support the existing evidences for a positive relationship between FFM and SI during growth. Furthermore, long-term FM gain was inversely associated with SI in boys, whereas opposing associations were observed across menarche in girls.

中文翻译：

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欧洲儿童和青少年身体成分与骨硬度指数纵向关联的性别差异

脂肪量 (FM) 和无脂肪量 (FFM) 可能对骨骼健康产生不同的影响。然而，关于 FM、FFM 和骨骼健康之间关联的现有证据并不一致，并且因性别和成熟度而异。本研究旨在评估 6 年随访的欧洲儿童和青少年 FM、FFM 和骨硬度指数 (SI) 之间的纵向关联。包括来自 IDEFICS/I.Family 的 2468 名儿童样本，使用跟骨定量超声重复测量 SI，使用皮褶厚度测量身体成分，使用自我管理的问卷调查久坐行为和身体活动。回归系数 (β) 和 99% 置信区间 (99% CI) 由性别指定的广义线性混合效应模型计算，以分析 FM 和 FFM z 分数（zFM 和 zFFM）与 SI 百分位数之间的纵向关联，并探索 zFM、zFFM 和成熟度之间可能的相互作用。观察到基线 zFFM 可预测 2 年后男孩 (β = 4.57, 99% CI: 1.36, 7.78) 和女孩 (β = 3.42, 99% CI: 0.05, 6.79) SI 百分位数的变化。此外，基线 zFFM（β = 8.72, 99% CI: 3.18, 14.27 在男孩和 β = 5.89, 99% CI: 0.34, 11.44 在女孩）和 zFFM 的变化（β = 6.58, 99% CI: 0.834, 12在男孩中，β = 4.81，99% CI：-0.41，在女孩中为 10.02）与 6 年后 SI 百分位数的变化呈正相关。相比之下，在 6 年后男孩的 zFM 和 SI 百分位数的变化之间观察到负相关（β = -3.70，99% CI：-6.99，-0.42）。此外，在 6 年随访时观察到 zFM 变化和初潮之间的相互作用对女孩 SI 百分位数的变化（p = .009），表明初潮前呈负相关，而初潮后呈正相关。我们的研究结果支持现有证据表明 FFM 和 SI 在生长过程中存在正相关关系。此外，长期 FM 增益与男孩的 SI 呈负相关，而在女孩的初潮中观察到相反的关联。暗示初潮前呈负相关，而初潮后呈正相关。我们的研究结果支持现有证据表明 FFM 和 SI 在生长过程中存在正相关关系。此外，长期 FM 增益与男孩的 SI 呈负相关，而在女孩的初潮中观察到相反的关联。暗示初潮前呈负相关，而初潮后呈正相关。我们的研究结果支持现有证据表明 FFM 和 SI 在生长过程中存在正相关关系。此外，长期 FM 增益与男孩的 SI 呈负相关，而在女孩的初潮中观察到相反的关联。