The study of BMD and the variables that influence it in children and adolescents is important because in these stages of life, the accumulation of bone minerals must be optimized [2, 24]. In this study, all participants had a normal BMD (> – 2 SD) [21], reflecting good bone health despite differences in the nutritional status of the participants.

As expected, we identified significant differences in weight and BMI values between groups; and also in height, which was higher in children with obesity, as has been reported due to a higher growth velocity in the prepubertal stage, difference that decreases during puberty and that results in a similar final height between subjects with and without obesity [25].

When comparing the BMDTBLH (Z score) between the three groups, we found a progressive increase as the category of nutritional status changed from normal weight to overweight and obesity; also, the BMDL1-4 (Z score) was higher in obese children compared to those of normal weight. These findings are consistent with what was reported in a systematic review with meta-analysis, which reported evidence (of moderate quality) that overweight and obese children have higher BMD than those of normal weight [7].

It has been noted that overweight and obese children have higher BMD because their higher body weight causes an increase in mechanical load on the bone, which stimulates bone shaping [12, 15, 26, 27]. However, as obese children generate greater muscle forces during physical activity [12, 28], bone strength seems to adapt more easily to dynamic muscular forces than to the static loads imposed by greater fat mass [29, 30]. According to this concept, Wetzsteon et al. (2008) [31] reported that although overweight and obese children had greater bone strength than those of healthy weight, it was not adapted to excess body fat but to greater muscle area. Gracia-Marco et al. (2012) [27] also concluded that overweight and obese adolescents had higher bone mass as a result of higher lean mass. Therefore, it is possible that obese participants in this study had higher BMD (total and lumbar spine) because their lean mass was greater compared to those of normal weight.

Regarding the relationship between BMD and body composition, a direct correlation was observed between lean mass and BMDTBLH (g/cm2) in the three study groups. The BMD (g/cm2) of the lumbar spine had a direct relationship with the lean mass only in normal weight and overweight children. Similar to our study, several studies have reported a direct effect of lean mass on BMD in children and adolescents [11,12,13,14, 16, 32, 33].

In contrast to lean mass, the results of the relationship between body fat and BMD have been conflicting [13, 16, 17, 34]. In the present work, BMDTBLH (g/cm2) correlated directly with the total and trunk fat mass only in normal weight and overweight children, with lower values in the latter group. In the same way, the percentage of total and trunk fat also correlated directly with the BMDTBLH (g/cm2) only in children with normal weight. However, in obese children, BMDTBLH (g/cm2) was not related to fat mass variables. We can speculate that as children accumulate body fat and go from normal weight to overweight and later to obese, the influence of fat mass on BMD decreases or disappears.

In the group of obese children, an inverse relationship between fat mass and lumbar BMD was identified: the greater the total and trunk fat mass, the lower the BMDL1-4 (Z score) (p < 0.05). Similar results were reported by Mosca et al. (2014) [16], who identified an inverse correlation between the percentage of body fat and BMD of the lumbar spine in overweight and obese adolescents. Similarly, Gallego et al. (2017) [35] reported that BMD of the total body and the lumbar spine decreased as the percentage of body fat and total fat mass increased. Recently, Rokoff et al. (2019) [4] identified that BMDTBLH was directly related to trunk fat mass in children when fat values were below the 85th percentile, while the BMDTBLH decreased – 0.17 SD for each kg of increment in trunk fat mass in those who were above this value. Therefore, there is a possibility of a threshold above which central adipose tissue becomes more metabolically active and has a negative impact on bone [4].

These findings suggest that in normal weight and overweight children, a higher fat mass favors the accumulation of bone minerals; however, in children with obesity, it seems to be detrimental for the gain of adequate bone mass in the lumbar spine, compromising the future health of the skeleton in cases of persistent obesity.

The multivariate models in the total sample found that total lean mass was the only variable that explained the variability of both BMDTBLH and BMDL1-4 (g/cm2). This finding aligns with previous studies that indicate that lean mass has a greater contribution to the variability of BMD than fat mass [12,13,14]. These findings highlight the importance of promoting and maintaining muscle mass through physical activity and a healthy lifestyle to optimize bone formation and maintain skeletal health [2, 13]. However, the BMD Z score model showed that 73.4% of the variability was explained by BMI, percentage of body fat and height-for-age index, which reflects the contribution of total body mass to BMD; in addition, the inclusion of height in the model could be explained by its relationship with age and with the BMI itself since, as noted, children with obesity showed higher height values.

The limitations of our study include the cross-sectional design and the lack of information on physical activity habits. Furthermore, the values obtained by DEXA are limited to measuring BMD (g/cm2) instead of volumetric BMD; the latter is important when the skeleton is still growing and bone size could influence BMD measurements.

One of the strengths of this work is the inclusion of the measurement of total body BMD and the L1-4 region of the lumbar spine, which are the sites recommended by the International Society for Clinical Densitometry for pediatric patients [21]. In addition, the analysis of the relationship between body composition and BMD according to nutritional status allowed us to identify how it changes as adiposity increases, which is reflected in changes in BMI.

In conclusion, the BMD in obese children was higher than that in normal weight children, which is explained by their greater lean mass and not by excess body fat. Fat mass had a direct relationship with BMDTBLH in normal and overweight children; however, in the obesity group, a higher fat mass was associated with lower lumbar spine BMD. Lean mass had a direct correlation with BMDTBLH in the three study groups and was the most important predictor of BMD versus fat mass, reflecting the importance of strengthening the muscular system through physical activity and practicing a healthy lifestyle.

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