The present study explores the effect of DHEAS at the age of 7 years on aBMD at the age of 10 years. Firstly, we found that aBMD at 10 years old correlated positively with DHEAS at 7 years old, after adjustment for age, sex, and BMI z-score. Secondly, using path analysis, we tried to distinguish a possible direct effect of DHEAS at age 7 on aBMD at age 10 from an indirect effect partially explained by sexual maturity or by aBMD at age 7. Although no direct effect of DHEAS at age 7 on aBMD at age 10 was observed, we found in girls, but not in boys, an indirect effect explained by sexual maturity, as higher DHEAS levels at 7 years old were associated with higher sexual maturity at 10 years old, which was further associated with higher aBMD, controlling for BMI.
To our best knowledge, this is the first study to address the longitudinal effect of DHEAS on aBMD in prepuberty and early puberty. So far, only a few cross-sectional studies have investigated the effect of circulating adrenal androgens on bone mass acquisition in mid-childhood, with mixed results, and a comparison with our findings is difficult due to different populations and methodological approaches. In accordance with our results, a positive effect of adrenal androgens on BMD was found in premenarcheal girls [47] and in two populations of children aged 5-8 years [15] and 6-18 years [32]. On the other hand, no association was found between DHEAS and bone mineral density in 255 children aged 7-8 years [31] and in a population of boys aged 6-14.5 years [26]. In a large cohort involving 472 Finnish children aged 6-8 years, the positive association of DHEAS with BMD disappeared after adjustment for fat and lean mass [33].
In our analyses, we have decided to study girls and boys separately, as we recognize the large sex differences in bone mass increase and the timing of puberty. At age 7, no statistically significant difference was found in aBMD between boys and girls, while, at the age of 10, girls presented higher aBMD than boys. Furthermore, at the age of 10, most of the girls had started puberty (78%), while 71% of the boys were still prepubescent. In girls, the effect of DHEAS at age 7 on aBMD at age 10 was partially explained by sexual maturity, as higher DHEAS at 7 years old was associated with higher Tanner stage at 10 years old, which was further associated with higher aBMD. In boys, no such effect was found, and some explanations for this sex discrepancy can be pointed out. Firstly, during puberty, girls accrue more bone mass than boys, and they do it in earlier Tanner stages [16]. Secondly, previous studies have shown that higher serum DHEAS at 7 years old is associated with earlier pubescent development in girls, but not in boys [11; 12]. Therefore, the indirect effect of DHEAS on sexual development and sexual development on aBMD is less relevant in boys than in girls, at this age.
It should be noted that the found effect of DHEAS on BMD at age 10, partially explained by sexual maturity, could also be the result of other unmeasured sexual hormones, such as estrogens. A direct effect of DHEAS on BMD, independent of estrogens, was not established.
Bone size and aBMD increase with height and weight [15; 16; 40; 41]. Hence, BMI, along with sex and the stage of puberty, was considered in our analyses. Obese and overweight prepubescent children present higher DHEAS levels [7; 9] and higher androgen levels are associated with changes in body composition, such as increased central adiposity and lean mass [19], which can affect the bone. The association between BMI and BMD in children is mostly determined by lean mass [33; 41], but adiposity also appears to play a role, despite contradictory findings [33; 48; 49]. Adiposity may augment BMD through an increased mechanical load exerted on the skeleton by fat mass [18], or the aromatization of androgens in fat [16], or through unmeasured cytokines, growth factors or other hormones (leptin, insulin and estrogens) [33], which may exert direct stimulatory effects on osteoblasts [19]. Although we had other measures of adiposity, like waist circumference or body fat, they were not included in the model due to multicollinearity.
Bone modelling and growth in childhood and early pubescent years are influenced by endogenous and exogenous factors. Exogenous factors include nutrition (mainly calcium and vitamin D) and weightbearing physical activity, while endogenous factors include hormones (growth hormone, sex steroids, and various growth factors), cytokines, and growth plate aging [50]. BMD is also affected by genetic and early growth. In a previous study involving 1853 participants from the same birth cohort, Generation XXI, weight and height velocities up to the age of 6 were associated with increased aBMD at 7 years with the strongest associations observed for growth in early childhood [51]. Moreover, in the same population, children that between zero and 4 years followed a trajectory of persistent weight gain, had clearly increased bone mass at 7 years old, and weight gain seemed slightly more beneficial when it occurred later than on a normal trajectory during the first years of life [52].
Strengths and limitations
The strengths of our study include the novelty, as previous longitudinal data on the study subject is minimal. Furthermore, we have used a population-based cohort, with detailed information regarding birth and early childhood, physical examination, anthropometry, biochemical data, and DXA evaluation, according to standardized procedures, at ages 7 and 10 years, as well as DHEAS levels in prepuberty. Consequently, our results cannot be generalized to other age groups, as they would differ because of the effect of increased growth hormone and sex steroid levels on BMD during puberty.
Nevertheless, some limitations must be acknowledged. DXA is a two-dimensional estimate of volumetric bone density, so differences in bone size may confound the androgen-BMD association assessed by this technique. Nevertheless, adjustment for BMI partially attenuates this effect. Bone age evaluation was not part of the research protocol due to radiation exposure, and therefore no conclusions regarding skeleton maturation can be drawn. It is possible that the effect of DHEAS on BMD is not fully evident at the age we have assessed, especially among boys, who start puberty later than girls. As we continue to follow this cohort, we may carry on further investigation in different age ranges.
We have used path analysis to answer our main objective, but it is worth noting that path analysis is not intended to prove causation but rather to test if observed results are consistent with a priori hypothesis. Our statistical model is necessarily oversimplified, given the complex relationships between the variables analyzed. These variables may be influenced by several genetic and environmental factors that were not measured in this study. Furthermore, it assumes that the observed relations follow a particular direction that may not be totally realistic. Thus, the observed statistical associations demand careful interpretation regarding causality.