Bone Structure and Volumetric BMD in Overweight Children: A Longitudinal Study

Abstract

ABSTRACT:The effect of excess body fat on bone strength accrual is not well understood. Therefore, we assessed bone measures in healthy weight (HW) and overweight (OW) children. Children (9-11 yr) were classified as HW (n ‫ס‬ 302) or OW (n ‫ס‬ 143) based on body mass index. We assessed total (ToD) and cortical (CoD) volumetric BMD and bone area, estimates of bone strength (bone strength index [BSI]; stress-strain index [SSIp]), and muscle cross-sectional area (CSA) at the distal (8%), midshaft (50%), and proximal (66%) tibia by pQCT. We used analysis of covariance to compare bone outcomes at baseline and change over 16 mo. At baseline, all bone measures were significantly greater in OW compared with HW children (+4-15%; p Յ 0.001), with the exception of CoD at the 50% and 66% sites. Over 16 mo, ToA increased more in the OW children, whereas there was no difference for change in BSI or ToD between groups at the distal tibia. At the tibial midshaft, SSIp was similar between groups at baseline when adjusted for muscle CSA, but low when adjusted for body fat in the OW group. At both sites, bone strength increased more in OW because of a greater increase in bone area. Changes in SSIp were associated with changes in lean mass (r ‫ס‬ 0.70, p < 0.001) but not fat mass. In conclusion, although OW children seem to be at an advantage in terms of absolute bone strength, bone strength did not adapt to excess body fat. Rather, bone strength was adapted to the greater muscle area in OW children.

How Does Bone Quality Differ Between Healthy-weight and Overweight Adolescents and Young Adults?

“…Importantly, differences in bone microstructure between overweight and healthy-weight males were attenuated or no longer significant after adjusting for LM. This result is consistent with previous DXA [26,37,42] and pQCT [14,25,29,50] studies and suggests enhanced bone microstructure in overweight youth adapts to their greater LM, not FM, consistent with the functional model of bone development [44]. It is unclear as to why we did not observe the same differences between groups for females.…”

“…More importantly, excess adiposity may actually compromise bone microstructure in overweight males. Similar negative relationships between FM and pQCT-derived measures of bone area, BMD, and bone strength were observed at metaphyseal and diaphyseal sites of the radius and tibia in males and females [17,18,25,39,40,50,51]. The mechanisms that underpin potential detrimental effects of excess adiposity on bone structure and strength are not well defined.…”

“…With peripheral quantitative CT (pQCT), one is able to assess associations between excess body fat and bone cross-sectional structure, volumetric BMD, and estimates of bone strength at the radius and tibia in children and adolescents [14,17,18,39,43,50,51]. Before adjusting for muscle force (or surrogates), several studies reported a positive association between body fat and pQCT-derived BMD, structure, and strength in females and males [14,17,25,43,50]. Importantly, after adjusting for lean mass (LM) or muscle cross-sectional area, this association weakened [29], became nonsignificant [14,17], or negative [25,39,40].…”

Fat mass accumulation compromises bone adaptation to load in finnish women: A cross-sectional study spanning three generations

“…This agrees with the existing paradigm that bone adaptation is driven primarily by changes of mechanical load, (43) which are generated largely by muscle force (surrogated here by lean mass), not fat mass. (32,44) Since body weight is composed principally of lean and fat mass, and since fat mass accumulation contributes more than lean mass to weight gain in women, (45,46) we can conclude that it is the underlying negative relationship between RBSI and fat mass that gives rise to the negative association between RBSI and body weight. Therefore, it is fat mass that compromises the mechanical competence of the bone.…”

“…The product of body weight and tibial length, reflecting the bending moment of body weight acting on the lever length of the tibia, was used as a surrogate for load. (32,33) Raw RBSI was multiplied by 10 7 for presentational convenience. Note that the higher the RBSI value, the stronger is the bone relative to the applied load.…”