Adiposity, Insulin Resistance, Cardiorespiratory Fitness, and Bone Health in Hispanic Children

CONTEXT

Childhood obesity disproportionately affects Hispanic youth. The skeletal system appears to be a target organ of the adverse effects of obesity. Yet, the relationship between adiposity and bone health in youth and the modulating factors are not well understood.

OBJECTIVE

This work aims to examine the relationship between adiposity, insulin resistance (IR), cardiorespiratory fitness (CRF), and bone mass in Hispanic youth.

METHODS

A total of 951 Hispanic youth (50% male), aged 4 to 19 years, participated in this cross-sectional design study from the Viva La Familia Study at Children's Nutrition Research Center. Bone mineral content (BMC) and density (BMD), lean mass (LM), total body fat mass (FM), truncal FM were obtained using dual-energy x-ray absorptiometry. Fasting glucose and insulin were obtained and the homeostasis model assessment of insulin resistance (HOMA-IR) was calculated. CRF was measured using a treadmill ramp protocol. We applied linear regression models and mediation analyses.

RESULTS

Adiposity measures were negatively related to BMC and BMD after accounting for LM and sex. IR negatively contributed whereas CRF positively contributed to the variance in BMC and BMD, more notably in the pubertal age group. In mediation analysis, HOMA-IR partially mediated the negative relationship of adiposity to BMC (standardized indirect effect [IE] = -0.0382; 95% CI, -0.0515 to -0.0264) whereas the sequential IE of HOMA-IR and CRF partially attenuated (IE = -0.0026; 95% CI, -0.0053 to -0.0005) this relationship. Similar findings were seen with BMD as the primary outcome.

CONCLUSION

IR mediates the negative relationship between adiposity and bone mass whereas CRF may partially attenuate it.

The Effect of Cardiorespiratory Fitness and Insulin Resistance on Bone Health in Hispanic Children

Obesity appears to have a negative impact on pediatric bone health, and insulin resistance may mediate this relationship. It is unclear if cardiorespiratory fitness (CRF) has a protective effect on bone in obese children. We tested the hypothesis that CRF attenuates the negative effect of obesity and insulin resistance on skeletal health in a large cohort of Hispanic youth.

We studied 413 (193 males and 220 females) children and adolescents from the Viva la Familia Study. They were all pubertal; mean age (SD) 13.4 ± 2.3 years; 27% were normal weight (NW), 19% overweight (OW) and 54% obese (OB). They underwent measurement of body composition, total body bone mineral content (BMC) and density (BMD) by DXA scan; VO2peak using the ramp protocol on a treadmill for CRF; fasting glucose and insulin. The homeostasis model assessment of insulin resistance (HOMA-IR) was calculated. BMC increased from NW to OW to OB (mean 1.35 ± 0.4, 1.41 ± 0.4, and 1.49 ± 0.4 kg, respectively, p=.005). Peak VO2 decreased from NW to OW to OB (41.3 ± 9.7, 35.5 ± 7.7, 28.9. ± 5.5 mL/kg per min, respectively, p <.001). After adjusting for sex, age and lean body mass, BMC was inversely related to fat mass (r = -0.34, p <.001) and HOMA-IR (r = -0.29, p <.001). Similar relationships were found for BMD. In a regression model with BMC as the dependent variable, lean body mass (standardized coefficient (β)=0.95, p <.001) was positively and fat mass (β=-0.18, p <0.001) negatively associated with BMC (model R²=0.88, p<.001). HOMA-IR (β=-0.07, p =0.001) and VO2peak (β=0.09, p =0.003) had significant and opposite associations with BMC (model p<.001) but fat mass was no longer a significant contributor. With BMD as the dependent variable, lean body mass (β=0.82, p <.001), HOMA-IR (β=-0.06, p =0.04) and peak VO2 (β=0.17, p <.001), but not fat mass, contributed to the variance in BMD (R²=0.79, p<.001).

In conclusion, lean body mass is the major determinant of BMC and BMD in Hispanic youth. Adiposity associated insulin resistance has a negative effect on BMC and BMD. CRF contributes positively to the variance in BMC and BMD. This suggests that CRF and higher lean mass attenuate the adverse effects of insulin resistance on bone health in children.