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 R(2)=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 (R(2)=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.