Associations of physical activity, cardiorespiratory fitness and fatness with low-grade inflammation in adolescents: the AFINOS Study.

Objective: To examine the independent associations of objectively measured physical activity (PA), cardiorespiratory fitness (CRF) and fatness with low-grade inflammatory markers in adolescents.
Design: Cross-sectional study in Spain.
Subjects: A sample of 192 adolescents aged 13-17 years.
Measurements: PA was assessed with an accelerometer for 7 days. A 20-m shuttle-run test was used to assess CRF. Skinfold thicknesses at six sites and WCs were measured. BMI was calculated from measured height and weight. C-reactive protein (CRP), interleukin-6 (IL-6) and complement factors C3 and C4 were assayed. The homeostasis model assessment of insulin resistance (HOMA-IR) was calculated from glucose and insulin. Regression analysis adjusted for potential confounders and HOMA-IR was used to determine the associations between PA, CRF and fatness with low-grade inflammatory markers.
Results: Total PA, vigorous PA and MVPA were positively associated with CRF (r=0.25-0.48), whereas vigorous PA was negatively associated with skinfolds (r=-0.27). CRF was inversely associated with fatness, (r=-0.30 to -0.48). CRF and fatness were inversely and positively associated with HOMA-IR (r=-0.16 and 0.21, respectively). PA variables were not independently associated with inflammatory markers. CRF and fatness were inversely and positively associated with CRP, C3 and C4, respectively. Only body fat explained a relevant amount of the variance of the model in CRP (4%) and C4 (19%), whereas CRP and body fat jointly explained the variance in C3 (25%). All these observations were independent of HOMA-IR.
Conclusions: These findings support the key role of CRF and fatness on low-grade inflammation, as well as the possible indirect role of habitual PA through CRF and body fat in adolescents.