Association between cardiorespiratory fitness level and insulin resistance in adolescents with various obesity categories

BACKGROUND: An association between cardiorespiratory fitness (CRF) and insulin resistance in obese adolescents, especially in those with various obesity categories, has not been systematically studied. There is a lack of knowledge about the effects of CRF on insulin resistance in severely obese adolescents, despite their continuous rise. AIM: To investigate the association between CRF and insulin resistance in obese adolescents, with special emphasis on severely obese adolescents. METHODS: We performed a prospective, cross-sectional study that included 200 pubertal adolescents, 10 years to 18 years of age, who were referred to a tertiary care center due to obesity. According to body mass index (BMI), adolescents were classified as mildly obese (BMI 100% to 120% of the 95(th) percentile for age and sex) or severely obese (BMI ≥ 120% of the 95(th) percentile for age and sex or ≥ 35 kg/m(2), whichever was lower). Participant body composition was assessed by bioelectrical impedance analysis. A homeostatic model assessment of insulin resistance (HOMA-IR) was calculated. Maximal oxygen uptake (VO(2)max) was determined from submaximal treadmill exercise test. CRF was expressed as VO(2)max scaled by total body weight (TBW) (mL/min/kg TBW) or by fat free mass (FFM) (mL/min/kg FFM), and then categorized as poor, intermediate, or good, according to VO(2)max terciles. Data were analyzed by statistical software package SPSS (IBM SPSS Statistics for Windows, Version 24.0). P < 0.05 was considered statistically significant. RESULTS: A weak negative correlation between CRF and HOMA-IR was found [Spearman’s rank correlation coefficient (r(s)) = -0.28, P < 0.01 for CRF(TBW); (r(s)) = -0.21, P < 0.01 for CRF(FFM)]. One-way analysis of variance (ANOVA) revealed a significant main effect of CRF on HOMA-IR [F((2200)) = 6.840, P = 0.001 for CRF(TBW); F((2200)) = 3.883, P = 0.022 for CRF(FFM)]. Subsequent analyses showed that obese adolescents with poor CRF had higher HOMA-IR than obese adolescents with good CRF (P = 0.001 for CRF(TBW); P = 0.018 for CRF(FFM)). Two-way ANOVA with Bonferroni correction confirmed significant effect of interaction of CRF level and obesity category on HOMA-IR [F((2200)) = 3.292, P = 0.039 for CRF(TBW)]. Severely obese adolescents had higher HOMA-IR than those who were mildly obese, with either good or poor CRF. However, HOMA-IR did not differ between severely obese adolescents with good and mildly obese adolescents with poor CRF. CONCLUSION: CRF is an important determinant of insulin resistance in obese adolescents, regardless of obesity category. Therefore, CRF assessment should be a part of diagnostic procedure, and its improvement should be a therapeutic goal.