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- Research Methods and Procedures
Objective: We examined whether the relationship between cardiorespiratory fitness (CRF) and insulin sensitivity (IS)/secretion is independent of adiposity in healthy African-American (n = 65) and white (n = 57) youth.
Research Methods and Procedures: IS and β-cell function were evaluated by a 3-hour hyperinsulinemic-euglycemic and a 2-hour hyperglycemic (12.5 mM) clamp, respectively. Total fat was measured by DXA and abdominal fat with computed tomography. CRF (peak volume of oxygen) was measured using a graded maximal treadmill test.
Results: Independent of race, CRF was inversely (p < 0.05) related to total and abdominal fat, fasting insulin and first phase insulin secretion, and positively (p < 0.05) related to IS. When subjects were categorized into low (≤50th) and high (>50th) CRF groups, IS was significantly (p < 0.05) higher in the high compared with the low CRF group independently of race. Furthermore, first and second phase insulin secretion were lower (p < 0.05) in the high CRF group in comparison with the low CRF group in both races. However, in multiple regression analyses CRF was not (p > 0.05) an independent predictor of IS and acute insulin secretion after accounting for total adiposity.
Discussion: Our findings demonstrate that low CRF is associated with decreased IS compensated by higher insulin secretion in both African-American and white youth. However, this relationship disappears after adjusting for differences in adiposity, suggesting that the association between fitness and IS is mediated, at least in part, through fatness.
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- Research Methods and Procedures
We evaluated the relationship of CRF to IS and β-cell function in the context of body composition using the gold standard methods in healthy African-American and white youth. The primary findings of this study are that low CRF is associated with higher total and abdominal fat, and lower IS compensated by higher insulin secretion independent of race; and beneficial influence of high CRF on IS/secretion is abolished once adiposity is controlled for. These observations suggest that in the pediatric population, the effect of high CRF on IS and secretion may be mediated, in part, by decreased adiposity.
Boreham et al. (18) reported that the strength of the relationships between fatness and cardiovascular disease risk factors is greater than those observed between fitness and the same risk factors in a large sample of European boys and girls. In addition, Shaibi et al. (19) demonstrated that CRF is not independently associated with any individual components of the metabolic syndrome in overweight Hispanic youth. Our findings in a biracial sample of children and adolescents using the gold standard methods of examining IS and β-cell function extend the previous observations (6)(18)(19) and demonstrate that fatness is a stronger predictor of insulin resistance and hyperinsulinemia than fitness in youth. This observation differs from Kasa-Vubu et al. (8), who reported an independent effect of CRF on insulin resistance in a small sample of adolescent girls (n = 53). The different findings could be attributed to differences in the cohorts studied and methodology employed. In that study (8), the subjects examined includes young women (mean age, 18.7 ± 1.3 years) from various ethnic backgrounds (African American, white, Asian, and Hispanic white). Moreover, insulin resistance was evaluated by homeostasis model assessment, which is not a true measurement of in vivo IS.
Unlike several pediatric studies, some studies in adults reported an independent effect of CRF on major health outcomes. In men, fitness is associated with all-cause and cardiovascular disease-related mortality and the incidence of metabolic syndrome independent of fatness (2)(4). Although health risk measures examined are different between studies in children (i.e., disease risk factors) and adults (i.e., incidence of CVD or mortality), they provide substantial support for the beneficial influence of having a high aerobic fitness on obesity-related health risks independent of age.
Evidence suggests that lower aerobic fitness is independently related to greater adiposity gain in growing prepubertal children (20). During 3 to 5 years of follow-up, Johnson et al. (20) have shown that there is a significant inverse relationship between initial aerobic fitness and increasing adiposity during maturation, and this observation was similar for both African-American and white children. Given that childhood fitness predicts adulthood fitness (21) and that childhood obesity tracks well into adulthood obesity together with cardiovascular risk factors (21), regular physical activity should be an essential component of weight management in overweight youth to improve CRF and health risk factors.
Limitations of this study warrant mention. First, the cross-sectional design of the present study does not allow us to infer a causal relationship. Thus, intervention studies with serial measurements of fitness, total fat, and metabolic profiles are needed to confirm our observation. Second, due to small sample size, we could not examine whether gender influences the relationship between fitness and fatness on the metabolic risks.
In conclusion, our findings suggest that in youth, low CRF is associated with lower IS compensated by higher insulin secretion, and these relationships may be mediated, in part, by increased fatness. These findings reinforce the recommendation that youths adopt a physically active lifestyle to improve aerobic fitness and body composition for preventing childhood obesity and related health risks.