Cardorespiratory fitness in youth:: Relationship to insulin sensitivity and β-cell function

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 beta-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.