Background: The obesity paradox refers to a category of subjects who may be less prone to develop co-morbidities, such as type 2 diabetes. Cardiorespiratory fitness (CRF) has been identified as one of the key factors. We aimed at exploring the difference in insulin metabolism between fit and unfit obese adolescents. Methods: We recruited 22 obese adolescents and assessed CRF during an incremental treadmill test. According to a cut-off at 80% of predicted maximal oxygen consumption (VO2max), subjects were separated into low or normal CRF. Body composition was determined by densitometry. Serum levels of insulin were measured sequentially during an oral glucose tolerance test and insulin secretion responses were calculated. Results: Compared to adolescents with normal CRF, the ones with low CRF had higher insulin resistance indices (p=0.023) and insulin secretion response (p=0.010), independently of the body mass index z-score. Conclusions: Interventions in obese adolescents should focus on the maintenance or improvement of CRF to at least 80% of predicted VO2max. Indeed, this cut-off was significantly related to insulin secretion responses, independently of the adiposity level. A CRF above the proposed cut-off may prevent the development of insulin resistance
