Context A close link between BMI and female pubertal onset is well established. However, observations in boys remain inconclusive. Objective We aim to determine whether BMI as well as total and central adiposity in pre-pubertal Chilean boys is associated with pubertal timing. Design Longitudinal study. Patients 494 boys from the Growth and Obesity Chilean Cohort Study were followed starting from birth and throughout puberty including five pre-pubertal visits. Main outcome measures: Anthropometric data and semi-annual clinical pubertal staging. The association between BMI, obesity (BMI SDS ≥ 2) and central adiposity (waist circumference ≥ 90th centile) with precocious puberty and age at gonadarche was analyzed using a survival- and logistic regression models. Results BMI, prevalence of total obesity and central obesity increased throughout childhood. 45 boys entered puberty before the age of 9 years (9.1%). Obesity at 4 to 7 years and childhood mean BMI SDS were significantly associated with precocious gonadarche. Mean age at testicular enlargement (≥4mL), was 11.0 years (95%CI: 10.9-11.1) and was inversely associated with BMI SDS, waist circumference and percent fat mass in almost all pre-pubertal visits. Age at testicular enlargement in normal weight, overweight and obese boys was 11.2 (11.0-11.3), 10.9 (10.6-11.1) and 10.7 (10.4-11.1) years, respectively. Conclusions Our observation supports the association of BMI SDS and obesity with pubertal timing and precocious gonadarche in boys, respectively. Early intervention controlling the obesity epidemic could be useful in decreasing detrimental impact on later health. Abbreviations Growth and Obesity Chilean Cohort Study (GOCS), hypothalamic-pituitary-gonadal (HPG), Body mass index (BMI), World health organization (WHO), Waist circumference (WC)

Introduction: Pubertal onset is triggered by multiple neuroendocrine interactions. The role of prepubertal IGF-1 in this process has not been explored in both sexes. Our objective is to analyze the association of prepubertal IGF-1 concentration with age at thelarche (B2) and menarche (M) in girls and age at gonadarche (G2) in boys. Methods: Longitudinal study (n=1196 boys and girls) within the Growth and Obesity Chilean cohort Study (GOCS). At ages ≈ 6.7 years blood sample for IGF-1. Subjects analyzed into 4 groups according to the onset age of the pubertal event. Results: Higher prepubertal IGF-1 levels were observed at earlier ages of thelarche (p=0.003) and menarche onset (p=0.041). A taller prepubertal height was observed at younger ages of thelarche and menarche (p=<0.001 and 0.002, respectively). The proportional hazard regression (HR) models showed that with an increase of 1 SD in IGF-1, the HR of presenting thelarche at younger ages was 1.25 and this association was maintained when adjusted for confounding variables. Similarly, the HR of presenting menarche at earlier ages was 1.21. This association was maintained only when adjusting for BMI but not using further confounders. In boys, prepubertal IGF-1 showed a tendency to be significantly higher in children with earlier gonadarche and taller height (both p<0.001). The HR of presenting gonadarche at younger ages was 1.22 and this association was maintained after adjusting for confounders. Conclusions: higher IGF-I levels in mid-childhood are associated with earlier puberty onset. The role of IGF-I in the onset of puberty requires further investigation.