People are generally lean during childhood and show more variability in body sizes and shapes later in life. Cortical development generally correlates with body growth. However, in children cortical growth may be impaired with oversized body growth. Inverse correlations between body mass index (BMI) and brain volumes suggest that lean bodies may be associated with increased cortical volume. To clarify the positive effects of a lean body on a child's cortical development, we used MRI to measure brain structures longitudinally in 107 children and adolescents aged 5-16 years. The relationships between changes in BMI and cortical volumes during 3 years of development were investigated, while controlling for age, gender and intracranial volume changes. Voxel-based morphometry analyses revealed that an increase in the volume of the right posterior medial temporal lobe - including the hippocampus and parahippocampal gyrus - was associated with lower BMI increases. No correlations were observed between higher BMI increases and cortical volumes. Our results suggest that keeping a lean body - or not getting fat - during childhood can induce an increase in regional cortical volume rather than impair growth. This is the first longitudinal study showing positive effects of a lean body on cortical development in children. (C) 2015 S. Karger AG, Basel