This paper evaluates severe normal trunk asymmetry (TA) by higher and lower body mass index (BMI) values in 5953 adolescents age 11–17 years (boys 2939, girls 3014) whilst standing forward bending (FB) and sitting FB during screening for scoliosis. TA was measured as angle of trunk inclinations (ATIs) across the back (thoracic, thoracolumbar and lumbar) with abnormality defined as 2 standard deviations or more. The findings for sitting FB position are reported because the readings express TA free from any leg-length inequality. Relatively lower BMIs are associated statistically with

excess of abnormal TAs, and

BMI is known to be linked to puberty timing and energy balance but not to TAs in healthy students. Similar to girls with adolescent idiopathic scoliosis, we suggest that severe TA is caused by a genetically-determined selectively increased hypothalamic sensitivity to leptin with asymmetry as an adverse hormetic response, exacerbated by presumed lower circulating leptin levels associated with relatively lower BMIs. The asymmetry is expressed bilaterally via the sympathetic nervous system to produce left-right asymmetry in ribs and/or vertebrae leading to severe TA when beyond the capacity of postural mechanisms of the somatic nervous system to control the shape distortion of the trunk