<qa>  What is the cause of obesity co-morbidities?The pediatric obesity epidemic continues unabated. Its cardio-metabolic complications are undisputed, including inflammation, insulin resistance IR, glucose intolerance and greater prevalence of type 2 diabetes T2D in youth. We contend an equally serious consequence of these obesity co-morbidities is their detrimental effects on bone development during adolescence, a critical time for mineral accrual and architectural modeling that underlies bone strength and fracture risk. This proposition has received little attention and the sparse results are mixed, with reports of augmented and impaired mineralization. In contrast, animal data demonstrates reduced mineral accrual and compromised architecture with insulin resistance and chronic inflammation. The conflicting results in youth are likely due to mixed samples and analyses that commingle obese youth with metabolic complications with so called metabolically healthy obese youth and the use of technology i.e., dual energy x-ray absorptiometry, DXA to measure bone outcomes that is confounded by the very changes that investigators seek to detect. We posit that the positive mechanical effect of excess adiposity on bone is countered by chronic low-grade inflammation and IR so that obese youth with these metabolic complications suffer impaired bone development whereas obesity in otherwise metabolically healthy youth augments development. A thorough understanding of the effects of adiposity and its co-morbidities on bone development is crucial to the development of efficacious interventions aimed at maximal mineral accrual and bone modeling. Thus, we propose primary aims designed to clarify the effects of obesity, insulin resistance and inflammation on bone around the time of peak height velocity. Adipose tissue AT distribution undoubtedly matters, especially abdominal visceral AT and skeletal muscle fat content, both strongly related to insulin resistance. Failure to characterize fat distribution is another important limitation of past studies. Consequently we will assess the effect of visceral AT and skeletal muscle fat along with whole body fatness and propose secondary aims designed to develop safe, cost effective methods that we and others can use for estimating AT distribution, a critical component of risk that has rarely been studied in youth relative to bone development. 