AbstractBackgroundOsteosarcopenic obesity was recently described as a variant phenotype of obesity, mainly observed in old age. This nested case–control study was performed to detect the differences in body composition between young, healthy overweight/obese and healthy lean populations of both genders. Our null hypothesis was that except for the fat mass, there would be absence of body composition differences, namely skeletal muscle and bone masses, between the groups.MethodsWe used an advanced bio‐impedance device to determine the body composition and measured circulating CRP (hsCRP) and diurnal salivary cortisol concentrations, as indices of inflammation and chronic stress, respectively. Overall, 2551 subjects aged 18–21 years participated in the study.ResultsThe healthy lean group included 1072 participants [900 males (84%) and 172 females (16%)], and the healthy overweight/obese group included 1479 participants [74 males (5%) and 1405 females (95%)]. Healthy overweight/obese participants presented with an increased fat mass (P < 0·001), as expected, but lower muscle (P < 0·001) and bone (P < 0·001) masses than lean controls. These findings were accompanied by increased extracellular water compartments, circulating hsCRP levels and evening salivary cortisol concentrations in the healthy overweight/obese group.ConclusionsOur study suggests that ‘osteosarcopenic’ elements exist even in very young populations. These may represent a ‘precursor’ or forme fruste of the osteosarcopenic obesity phenotype in young healthy overweight/obese subjects, who may progressively develop osteosarcopenia in its full form at an older age. Our study highlights the significance of body composition analysis in medical practice, improving prevention and alleviating later health‐related economic burden.