Sedentary behaviour (SB) is increasing in Western societies and some studies suggest a deleterious effect of SB on bone. The aim of this systematic review was to examine the association between SB and bone health in children, adolescents and young adults. Electronic databases (PubMed, MEDLINE, PsycINFO and Science Citation Index) were searched for relevant articles up to January 9, 2017. Studies were included when results on bone health (e.g. strength, mass and structure) and either subjectively (questionnaires) or objectively (accelerometry) measured SB were reported in healthy participants ≤24 years. Two reviewers independently screened titles and abstracts for eligibility, rated methodological quality and extracted data. Seventeen observational studies were included. Several studies that used DXA or quantitative ultrasound suggested that objectively measured SB was negatively associated with lower extremity bone outcomes, such as femoral neck bone mineral density. The magnitude of this negative association was small and independent of moderate-to-vigorous physical activity. In contrast to the lower extremities, there was insufficient evidence for an association of lumbar spine bone outcomes with objectively measured SB. In high-quality studies that used DXA, no association was observed between objectively measured SB and total body bone outcomes. In studies using questionnaires, none of these relationships were observed. Well-designed longitudinal studies, objectively measuring SB, are needed to further unravel the effect of SB, physical activity and their interaction on bone health.Electronic supplementary materialThe online version of this article (doi:10.1007/s00198-017-4076-2) contains supplementary material, which is available to authorized users.

Background: Sedentary behaviour (SB) is a potential risk factor for suboptimal bone deposition in youth. Results: Total SB was negatively associated with lower extremity bone outcomes, while no association was observed with total body bone outcomes. Insufficient evidence was found for an association between total SB and lumbar spine bone outcomes. Conclusion: This review highlights the heterogeneity of the available evidence and emphasizes the need for well-designed studies.Keywords Adolescent . Bone . Child . Osteoporosis . Sedentary behaviour . Unloading AimThe aim of this systematic review was to examine the association between SB and bone health in children, adolescents and young adults [1]. Key findingsBased on studies that used DXA and quantitative ultrasound, we found moderate evidence for a negative association between objectively measured SB and lower extremity bone outcomes in schoolchildren that was independent of moderate-to-vigorous physical activity (MVPA). We calculated that 1 h less sedentary time per day is associated with the same effect on femoral neck bone mineral density (BMD) as 18 min of MVPA (based on one high-quality study). Based on high-quality studies that used DXA, there was strong evidence to suggest no association between objectively measured SB and total body bone outcomes in schoolchildren, and insufficient evidence for an association with lumbar spine bone outcomes. For questionnaires, none of these relationships were observed. Importance, timeliness and interestThis systematic review suggests that objectively measured total sedentary time is negatively associated with b o n e o u t c o m e s o f t h e l o w e r e x t r e m i t i e s i n schoolchildren.The electronic supplementary material presented here complements the article BSedentary behaviour and bone health in children, adolescents and young adults: a systematic review^ [1].