Decreased levels of physical activity in adolescents with down syndrome are related with low bone mineral density: a cross-sectional study

Background: Down syndrome (DS) has been described as one of the main contributors for low bone mineral density (BMD). Physical activity (PA) is a key factor in skeletal health and thus, PA levels might be associated to the risk of developing osteoporosis. Therefore, the aims were (1) to describe PA patterns in adolescents with DS compared to their counterparts and (2) to determine the relationships between PA and the risk of having low bone mass in adolescents with DS. Methods: Nineteen adolescents (10 girls) with DS and 14 without disabilities (7 girls) participated in the study. Minutes in different PA intensities were objectively assessed with accelerometers (ActiTrainer). Moreover adolescents with DS were classified into PA tertiles taking into account the amount of total minutes of PA at any intensity, resulting in those performing low, medium or high of PA (lowPA, medPA and highPA). BMD was measured at the whole body, hip and lumbar spine with dual-energy X-ray absorptiometry and the BMD Z-score was calculated for each region taking into account age-and sex-matched reference data. Student's unpaired t-tests and analysis of covariance were used to compare variables between different conditions (DS vs. control) and PA levels (low, medium and high). Results: None of the adolescents with DS achieved the minimum of 60 min of daily moderate to vigorous PA (VPA) intensity recommended by PA guidelines; adolescents with DS group spent less time in sedentary and in VPA and more time in light PA than those without DS (p < 0.05). Adolescents with DS showed lower BMD Z-score values than those without (p < 0.05). Those adolescents with DS allocated in the lowPA tertile showed significant lower BMD Z-score at the hip and a general tendency towards lower BMD Z-score was found at whole body and lumbar spine compared to those in highPA tertile and (p < 0.05). Conclusions: Adolescents with DS in the highPA tertile showed lower risk of developing future osteoporosis by having higher BMD Z-score at the hip. This data provides an idea regarding the importance of accumulated minutes of PA, and not only moderate or vigorous in the bone health in adolescents with DS.