Heterogeneity of fracture pathogenesis in urban South African children: the birth to twenty cohort.

South African black children fracture less than white children. Differences in bone mass, body composition, and physical activity may be contributing risk factors. This study aimed to investigate the association between fracture prevalence, bone mass, and physical activity in South African children. Using the Bone Health cohort of the Birth to Twenty longitudinal study, we retrospectively obtained information of lifetime fractures until age 15 years in 533 subjects. Whole-body bone mineral content (BMC), bone area (BA), fat mass (FM), and lean mass (LM) (measured by dual-energy X-ray absorptiometry [DXA]), anthropometric data, physical activity scores, and skeletal maturity were obtained at ages 10 and 15 years. Nonfracturing black females were used as the control group and comparisons were made between those who did and did not fracture within the same sex and ethnic groups. Of the 533 subjects, 130 (24%) reported a fracture (black, 15%; white, 41.5%; p  <  0.001). White males who fractured were significantly taller (10 years, p  <  0.01), more physically active (15 years, p  <  0.05) and had higher LM (10 years, p  =  0.01; 15 years, p  <  0.001), whereas white females who fractured were fatter (10 and 15 years, p  =  0.05 and p  <  0.05, respectively), than their nonfracturing peers. White males who fractured had greater BA and BMC at all sites at 10 and 15 years compared to their nonfracturing peers after adjusting for differences in height and weight; BA and BMC were similar in each of the other sex and ethnic groups. No anthropometric or bone mass differences were found between black children with and without fractures. The factor associated with fractures in white males appears to be participation in sports activities, while in white females obesity appears to play a role. No contributing factors in black males and females were found, and needs further elucidation.