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Since most of our current knowledge about the concept of bone gain during adolescence or early adulthood comes from the cross-sectional studies, we aimed to evaluate natural course of changes in bone mineral density prospectively during early adulthood in a cohort. Twenty-two volunteers (15 female, 7 male) with the mean age of 20.0 ± 0.69 years were enrolled into the study. Subjects with any known secondary cause of osteoporosis were excluded from the study. Demographic variables, anthropometric measurements were recorded. Subjects underwent an interview about risk factors for osteoporosis. Physical activity level and calcium intake were evaluated by European Vertebral Osteoporosis Study questionnaire. Lumbar and femoral bone mineral densities were measured using Dual Energy X-ray Absorptiometry (DXA) at baseline and two years after. L1-4 BMD increased from 0.925 ± 0.099 g/cm² to 0.957 ± 0.094 g/cm²(p = 0.002) and total femur BMD increased from 0.866 ± 0.113g/cm² to 0.908 ± 0.108g/cm²(p = 0.0001). BMC of L1-4 increased significantly (p = 0.022), however there was no change in bone area measurements (p = 0.82). In case of femoral total measurements, both BMC and bone area demonstrated significant increase over the study period (p = 0.0001 and p = 0.0001). There were significant positive differences in bone mineral densities for L3, L4 measurements in males and for L3 measurements in females (p = 0.028, p = 0.018 and p = 0.02 respectively). Different from the bone gain during adolescent years which is usually attributed to volumetric expansion of the bones; we were able to demonstrate that bone gain continues through early years of adulthood. It would be rational to extend preventive measures beyond adolescence, into early adulthood to optimize the bone mass.

In this study we identify multiple trajectories of school–work paths for Chinese young adults and assess how much the urban–rural hukou (household registration) origin explains the between-group difference in the life trajectories. Results based on the 2006 and 2008 waves of the Chinese General Social Survey show a stark disparity in school–work trajectory patterns between those with urban and rural origins, and there is a strong path dependence from one transition point to another. More than three-quarters of those with a rural origin did not attend high school, of whom 60% became agricultural laborers, making this the most dominant life path for rural youths. In contrast, about 70% of urban youths went on to high school and half of them completed a college degree or higher. One-third of urban youths in our sample became managers or professionals by the age of 35 compared to 9% of their rural counterparts. There is a clear long-term implication of these life pathways for income in early adulthood, which may continue to increase in later life stages. Even when taking the same school–work path, an individual with a rural-hukou origin still earns significantly less than one who has an urban origin.