Bierhals IO, et al. - Researchers examined how body composition components (Fat mass index and Lean mass index) and body mass index [BMI] at 18 and 22 years and obesity are associated with bone density. Data on 2968 members of the 1993 Pelotas Birth Cohort from follow-ups at 18 and 22 years of age was prospectively studied. Using dual-energy X-ray absorptiometry, they determined areal bone mineral density for whole body, lumbar spine, and femoral neck at 22 years. Findings suggest that BMI influences bone mass, however, lean mass and fat mass differed in their impact, with a largest effect noted for lean mass. At 18 years, the strongest associations for both body composition components were observed. According to obesity classification, men who were “always obese” show a negative effect in the lumbar spine. For women, who were obese “only at 18 years” had a negative effect in the whole body. A density increase in whole-body and femoral neck were noted between those who were obese “only at 22 years” and “always obese”. These findings highlight the differential effect of the body composition components on bone mass and raise questions about the effects of fat mass at young ages, particularly whether the longer time of adiposity exposure may have harmful consequences for bone health.