O'Brien KM, et al. - Researchers determined the impact of a breast cancer diagnosis on serum 25-hydroxyvitamin D (25[OH]D) concentrations in Sister Study participants: 827 women with an intervening breast cancer and 771 without. The investigators measured participant total serum 25(OH)D levels at baseline (2003-2009) and 4-10 years later (2013-2015). In both groups, increased 25(OH)D concentrations were detected, although modestly correlated over time; however, they observed larger increases among women with breast cancer vs those without. Consequently, whether baseline or second blood draw measures were used was found to be a decisive factor for the estimated link between 25(OH)D and breast cancer. Findings revealed association of concentrations with regular (> 4 times/week) vitamin D supplement use, which became more common over time. In patients with breast cancer (56% to 84%) vs in control participants (56% to 77%), greater increases were observed. Overall, previously observed differences between retrospective and prospective studies were not explained by the findings but do show how reverse causation and temporal trends in exposure can distort inference.