Associations of Physical Activity, Sedentary Time, and Insulin with Percent Breast Density in Hispanic Women

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In a previous study of Hispanic women, we reported a positive association between sedentary time and percent breast density, a marker of breast cancer risk. It is unclear whether associations between sedentary time or physical activity and percent breast density are mediated through serum insulin levels or insulin resistance, factors also associated with physical activity and breast cancer risk. In the Chicago Breast Health Project phase II pilot study, detailed information on health and lifestyle factors, including sitting time and total physical activity over the previous 7 days, was collected from 95 Hispanic women aged 40-77 years. We also assessed percent breast density and measured fasting serum insulin and glucose to calculate the Homeostasis Model Assessment (HOMA) index, a measure of insulin resistance. Multivariable linear regression was used to assess associations of total physical activity, time spent sitting, serum insulin, and HOMA index with percent breast density. There was no association between total physical activity and percent breast density (p = 0.98), whereas there was a positive association between sedentary time and percent density (beta = 0.25% per 100 metabolic equivalent (MET)-minutes/week, p = 0.06). Percent breast density was not associated with insulin or with HOMA index. The strength of the association between time spent sitting and percent density was unchanged after inclusion of insulin or the HOMA index in the model. These results are consistent with our previous finding of an association between sedentary time and percent breast density and suggest that insulin or insulin resistance is unlikely to mediate this relation.