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Background: The inactivity physiology paradigm proposes that sedentary behaviors, including sitting too much, are independent of the type of physical activity delineated for health in the Physical Activity Guidelines for Americans. Thus, we hypothesized that, when accounting for behaviors across the entire day, variability in the amount of time spent sitting would be independent of the inter-and intra-individual time engaged in sustained moderate-to-vigorous physical activity (MVPA). Methods: Ninety-one healthy women, aged 40-75 years, completed a demographic questionnaire and assessment of height and weight. Participants wore the activPAL activity monitor for one week and time (minutes/day) spent sitting, standing, stepping, and in sustained bouts (bouts ?10 minutes) of MVPA were quantified. The women were then stratified into groups based on weekly sustained MVPA. Additionally, each day of data collection for each participant was classified as either a "sufficient" MVPA day (? 30 min of MVPA) or an "insufficient" MVPA day for within-participant analyses. Results: Time spent sitting, standing, and in incidental non-exercise stepping averaged 64, 28, and 11 hrs/week, respectively, and did not differ between groups with individuals meeting/exceeding the current exercise recommendation of 150 min/week of sustained MVPA in ≥10 minutes bouts (M = 294 min/week, SD = 22) compared to those with none or minimal levels (M= 20min/week, SD = 4). Time spent sitting (M = 9.1 hr/day, SD = 0.19 vs. M = 8.8 hr/day, SD = 0.22), standing (M = 3.9 hr/day, SD = 0.16 vs. M = 3.9 hr/day, SD = 0.15), and in intermittent stepping (M = 1.6 hr/day, SD = 0.07 vs. M = 1.6 hr/day, SD = 0.06) did not differ between days with (~55 min/day) and without recommended MVPA. Conclusions: This study provides the first objective evidence that participation in sustained MVPA is unrelated to daily sitting duration in relatively healthy, middle and older-aged women. More research is needed to extend these findings to other populations and to inform distinct behavioral recommendations focused on sedentary time. [ABSTRACT FROM AUTHOR]

Background: 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., Methods: 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., Results: 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., Conclusions: 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.