Study Overview
Objective. To evaluate the association of number of steps taken per day and stepping intensity with all-cause mortality in older women.
Design. This was a prospective cohort study of US women participating in the Women’s Health Study (WHS). Participants wore an accelerometer device (ActiGraph GT3X+, ActiGraph Corp, Pensacola, FL) on the hip during waking hours for 7 consecutive days between 2011 and 2015. The accelerator data were collected at 30 Hz and aggregated into 60-second, time-stamped epochs. Data from participants who were adherent with wearing devices (defined as ≥ 10 hours/day of wear on ≥ 4 days) were used in an analysis that was conducted between 2018 and 2019. The exposure variables were defined as steps taken per day and measures of stepping intensity (ie, peak 1-minute cadence; peak 30-minute cadence; maximum 5-minute cadence; and time spent at a stepping rate of ≥ 40 steps/minute, reflecting purposeful steps).
Setting and participants. In total, 18,289 women participated in this study. Of these, 17,708 wore and returned their accelerometer devices, and data were downloaded successfully from 17,466 devices. Compliant wearers of the device (≥ 10 hours/day of wear on ≥4 days) included 16,741 participants (96% compliance rate of all downloaded device data).
Main outcome measure. All-cause mortality as ascertained through the National Death Index or confirmed by medical records and death certificates.
Main results. In this cohort of 16,741 women, average age at baseline was 72.0 ± 5.7 years (range, 62 to 101 years) and the mean step count was 5499 per day (median, 5094 steps/day) during the 7-day data capture period between 2011 and 2015. Not taking steps (0 steps/minute) accounted for 51.4% of the recorded time, incidental steps (1 to 39 steps/minute) accounted for 45.5%, and purposeful steps (≥ 40 steps/minute) accounted for 3.1%. The mean follow-up period was 4.3 years; during this time, 504 participants died. The median steps per day across quartiles were 2718 (lowest), 4363, 5905, and 8442 (highest). The corresponding quartile hazard ratios (HRs) associated with mortality adjusted for confounders were 1.00 (reference; lowest quartile), 0.59 (95% confidence interval [CI], 0.47-0.75), 0.54 (95% CI, 0.41-0.72), and 0.42 (95% CI, 0.30-0.60; highest quartile), respectively (P < 0.01). A higher mean step count per day, up to approximately 7500 steps/day, corresponded with progressive and steady decline in mortality HRs using spline analyses. Similar results were observed using sensitivity analyses that minimized reverse causation bias. While the adjusted analysis of measures of stepping intensity showed an inverse association with mortality rates, these associations were no longer significant after accounting for steps per day. Specifically, adjusted HRs comparing highest to lowest quartile were 0.87 (95% CI, 0.68-1.11) for peak 1-minute cadence; 0.86 (95% CI, 0.65-1.13) for peak 30-minute cadence; 0.80 (95% CI, 0.62-1.05) for maximum 5-minute cadence; and 1.27 (95% CI, 0.96-1.68) for time spent at a stepping rate of ≥ 40 steps/minute.
Conclusion. Older women who took approximately 4400 steps per day had lower all-cause mortality rates during a follow-up period of 4.3 years compared to those who took approximately 2700 steps each day. Progressive reduction in mortality rates was associated with increased steps per day before leveling at about 7500 steps/day. Stepping intensity, when accounting for number of steps taken per day, was not associated with reduction in mortality rates in older women.
Commentary
The health and mortality benefits of exercise are well recognized. The 2018 Department of Health and Human Services Physical Activity Guidelines (DHHS-PAG) recommend that adults should do at least 150 to 300 minutes of moderate-intensity aerobic physical activity per week, or 75 to 150 minutes of vigorous-intensity aerobic physical activity per week, in addition to doing muscle-strengthening activities on 2 or more days a week.1 Importantly, the guidelines emphasize that moving more and sitting less benefit nearly everyone, and note that measures of steps as a metric of ambulation can further promote translation of research into public health recommendations for exercise interventions. Despite this recognition, there is limited information centering on the number of daily steps (step volume) and the intensity of stepping that are needed to achieve optimal health outcomes in older adults. The study reported by Lee and colleagues adds new knowledge regarding the relationship between step volume and intensity and mortality in older women.