Mortality risk reductions for replacing sedentary time with physical activities

Abstract

Introduction

Regular moderate-vigorous intensity physical activity (MVPA) is associated with a lower risk of cardiovascular disease (CVD), certain cancers, and premature death.^1–3 Distinct from physical inactivity, the amount of time spent sedentary is also associated with a higher risk of morbidity and mortality.^4–9 This may be due, at least in part, to the displacement of physical activities (PA) with sedentary behaviors.

There is a finite amount of time in a day, so it is necessary to consider how time spent on one behavior displaces time spent on another.¹⁰ Most previous studies explored associations with sedentary time without considering the PA it displaces, leaving a gap in our understanding of healthful time use. Using isotemporal substitution models (ISM), it is possible to estimate the mortality risk reductions associated with replacing sedentary time with an equivalent amount of PA.^11,12

The primary aim of this study was to estimate the mortality risks associated with replacing 30 min·day⁻¹ of sitting time with 30 min·day⁻¹ of either light (LPA) or MVPA in a large prospective cohort of U.S adults over 14 years of follow-up. Secondary aims included estimating these mortality risks: 1) among low, moderate, and high active participants separately, 2) for cancer, CVD, and other causes of death, and 3) stratified by sex, age, and body mass index (BMI).

Methods

Results

During 14 years of follow-up, 14,415 men and 13,358 women died. Participants reporting more MVPA were leaner, had a higher educational attainment, and were less likely to be current smokers (Table 1). Sitting time largely included watching TV (39%) and reading (20%).

Overall, reallocation of 30 min·day⁻¹ of sitting to LPA (HR=0.94, 95% CI 0.92-0.96) or MVPA (HR=0.92, 0.90-0.93) was associated with significant reductions in mortality risk. However, there was significant variation by underlying MVPA level (Figure 1). In the stratified analysis, replacement of sitting with LPA was associated with a reduced mortality risk for only low (HR=0.86, 0.81-0.89) and moderately (HR=0.94, 0.91-0.97) active participants, but not high active participants. Benefits were greater when replacing sitting time with MVPA (HR=0.55, 0.47-0.62 low active; HR=0.83, 0.76-0.88 moderate active). Results for grouped causes of death were similar (Table 2).

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Figure 1.

Estimated risk (HR) for all-cause mortality associated with replacement of 30 minutes of sitting time with physical activity.

Models adjust for age (continuous), sex, race (white, other), alcohol use (non-drinker, <1, 1, ≥2 drinks/day, unknown), smoking status (never, current, former), years since quitting among former smokers (<10, 10-19, ≥ 20 years), cigarette frequency and smoking duration among current smokers (<20 cigarettes/day for ≤35 years, <20 cigarettes/day for >35 years, 20+ cigarettes/day for ≤35 years, 20+ cigarettes/day for >35 years), aspirin use (non-user, <15, 15-29, 30+ pills/month, unknown), education (high school or some college, college graduate or higher, unknown), occupational status (employed, not employed/retired), ACS dietary guidelines adherence score (0-<3, 3-<6, ≥6, unknown),¹⁶ comorbidity score (0, 1, ≥2 comorbidities, including high blood pressure, diabetes, and high cholesterol), and BMI (continuous, [kg/m²]). MVPA categories split into approximate tertiles: low active ≤ 17 MVPA min/day, moderate active > 17 to ≤ 38 MVPA min/day, high active > 38 MVPA min/day.

Given the significant interaction by underlying MVPA, analyses by sex, age, and BMI were restricted to the low and moderately active groups (Table 3). Results were similar when stratified on sex and BMI. However, significant interactions by age revealed larger mortality benefits for older adults when sitting was replaced with LPA.

Discussion

In this large prospective study, replacement of 30 min·day⁻¹ of sitting with LPA or MVPA was associated with lower mortality risk, but results varied substantially by underlying MVPA level. Among low to moderately active adults (MVPA ≤38 min/day), replacing sitting with LPA was associated with a reduction in cancer, CVD, and all-cause mortality. Additionally, replacement of sitting with MVPA resulted in larger mortality benefits. However, among the highly active participants, the same reallocation of sitting to LPA or MVPA was not associated with additional mortality benefits.

Application of the ISM in this study revealed a significant association of reallocation of sitting time to LPA with mortality in low and moderately active participants. This finding is relevant to public health as LPA may be more attainable for certain groups failing to meet PA guidelines (e.g., older adults), and suggests an important strategy for gaining further health benefits from PA for many already engaging in MVPA. This study was one of the first to explore the role of age on the replacement of sitting time and found that older adults may benefit more from the replacement of sitting time with LPA. However, it is possible that the assessed LPAs may be of higher relative intensity for adults over age 65 years, resulting in larger than expected mortality risk estimates. It is also possible that participation in purposeful, leisure-time PA later in life may represent better general health.

Conclusions

Among the least and moderately active, replacing 30 min·day⁻¹ of sitting with 30 min·day⁻¹ of LPA or MVPA was associated with longevity, and reallocating sitting time to MVPA provided the most benefit for time invested. As many adults do not engage in any MVPA, evidence of associated benefits for LPA highlights the potential for targeting lower intensity PA behaviors to improve health.

Acknowledgements

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