Associations of discretionary screen time with mortality, cardiovascular disease and cancer are attenuated by strength, fitness and physical activity: findings from the UK Biobank study

Abstract

Background: Discretionary screen time (time spent viewing a television or computer screen during leisure time) is an important contributor to total sedentary behaviour, which is associated with increased risk of mortality and cardiovascular disease (CVD). The aim of this study was to determine whether the associations of screen time with cardiovascular disease and all-cause mortality were modified by levels of cardiorespiratory fitness, grip strength or physical activity.

Methods: In total, 390,089 participants (54% women) from the UK Biobank were included in this study. All-cause mortality, CVD and cancer incidence and mortality were the main outcomes. Discretionary television (TV) viewing, personal computer (PC) screen time and overall screen time (TV + PC time) were the exposure variables. Grip strength, fitness and physical activity were treated as potential effect modifiers.

Results: Altogether, 7420 participants died, and there were 22,210 CVD events, over a median of 5.0 years follow-up (interquartile range 4.3 to 5.7; after exclusion of the first 2 years from baseline in the landmark analysis). All discretionary screen-time exposures were significantly associated with all health outcomes. The associations of overall discretionary screen time with all-cause mortality and incidence of CVD and cancer were strongest amongst participants in the lowest tertile for grip strength (all-cause mortality hazard ratio per 2-h increase in screen time (1.31 [95% confidence interval: 1.22-1.43], p < 0.0001; CVD 1.21 [1.13-1.30], p = 0.0001; cancer incidence 1.14 [1.10-1.19], p < 0.0001) and weakest amongst those in the highest grip-strength tertile (all-cause mortality 1.04 [0.95-1.14], p = 0.198; CVD 1.05 [0.99-1.11], p = 0.070; cancer 0.98 [0.93-1.05], p = 0.771). Similar trends were found for fitness (lowest fitness tertile: all-cause mortality 1.23 [1.13-1.34], p = 0.002 and CVD 1.10 [1.02-1.22], p = 0.010; highest fitness tertile: all-cause mortality 1.12 [0.96-1.28], p = 0.848 and CVD 1.01 [0.96-1.07], p = 0.570). Similar findings were found for physical activity for all-cause mortality and cancer incidence.

Conclusions: The associations between discretionary screen time and adverse health outcomes were strongest in those with low grip strength, fitness and physical activity and markedly attenuated in those with the highest levels of grip strength, fitness and physical activity. Thus, if these associations are causal, the greatest benefits from health promotion interventions to reduce discretionary screen time may be seen in those with low levels of strength, fitness and physical activity.

Keywords: Cardiovascular; Fitness; Mortality; Physical activity; Screen time; Screen-time; Strength.

Fig. 1

Cox proportional hazard model of the association of 1-h increments in overall discretionary screen time, TV viewing and leisure PC screen time with all-cause mortality and incidence CVD and cancer. Data presented as adjusted hazard ratio (HR) (95% CI) per 1-h increments in discretionary screen time, TV viewing and PC screen time per day. CVD cardiovascular disease, CI confidence interval, HR hazard ratio, PC personal computer, TV television

Fig. 2

Cox proportional hazard models of the association of overall discretionary screen time with all-cause mortality, and incidence of CVD and cancer by physical activity, fitness and handgrip strength strata. Data are presented as adjusted hazard ratio (HR) (95% CI). Reference category was defined as those participants with < 2 h.day^− 1 of screen time and who were in the highest tertile for physical activity, fitness or grip strength. Within-tertile HR trends, with p values for these trends also shown for each physical activity, fitness and physical activity strata. P-interaction indicates the p value for the interaction between screen time and tertile of physical activity, fitness or strength. CVD cardiovascular disease, CI confidence interval, HR hazard ratio, PA physical activity

Fig. 3

Cox proportional hazard models of the association of TV viewing with all-cause mortality, and incidence of CVD and cancer by physical activity, fitness and handgrip strength strata. Data presented as adjusted hazard ratio (HR) (95%CI). Reference category was defined as those participants with < 2 h.day^− 1 of TV viewing and who were in the highest tertile for physical activity, fitness or grip strength. Within-tertile HR trends, with p values for these trends also shown for each physical activity, fitness and physical activity strata. Analyses were adjusted for age, sex, ethnicity, deprivation index, professional qualifications, income, employment, smoking status, sleep duration categories, dietary intake (alcohol, red meat, processed meat, fruit and vegetable and oily fish intake), systolic blood pressure, prevalent diabetes, hypertension and medication for diabetes, hypertension, and cholesterol. Analyses were all performed as landmark analysis with follow-up commenced 2 years after recruitment and only including participants who were event-free at this time. Participants with comorbidities at baseline were excluded from all-analysis (n = 103,755). P-interaction indicates the p value for the interaction between T-viewing and tertile of physical activity, fitness or strength

Fig. 4

Cox proportional hazard models of the association of leisure PC screen time with all-cause mortality, and incidence of CVD and cancer by physical activity, fitness and handgrip strength strata. Data presented as adjusted hazard ratio (HR) (95%CI). Reference category was defined as those participants with < 2 h.day^− 1 of PC screen time and who were in the highest tertile for physical activity, fitness or grip strength. Within-tertile HR trends, with p values for these trends also shown for each physical activity, fitness and physical activity strata. Analyses were adjusted for age, sex, ethnicity, deprivation index, professional qualifications, income, employment, smoking status, sleep duration categories, dietary intake (alcohol, red meat, processed meat, fruit and vegetable and oily fish intake), systolic blood pressure, prevalent diabetes, hypertension and medication for diabetes, hypertension, and cholesterol. Analyses were all performed as landmark analysis with follow-up commenced 2 years after recruitment and only including participants who were event-free at this time. Participants with comorbidities at baseline were excluded from all-analysis (n = 103,755). P-interaction indicates the p value for the interaction between PC screen and tertile of physical activity, fitness or strength