Independence of physical activity and screen time as predictors of cardiorespiratory fitness in youth

BACKGROUND: Screen time (ST) is associated with physical activity (PA) levels. We aimed to investigate the relationship between ST and fitness. METHODS: We assessed fitness and daily ST in 7,466 10-16-y-old schoolchildren from the east of England. The differences in fitness between ST groups, and the association between ST and fitness were assessed. RESULTS: ST was negatively associated with fitness in boys of all ages (P < 0.001) and girls in grades 6-9 (P < 0.005). Boys were less likely to be fit if they reported >2 h/d ST (odds ratio (OR): 0.70, 95% confidence interval (CD: 0.58-0.85) or >4 h/d (OR: 0.45, 95% CI: 0.35-0.57) ST, as were girls reporting >4 h/d ST (OR: 0.58, 95% CI: 0.43-0.78). Controlling for PA levels attenuated these odds in boys reporting >2 h/d ST (OR: 0.81, 95% CI: 0.66-0.91) or >4 h/d ST (OR: 0.65, 95% CI: 0.50-0.84) and in girls reporting >4 h/d ST (OR: 0.68, 95% CI: 0.50-0.93). CONCLUSION: These first data from English children show a negative association between ST and fitness in youth. Of note, very high levels of daily ST (>4 h) are associated with poor fitness. Some of the associations were mediated by PA levels. Our data support international recommendations to limit ST to <2 h/d; we believe such guidance should be issued in the United Kingdom.