Background: The current evidence for higher physical activity and better cognitive function and lower risk of dementia is strong but not conclusive. More robust evidence is needed to inform public health policy. We provide further insight to discrepancies observed across studies, reporting on habitual inactivity including that during work. Methods: We examined cross-sectional and prospective relationships of physical inactivity during leisure and occupation time, with cognitive performance using a validated physical activity index in a cohort of 8585 men and women aged 40-79 years at baseline (1993-1997) for different domains using a range of cognitive measures. Cognitive testing was conducted between 2006-2011 (including pilot phase 2004-2006). Associations were examined using multinomial logistic regression adjusting for socio-demographic and health variables as well total habitual physical activity. Results: Inactivity during work was inversely associated with poor cognitive performance (bottom tenth percentile of a composite cognition score); Odds Ratio (OR) = 0·68 (95% Confidence Interval (CI) 0.54, 0·86) P=0·001. Results were similar cross-sectionally; OR = 0·65 (95% CI 0·45, 0·93) P=0·02. Manual workers had increased risk of poor performance compared to those with an occupation classified as inactive. Inactivity during leisure time was associated with increased risk of poor performance in the cross-sectional analyses only.
Conclusions: The relationship between inactivity and cognition is strongly confounded by education, social class and occupation. Physical activity during leisure may be protective for cognition, but work related physical activity is not protective. A greater understanding of the mechanisms and confounding underlying these paradoxical findings is needed.