Background:
Insufficient physical activity is a leading global cause of premature mortality, yet how light-, moderate-, and vigorous-intensity activity differ in their associations with mortality and the implications of reallocating time across intensities remain unclear.

Methods:
We prospectively analysed individually pooled baseline device-measured physical activity from 147,414 participants aged 40–74 years in the German National Cohort (NAKO) and UK Biobank, the largest pooled accelerometer dataset to date. Multivariable Cox models estimated hazard ratios (HR) and 95% confidence intervals (CI) for mortality across activity intensities and under time-reallocation models holding total active time constant.

Results:
Over a mean follow-up of 7.9 years, 5,545 deaths occurred. Compared with no activity, 150 min per week of moderate-intensity activity was associated with lower mortality (HR: 0.70; 95% CI: 0.67, 0.72). Similar associations were observed for approximately 510 min per week of light-intensity activity or 12 min per week of vigorous-intensity activity. Additional activity beyond these levels was associated with further risk reduction. Replacing 15–60 min per week of light- or moderate-intensity activity with vigorous-intensity activity was associated with 18–30% lower mortality risk.

Conclusions:
Physical activity at all intensities was associated with lower mortality, with vigorous-intensity activity showing strong inverse associations even at low volumes. Shifting modest amounts of active time toward higher intensities was associated with further risk reductions. These findings provide quantitative evidence to inform the interpretation of physical activity recommendations.