Tortoise or Hare? The Associations between Physical Activity Volume and Intensity Distribution and the Risk of All-Cause Mortality: A Large Prospective Analysis of the UK Biobank

Analysis methods to determine the optimal combination of volume and intensity of objectively measured physical activity (PA) with prospective outcomes are limited. Participants in UK Bi-obank were recruited in UK between 2006-2010. We linked questionnaire and accelerometer with all-cause mortality data from the NHS Information Centre and NHS Central Register up to April 2021. We developed a novel method, extending the penalized spline model of Augustin et al[1] to a smooth additive Cox model for survival data, and estimated the prospective relation-ship between intensity distribution and all-cause mortality, adjusting for overall volume of PA. We followed 84 166 men and women (aged 40–69) for an average of 6.4 years (range 5.3-7.9), with an observed mortality rate of 22.2 deaths per 1000. Survival rates differed by PA volume quartile, with poorer outcomes for lowest PA volumes. Participants with more sedentary to light intensity PA (<100 milligravities (mg)) and/or less vigorous intensity PA (>250mg) than average for a given volume of PA, had higher mortality rates than vice versa. Approximate hazard ratios were 0.83 (95% credible interval [CI]:0.79,0.88) for an average-risk profile compared to a high-risk profile, and 0.80 (95%CI:0.74,0.87) for a low-risk profile compared to an average-risk profile. A high- versus low-risk profile has the equivalent of 15 minutes more slow walking, but 10 minutes less moderate walking. At low PA volumes, increasing overall volume suggests most benefit in reducing all-cause mortality risk. However, at higher overall volumes, substituting lighter with more vigorous intensity activity suggests greater benefit.