Measured activity from wearable devices predicts mortality better than any questionnaire, and physical activity now has the cleanest new evidence in behavioural epidemiology. In every study, the largest gains come from moving from almost nothing to a little: a few thousand steps a day, a few minutes of brisk movement. The published estimates attribute five to eight per cent of deaths to inactivity, and the fraction rises with age and with sedentary work. That said, the surveys that track it are being replaced by devices whose wearers are healthier and richer than average, which flatters the trend.
Diet is where the evidence is broadest and least settled. The associations between dietary pattern and mortality are consistent across the large cohorts — more plants, less processed meat, less sugar — and the attributable estimates are large. That said, almost all of it is observational, self-reported and confounded by income. The live argument is ultra-processed food: whether the processing itself harms, or the fat, sugar and salt it carries. What has been shown to change diet at scale is reformulation under pressure: sugar levies halved the sugar in soft drinks in the countries that imposed them. The effects on weight are modest and mostly measured in children.
How far smoking has fallen and how many deaths it still causes, what vaping is doing to the people who take it up, and what generational bans will show.
