Reacción a "Small and feasible changes in physical activity are associated with lower mortality risk"

This study has strong methodological quality, as it is a meta-analysis with individual-level data from several prospective cohorts in which physical activity and sedentary behavior are measured objectively using accelerometry devices. This approach reduces self-report bias and allows the dose–response relationship to be modeled. The focus of the work is not to demonstrate actual changes in each individual, but rather to estimate the potential impact of shifting the population distribution based on observed levels—for example, what proportion of deaths could be prevented if the least active individuals accumulated 5 or 10 minutes per day of moderate-to-vigorous physical activity, or if sedentary time were reduced by 30–60 minutes per day. This scenario-based logic is clearly laid out by the authors and helps convey that small goals could yield meaningful public health benefits.

That said, the article itself addresses the main caveats: although deaths in the first two years are excluded to reduce reverse causality, and sensitivity analyses are conducted excluding individuals with mobility limitations, the authors acknowledge that residual confounding or reverse causality cannot be completely ruled out in an observational study. In addition, they emphasize an important point for interpreting the message of “small changes”: exposures are measured only at baseline, so it is not directly observed whether these hypothetical increases actually occur and are sustained over time, which may affect the estimation of the effect (discussed as a limitation related to baseline measurement and the assumption of consistency).

Overall, the study reinforces a cautious message: moving a little more and sitting less is likely associated with lower mortality, especially among the least active, but the percentages should be communicated as potential impact under certain assumptions, not as an individual-level causal prescription.