The oversimplified risk factor

Physical activity is a complex multidimensional behaviour whose health effects are determined by its intensity, duration, type (eg, aerobic vs strength training) and domain.4 GBD considered a single physical activity indicator that captures only total volume. This provides a stark contrast with how GBD treated other behaviours such as diet, which was addressed by 15 different risk factor indicators, including a diet low in seafood omega-3 fatty acids intake and low milk intake.2 One immediate consequence of such contrasting approaches was that low physical activity appeared to be a relatively simple, yet unimportant, prevention indicator.

Incomplete capturing of health risks

GBD 2019 calculated low physical activity relative risks using a single and relatively outdated (covering literature to early 2016) internal GBD meta-analysis of 174 studies,5 which addressed only five outcomes: breast and colon cancers, diabetes, ischaemic heart disease and ischaemic stroke. This is also counterintuitive considering the large amount of literature accumulated over several decades consistently linking physical inactivity to dozens of health outcomes, including 13 cancer sites.6 Such a breadth of health outcomes was reviewed by the WHO’s physical activity Guidelines Development Group in 2020,4 and the equivalent US 20187 and UK 2019 initiatives. By contrast, GBD 2019 included 44 new dietary meta-analyses covering a very long list of health outcomes. Moreover, GBD 2019 did not consider the causal influence of physical activity on key intermediate risk factors (eg, elevated blood pressure, body mass index), an omission which may have weakened further the disease burden position of low physical activity.

Black box methods?

GBD 2019 used intense, yet not clearly explained, data curation and analytical methods which are described in nearly 9000 pages of appendices.2 Online supplemental figure 1 summarises the 14-stage physical activity data curation. Other scientists and media workers alike8 have voiced concerns about this ‘black box’ approach as being non-replicable. Methodological opacity is amplified by changes in GBD standards over time, for example, the new Summary Exposure Value (SEV) metric introduced in GBD 2019 which essentially determines the ranking of each factor. SEV takes into account the extent of exposure by risk level and the ‘severity of that risk’s contribution to disease burden’. Using an undocumented method2 and imputed data for an unknown number of countries/territories, GBD 2019 used machine learning to harmonise different questionnaires’ data producing an SEV estimate of 3.5% for low physical activity. Although SEV is not directly comparable with conventional prevalence estimates, the 3.5% SEV value is 1/8 of the global aerobic physical inactivity prevalence estimates (27.5%)9 and 1/24 of the prevalence of physical inactivity when strength exercise is also considered (85.0%).10 In comparison, the SEV for the composite 15 dietary risk factors was 47%, a striking 13 times higher than that of low physical activity.

How realistic are the GBD relative risks?

The GBD estimates3 are based on the concept of population attributable fractions, which are determined by the prevalence of a risk factor and the risk factor-outcome association (eg, relative risk). Both metrics are determined by what analysts chose as the optimal reference category (eg, lowest-risk physical activity category).

The GBD use methods that deviate substantially from established thresholds for optimal and minimal physical activity: while it is not entirely clear from the published GBD methods, the optimal theoretical minimum-risk (reference) level of physical activity appears to have been set to 8000 MET-minutes per week (equivalent to 5.5 hours of moderate intensity physical activity per day). This amount is 10 times higher than the latest accelerometry-based data indicating that the optimal protection against all-cause mortality occurs at 800 MET-minutes per week.11 The GBD relative risk calculations involved comparisons of the 8000 MET-minutes per week reference level with the also GBD-defined low physical activity (<4000 MET-hours per week, equivalent to 2.7 hours of moderate intensity physical activity per day), which is nearly six times the WHO-defined minimum physical activity threshold4 for health benefits.

Potential consequences

The issues we outlined above collectively contribute to a significant, yet entirely implausible, downgrading of low physical activity as a risk factor in GBD 2019. Left undiscussed, such downgrading could weaken the well-established role physical activity plays in disease prevention,3 4 and decades of evidence building and translation efforts . We are particularity concerned that GBD 2019 may undermine WHO’s Global Action Plan on Physical Activity 2018–203012, by misleading clinicians and policy makers into trivialising physical inactivity as a health problem.