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Are Physical Activity Guidelines Evidence Based?… No, they are a Value Judgement by Scientists!

Another study has been published this week that shows that physical activity improves health through the proxy measure of reducing all-cause mortality risk (risk of death). It compares all-cause mortality risk among those who exercise at or above the government’s guideline level of 150 minutes of moderate intensity physical activity per week, with the all cause mortality risk of those who exercise below this level. However, what this study does not tell us is how great the health benefits are at different levels of physical activity – for that we need to look at a dose-response curve:

ALL CAUSE MORTALITY CURVE

This curve shows us that, compared to those who exercise for 30 minutes or less per week, exercising for 60 minutes per week reduces risk of all cause mortality by 15%, exercising for 90 minutes reduces risk by 20%, for 150 minutes by 25%, and so on up to 420 minutes (7 hours) which reduces risk by 38%. This is the evidence that informs physical activity guidelines in the USA, UK and Australia.
 
However, the decision to recommend 150 minutes as the “minimum required for substantial health benefits” is not based on scientific evidence, rather it is a value-judgment made on our behalf by the medical scientists that advise governments that a 25% risk reduction is what we (the general public) would consider to be a substantial health benefit.
 
But what if we consider a 20% risk reduction to be a substantial health benefit? For just a 5% lower risk reduction we could reduce our weekly amount of physical activity by 40% from 150 minutes to 90 minutes, or if a 15% risk reduction was good enough for us, we need only exercise for an hour a week.
 
Who knows, if the guidelines were set at 60 minutes, then perhaps this would be seen as a more achievable target by the least active, and we might actually succeed in shifting the curve of physical inactivity, something that the value judgment that has set the current guidelines has singularly failed to do.
 

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