The diagram displayed below comes from a meta-analysis published by The Lancet in 2016 titled Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality?. Properly understood, it is all the motivation you’d ever need to exercise.
How to read the diagram
In order to read it, you need the following definitions:
- Hazard ratio: A measure of the relative risks of death in two populations, A (the reference) and B.
- Metabolic Equivalent of Task (MET: A normalized measure of caloric expenditures for various task.
- Hours/day: Corresponds to hours of sitting (or more generally of activities under 3 MET/hour, see below)
Assume that you want to evaluate the relative risks of death in two populations, A (the reference) and B, over a given period of time, you would:
- take the death rate of A over a period of equivalent length, normalize it to 1;
- take the death rate of B over that same period, and normalize it relative to that ‘1’;
- Compare then numbers.
Population A has the reference hazard ratio (HR 1) to which population B is compared. If the HR of B is higher than 1, a member of B is at a higher risk than a member of A.
A value of 1 on the MET scale is equivalent to an energy expenditure of:
1 kcal per kilogram of bodyweight per hour (1 kcal/kg/h)
It is roughly equivalent to the metabolic cost of sitting quietly for one hour (which is quite ironic, when you think of it). The MET acknowledges only 3 intensities:
- Low-intensity physical activity (MET≤3): walking at or around 3 km/h (shopping, commuting, walking in a house or at the office); biking or around 12km/h (commuting, leisure biking).
- Moderate-intensity physical activity (3<MET≤6): brisk walking at or above 5.5 km/h or more(fast military march) or leisure biking at 16 km/h (both MET 5); walking at 3-5 km/h while carrying a load of 10-20% of one’s body weight (groceries, school books, etc.).
- Vigorous physical activity (6<MET): sprinting, bicycle racing, backpacking with a load above 25% of one’s body weight (more than 20 kg for most people).
MET values can be accrued and are often specified as MET-hours (MET-h) over a given period and sometimes as MET-minutes (divide by 60 and you get the MET-h, but see A Cut-Off below.
In the study, they are hours of sitting.
But since what really matters is the metabolic cost, and since the metabolic cost of standing at a desk (or sitting in a car, etc.) is the same as the metabolic cost of sitting at a desk, you might as well count hours of standing at a desk (counter, cash register, etc.) as well.
The study recommends 35+ MET/hours per week to anyone who sits more than 4 hours/day, which is twice as much as currently recommended by the WHO. Since MET-hours can be accrued it may seem that 24 hours of sitting at a desk (1.5 MET) would accrue the required MET-hours.
Unfortunately, in practice, activities with an expenditure under 3 MET-hours are not intense enough to give health benefits. For instance, while the practice of long walks has taken up in the U.K., people are not getting enough exercise from long walks.
A visual summary
The following diagram sums up the above information. Patreon supporters of The Older Avocado (who provided the illustration) can access a hi-resolution version here.