1 MET (Metabolic Equivalent) corresponds with RMR (Resting Metabolic Rate) that can be obtained quietly sitting. So why is Oura using 1.5 METs as the lower limit for active calories? What are the inactive calories between 1 MET and 1.5 MET?
During sedentary activities, your metabolic level increases from 1 MET to 1.5 METs. This addition gives meaning to the calories that we burn in everyday life while in sitting position, typically ranging from 6 to 12 hours of our 24-hour day. Sedentary activities can include typing on your PC at work, dining or doing other activities while not moving around. In contrast, active calories are connected with for example doing household chores and other physical activities or playing sports. Physical activities increase calorie burn to above 1.5 METs.
Active calories above 1.5 METs are typically associated with health-promoting energy burn, however, science has shown that the role of low intensities is smaller than higher intensities. Using 1.5 METs as the threshold for active calories prevents accumulating large amounts of active calories in low intensity activities. This increases accuracy with the expected health impact.
To summarize, sedentary activities accumulate total calorie burn, but they do not promote your health. Therefore, they are not included in active calories. Additionally, using 1.5 METs instead of 1 MET takes the low intensity activities' positive impact on health into account at a realistic scale in comparison to moderate and high intensity physical activities and sport.