Calorimetric detection of BMR and ppTh

The basal metabolic rate is best determined by calorimetric methods. There have been used two different approaches to measure BMR, depending on the available instruments. In every case the subjects are instructed or enforced not to eat overnight. In case of a canopy or mask system the su ojects are connected 

Subjects not explicitly enforced to rest and relax within the measurement of ppTh do perform some locomotor activity. Even instructed subjects have considerable difficulties to stay relaxed but awake for periods of 6 to 8 h without performing some physical workout. So there is another approach in which it is assumed that BMR and the uncontrolled locomotor activity of subjects are comparable in days with or without food. The ppTh is then calculated simply from the difference of EE on a fed and an unfed day. 

Some empirically interconnections have been found between locomotor activity and energy expenditure in free living subjects of a given lifestyle. But these connections are too weak to be used in calorimetric experiments. They are often used in balanced food intake experiments or studies performed utilising the doubly labelled water method. 

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