I'm aware that metabolism can be measured in multiple ways, using both direct and indirect calorimetry. It can also be approximated using a combination of bodyweight and other measures, like with the Penn State equation.
However, these methods require expensive and cumbersome equipment. Is there a way to (actually) measure metabolism using simple tools like a weight scale, an electronic thermometer, and a stethoscope (heart-rate)? And how do you know that these measures are not affected by transient effects, like an increase in body-temperature as a consequence of infection?
According to this text (that I have not verified) the human body loses heat through conduction, convection, evaporation and radiation. The proportions seem to be 5%, 15%, 20% and 60% respectively. So, in order to measure the metabolic rate efficiently, I suppose it would be enough to detect the level of radiation (approximating by black body radiation) emitted by the body. This could potentially be done with infrared sensors I imagine. Any ideas?
According to this study: Body Temperature Measurements for Metabolic Phenotyping in Mice, the relationship between body-temperature and metabolism is not always direct. For example, mice that had a mutation, had increased heat loss and lower body temperature because of failure of the vasoconstriction mechanism in the tail, yet, at the same time, they had increased metabolism and brown fat activity, as a compensatory mechanism to achieve euthermia. Infrared sensors can be used differentially to assess BAT (brown addipose tissue) thermogenesis.
So it seems not even body temperature is a good measure. Whole body metabolism has to be related to ATP and its waste products. By measuring all of them you should be able to estimate the metabolic rate.