Direct Calorimetry: Accurately Measuring Energy Expenditure in Human Metabolism?

[kbm]

I just have one quick question on using direct calorimeters on humans, and it might be a dumb question but its got me a bit confused.

From my understanding the calorimeter will measure the change in temperature of water from the heat that a person's body gives off.

I understand that when molecules are broken down by metabolism they give off heat as energy. My confusion is that, if all of a molecule's energy is given off as heat, then what energy is left for our body to use from it? For example, if someone burned 100 calories of energy internally, and the calorimeter detected that 100 calories were burned based on the increase in temperature, then it seems that all of the energy produced by the body is only being used for heating up the water and none of it is used for bodily processes.

So I guess what I am asking is, would direct calorimetry underestimate the amount of calories burned, because some fraction of the energy released is used by the body and another fraction is used to heat the surrounding water?

Any help and/or other resources to read on this would be appreciated

 

[jim mcnamara]

Direct calorimetry (especially for cardiology and exercise research and applications ) measures carbon dioxide production. It is measured in units called mets.

One met is defined as 1 kcal/kg/hour and is roughly equivalent to the energy cost of sitting quietly. A met also is defined as oxygen uptake in ml/kg/min where the oxygen cost of sitting quietly is equivalent to 3.5 ml/kg/min.