[mikekchar]

The thing is that because of the 2nd law of thermodynamics, that energy does not go away when we use it. Moving muscles, pumping blood, etc uses energy and transforms it to heat. It's a bit weird that energy doesn't go away when we "use" it, but that's how the universe works :-) What can happen is that energy is locked up in chemical structures (or, I guess as physical potential energy). So if you evaporate water, the energy goes into the water and stays there until the vapour condenses. Things like that. But it's pretty easy to isolate a person and measure the amount of heat they produce (to a gross level like we are talking about here).

But, yes. Exercise will drive your metabolism and cause you to be warmer.

[covidSurvivor19]

Yeah, completely agree, no one is denying the 2nd law :) Let's just say biochemistry is quite complicated with myriad metabolic pathways. I find it plausible that the body does not necessarily use all calories ingested all the time. Some probably remain in the digestive tract.

[StavrosK]

It's easier to measure the volume of exhaled air and extrapolate calories from that, via CO2 expelled.

[usrusr]

Different fuels create different amounts of CO2 per joule though. This is not just a truism in combating climate change but also a key tool when observing metabolism. Whenever someone gives numbers for how much of some exercise energy consumption is from fats and how much is from carbs it is determined by measuring CO2 output and comparing that to measured energy output.

[StavrosK]

Oh huh, I hadn't realized that, thank you!