[covidSurvivor19]

Your analysis seems to be in the right direction but I think that the body uses this energy in many more ways than just heat. Moving muscles, pumping blood, keeping ionic gradients in the kidneys, and in cells, producing waste products, do seem to take energy. Also, consider the water we lose due to evaporation just by breathing, we are constantly creating a significant amount of heat and dissipating it into the environment. This extra 1000Kcal would be roughly a 50 watt increase in average metabolic rate during a whole day.

It does seem a bit high but anecdotally, my body temperature is 1C higher on days I do some tennis early in the morning. And more importantly my oftenly cold hands go away, so radiating more of it away.

[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!

[samatman]

> This extra 1000Kcal would be roughly a 50 watt increase in average metabolic rate during a whole day.

This calculation makes the theory implausible. A resting human burns about 100 watts[0], this would imply 50% more energy output than that, over the whole day.

I too find that I'm warmer after a workout, and sweat lightly for up to a few hours after my heart rate returns to normal, which itself takes almost an hour. So burning more energy than the workout itself? Absolutely.

1000 calories extra seems high for ~4 minutes of cycle sprinting per hour. The HIIT pacing of it is probably ~just as good as spending the whole 4 minutes cycling steadily though.

All energy leaves the body as either work or heat, so of the processes you mentioned, moving things or yourself with your muscles is the only one that wouldn't raise temperature.

[covidSurvivor19]

I'm not denying the 2nd law, but metabolism is more complicated than just heat or mechanical work. One example, the process by which the body transforms ammonia into urea, requires energy. Not 100% sure of the details but some of this energy might be captured chemically, not sure if urea has a higher (free energy? I don't remember the exact term). Furthermore with this example, since urine is hyperosmotic, the body has to do a process against normal ion/water diffusion, in order to excrete minerals and not excessive water. This osmotic pressure, contains some potential energy.