[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.