How do you find out energy required per day?

[Avichal]

Human body needs about 2000-2500 calories per day. How do you find out this number?
What sort of experiments do people do to figure out this number?

Also, in general how do you find things like - energy spent while walking, talking, blinking? It almost seems impossible to find out these things but I find numbers in books.

 

[mfb]

Some things you can try:
- measure the produced heat of the body during those activities (and at rest)
- measure the required oxygen, as this is related to the consumed energy
- measure the required food (long-term measurement)
- use some model of the muscles to find an estimate
- probably some concepts I forgot or do not know

 

[Avichal]

I find it extremely hard to believe that you can actually calculate energy we require for doing activities. There are just too many things involved. Does this actually work?

 

[mfb]

It is possible to find approximations. The actual power/energy will vary a bit between different persons, details of the activity and other circumstances. And you always have a measurement uncertainty, of course.
It is just interesting how precise those values are. To find some values is easy.

 

[Avichal]

Just for the sake of an example, say I want to calculate energy it takes for an average man to climb 30 stairs (30 steps).
I just calculated using E = m.g.h. Will that be a correct approximation? Why/why not?

 

[mfb]

It is a lower limit. The human body is far away from 100% efficient.

 

[Ryan_m_b]

There will also be other complications like the fitness of the person in question. An unfit person will take longer to return to a basal (resting) metabolic state so in addition to the energy expended climbing the stairs there's the period after characterised by faster heart rate, heavy breathing etc.

 

[Borek]

How is it different from measuring heat of any process? Technically you just use a larger calorimeter (think thermally isolated room). I believe that's how the early experiments were designed.