Dear /sci/

I've been told numerous times that we do not use food for energy alone. Cells are made out of fat and proteins, and a certain percentage of our food is used not as "building material".

However I cannot find any specific numbers. The closest thing I can think of is that the BMR account for this "building material" but would have no idea how to figure out what percentage of food that is.

Anyone know how to get more information about how our body uses food?