Human "Technology"

I have been bothered for a while by the calories in/calories out model of weight thinking. I have finally articulated at least to myself why I have difficulties:

• My scale doesn't weigh "calories" it is all about mass.
• Energy seems to be a proxy for weight (and no I am not in the e=mc^^2 realm of thinking because that deals at entirely the wrong level!)
• The energy in/energy out doesn't account for a few things (which I will detail out in the body of this posting)

So, here's the fundamental set of thinking. Please comment and point out the holes.

Taking the assumption that I am interested in what the scales say, I should be thinking in terms of the total inputs/outputs. Even without understanding any digestive processes.

Weight at any point in time = weight at previous point in time + weight of all inputs since that time  - weight of all outputs since that time.

It's the all inputs/all outputs that we need to worry about.

At the simplest level the inputs consist of:

• All solids (food)
• All liquids (drink)
• All gases (air breathed in, water vapor in the air, etc.)
• Supplements

At the simplest level the outputs consist of:

• All solid (fecal) material 
• All liquid (urine) material including dissolved solids
• Vomit
• Sweat
• Respiration products (gases, especially Nitrogen and Carbon Dioxide, but total of all gases)
• Water vapor in respiration products
• Anything else we can think of (dead cells, sputum, tears, ear wax....)

It's no wonder we use energy as a proxy!

Now for the area where I have trouble with energy in/energy out. There is a move afoot to extract energy from human waste. That must mean that there is some residual energy in some of our outputs. Especially, I suspect in our feces. But maybe elsewhere too.  So the big questions for me are:

• How much of the available energy is in our waste products?
• Does the amount of available energy in our waste depend on our eating habits? If so how?
• Does the amount of available energy in our waste depend on any conditions (eg Celiac disease) we have? If so how much effect?

Any thoughts/comments gratefully received.

Chris