This is actually pretty interesting point of view. In essence, you should try to breath out all the carbon you ingest. Measuring the carbon dioxide output would be a pretty accurate measure of the actual energy consumption in the body.
I wonder if you could proxy your carbon dioxide output off of your breathing, with some calibration? When I run, I breathe harder; am I expelling more or less carbon dioxide per breath than when I am sitting in a chair? Counting breaths seems pretty technically feasible.
Maybe your heartbeat could also work; that's also pretty easy to collect.
It'd be awesome to have a running total of how many calories I'm burning at all time.
Polar heart rate monitors give you an estimate of the burned calories based on your pulse. That can't be too accurate though, because some people have bigger hearts which pump more blood per beat.
