Anyone know how they figure the calories in food?
The naive way would be to notionally "burn" the food, and calculate what the energy output of the combustion equation is (this could be done very accurately), and I wouldn't be surprised to discover that this is exactly how the number of calories advertised in food items is calculated. If so, I think you'd find that this massively overstates "effective calories", because our bodies don't burn things all that efficiently. From a "calorie counting" point of view, this needn't be a problem if there's a simple relationship between actual "calories in" to "calories available", because the studies the counting was based on would accidentally take the ratio into account (I think?). I would actually guess that there is something like this, just because calorie counting seems to work, even if the figures may be overstated.
What's interesting to me is more to do with how calories spent on effort would match up to calories consumed as fuel. What I've heard is that we don't have a tremendous peak power output. We don't "burn calories" terribly well. And yet you keep hearing of people's waistlines starting to reduce quite rapidly with exercise. If we're not burning more than the estimates suggest, how's that possible?
And for what it's worth, here's what I think: When it comes to the amount of energy we convert while doing exercise ("output"), we actually do burn what it says on the container. For instance, if I'm on my spinny thing and it says I'm putting out 150 Watts, it means that at the generator out there, the power output is exactly that. That's where the reading is done, and it can be done very accurately (I suspect they've managed to wangle things so that it flatters the user quite a lot, but that's another issue).
So when you say you burned 500 calories on a run, you're physically correct. That's exactly what the external measurements would say; however, when you say you "ate 3500 calories", you're probably skipping out the detail that you threw 2000 of that away. Maybe. It would be interesting to know.