Calorie question

[duk]

This comes from one of the fat threads, but seems like it might be worthy of an independent line of discussion.

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A calorie is a calorie, the source does not matter----IT IS A MEASUREMENT OF ENERGY AND NOTHING ELSE. A calorie = a calorie. A watt = a watt. A joule = a joule.

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Calories are measured by the amount of energy required to heat water, right? So a donut produces 500 calories when they burn in a fire (or however these things are measured), but may produce a completely different number of calories when your body processes it.

It doesn't seem too far fetched that some sorts of food items may produce calories at different rates outside and inside the body. That is, it seems possible that what is measured to be "500 calories of donut" might produce 400 calories in one's body while "500 calories of steak" might produce 200 calories in that same body, no?

If this is true, then eating 2000 calories as measured by the external calorie measurement process might actually provide one's body with very different amounts of calories depending on the food one eats, and how efficient one's body is at breaking it down.

Or perhaps 500 measured calories of donut equals 500 measured calories of steak regardless of the process used to release those calories. Someone who knows this stuff tell me what's up.