I would guess that the heat chemically alters or breaks down (denatures might be the right term?) some of those macronutrients into molecules that can no longer provide caloric energy.

“Calories” measures the amount of energy a human can extract from the food. Interestingly, if you cook.a steak (for examples le) it can gain calories, not lose them.

Does it really? I thought it was a simple chemical test where you burn a set amount of it and check how much it heats a given amount of water.

That's the old, less accurate way. By that measure, fiber tends to have quite a lot of calories, as it's a carbohydrate, but that is now known to be wrong - fiber passes through our digestive systems almost completely undigested, and yields almost 0 calories.

Today, calorie counts are usually obtained by checking the protein, fat, sugar and fiber contents, and using known values for calorie/g of each. Those well-known values are ideally obtained either through human calorimetric studies, or through burning protein, fat, sugar directly.

Citation for the stake numbers? I can see it being possible, if cooking increases bioavailability

https://news.harvard.edu/gazette/story/2011/11/why-cooking-c... Is one.

The chemical breakdown of bonds also generates thermal energy, so the milk emits black-body radiation slightly more than input heat alone accounts for.