You have to burn all the calories you absorb, which depending on a variety of other factors can be different from the number of calories released by burning the food and measuring the heat.
That's only because we eat so much of it. And calories in, calories out, is physics. Conservation of energy. You cannot gain weight unless you eat more calories than you burn; and you cannot lose weight unless you burn more calories than you eat. Anything else is magic.
If you think [that] a calorie of broccoli, a calorie of steak and a calorie of coke are the same, I'd say you are lost.
Straw man. I never said the foods are the same.
There are many differences in foods and their nutritional content. And there is much more to health than just weight. We need a lot of different nutrients for health. But to lose weight you have to burn all the calories you eat and a few more.
Even when we just focus on caloric balance and weight gain/loss, the kinds of foods you eat can affect the absorption and utilization of calories in more complex ways than might be expected by simply counting calories and keeping track of the average amount of energy various exercise activities require.
I sent an email to the SGU asking about exactly this issue, after a friend of mine argued that absorption and ingestion are different numbers. To my surprise, I got a personal reply from Dr. Novella. His comment was that this is true, but that the difference is small.
Further, I don't think that the medical community is so naive as to count the calories in, for example, cellulose, which some animals can digest but humans cannot. Individuals differ in their basal metabolism, and in their level of exercise, and both of these count in the equation. But you still have to burn the calories you absorb, and any difference between this and the calories you eat is not large enough to make much of a difference.
I believe the woo community has a lot of notions that eating certain foods will prevent you from absorbing the calories, but except for materials we know are not digestible, this idea doesn't fly.
I reject the notion that doctors, as a class, are too stupid or too stubborn to look at all the evidence, including the evidence from nutrition science, and analyze it more accurately than untrained laypersons.