Are all the calories in food absorbed?

We put this to Susan Jebb from the Medical Research Council's Unit of Human Nutrition Research in Cambridge.

Susan - Not all of the calories that are actually in a food will be absorbed, digested and available for the body to use. What happens is that the calories which are in food, which will be released if we were just to burn it, as we might do in the bomb calorimeter in the laboratory, those calories cannot all be absorbed by the body. Some will be lost in the faeces and the remainder will be digested. About, perhaps, 10% of the total calories we consume might actually appear at the other end of the gut.

Once calories have been absorbed, again, they're not all fully available. Some will be lost in urine for example. The final loss of calories happens because some of the energy is fermented by the bacteria in the gut and so, it's not available to humans. It's actually burned off by the bacteria that are living inside us.

And so the consequence of all of that is that not all the calories that are actually in the food will be available for the body to use. But in fact, the losses are proportionately quite small. The calories that you see written on the back of a food pack have already had all of these adjustments made for the amount that will be digested and absorbed.

And so, the calories you see on the packet is actually not the total calories in that food. It's the so-called metabolisable energy, the amount of energy which is going to be available to the body...