How are calories in food calculated?

Chris - I'll start with the simple aspect to this, probably good for me, which is that the way scientists work out how many calories are in food is quietly literally by burning it. You use something called a bomb calorimeter. And you put the food inside a container, you use an electrical element to ignite the food, and that's because you can log exactly how much electricity you fed to the element. So you can work out how much energy it took to get the food ignited. And you soak up all of the heat that comes out of the burning food by water, and you measure the temperature rise in the water. We know how much energy you have to put in water to make the temperature rise by a certain amount, so we can work at how much energy comes out of the food. So if you subtract how much energy you have to put in to light in the first place and how much came out at the end of the day that tells you the gross amount of energy you could get out of the food. But it is a bit more complicated, isn't it, Dave?

Dave - Yeah. The problem is that you don't absorb all the energy which you get from burning a food. So things like dietary fibre which you can't digest - it goes straight through you and you don't get energy out of it. Some things take more energy to digest than others. So, I think, actually when they work it out, they do it from a great big table of ingredients and they work it out. I'm not exactly sure how they work out how much food is absorbed from each ingredient.

Diana - Yeah. I think when we had Susan Jebb on to answer the question about how many calories are actually absorbed from food, she said that a lot of it does end up in your urine and your poo. And also that bacteria will end up digesting a lot of calories in your gut and that the amount of calories that bacteria will get through actually varies between people's guts because they have different species in there.