How efficiently do we use calories?

We put this to Giles Yeo, from MRC Metabolic Diseases Unit...

Giles - It's a very good question. In essence is every calorie a calorie? From a physics point of view, clearly a calorie is a unit of energy so every calorie is a calorie.

Kat - Could we just unpack what is a calorie?

Giles - A calorie is the amount of energy you need to raise 1 litre of water by 1 degree. That's correct - I'm getting the thumbs up.

Kat - So it's literally how much energy is in this particular amount of food?

Giles - Yes.

Kat - So that's calories. So a calorie is a calorie, is a calorie, or not a calorie?

Giles - A calorie is a calorie once you actually get access to it. It's all about caloric availability. Let me give you the example: 100 calories of sugar - you would get 100 calories out of it because it's our base fuel. 100 calories of sweet corn - now anyone who's ever sweetcorn, and given the topic of the conversation had a peak down the next day can see that you do not digest 100 calories of sweet corn. If you take that sweet corn, however, and actually dry it up and impound it and make it into like a cornbread, then you get a lot more accessibility. So how you actually treat the calories, the type of calorie, and in what form it is, gives you different caloric availability and, therefore, people think different calories differ, which they do from that perspective.