Are all calories created equal?
Are all calories created equal? Is a calorie from a chocolate bar worse than a calorie from some celery?
Giles Yeo answers this question...
Giles - Okay, so what is a calorie first? A calorie is the... it's not an SI unit, SI units are in joules, but a calorie is the amount of energy it takes to raise one litre of water one degree Celsius at sea level. So in a sense, a calorie is a unit of energy, so all calories are equal. But - and here's the but - if you actually take calories in food, we cannot extract every single calorie from every type of food that we eat. It's down to caloric availability. So three examples, which I use - if you have a hundred calories of sugar, if you ate a hundred calories of sugar, you would get a hundred calories out, pretty close to that. Whereas if you ate a hundred calories of sweetcorn and then you peeked in the loo the next day, it's clear you haven't absorbed anywhere close to a hundred calories of sweet corn. But yet if you take sweetcorn, you desiccate it, you turn it into a corn meal and then convert it into a corn tortilla, suddenly a far greater percentage of the calories are suddenly available to you. And so in that sense where a calorie comes from actually matters, even though when you've absorbed the calorie, they're all equal.
Chris - Therefore, when people talk about the glycaemic index of something, what does that actually mean and how does that translate into good diet practice?
Giles - So the glycaemic index is the speed at which the blood glucose, your blood glucose levels, go up after a specific meal. And it's based on the maximum, so the 100% is sugar because obviously it's pure glycaemic, okay, so you eat it and it goes up. So that's set at a hundred. Everything else is then based on the time after that. And so something which is going to be high in refined carbohydrates is gonna have a high glycaemic index; something which is higher in things like fibre or starches will have a lower glycaemic index. But the interesting thing is you can't just look at it in isolation, because the glycaemic index, the problem with that, is it looks at individual foods. Whereas if you actually mixed foods, say for example if you fry something very oddly and you introduce the elements of fat, fat slows down the release of sugar. So it lowers the... for potatoes, for example. A crisp will have a lower glycaemic index than a boiled potato. So I think just looking at the glycaemic index for health is not necessarily a good thing, because no one here is going to argue that a crisp is healthier for you than a boiled potato.