So your body runs on glucose. Glucose belongs to a class of chemicals called carbohydrates. And before we go any further we must look at what carbohydrates are and how they work.
Carbohydrates are very sensibly named – as their most basic structure is carbon + water. That is, their basic formula is CH2O, and every carbohydrate has this basic formula, but only in multiples of 6. So glucose is C6H12O6.
This 6 – carbon unit is the most basic structure possible, so it is referred to as a monosaccharide.
Table sugar (sucrose) is C12H24O12 and looks like this:
So sucrose is a disaccharide, and is composed of two monosaccharides.
And the sky’s the limit – saccharide units can be added together to infinity, like chemical lego blocks. An example of a polysaccharide is starch:
So a complex carbohydrate like starch is made up of many glucose units. Before your body gets fuel, therefore, it must break these complex molecules into its component glucose molecules.
But the real question is – how quickly does this happen? If the breakdown happens quickly, then your blood is flooded with glucose. This can cause a serious medical condition called hyperglycemia, so your body removes the excess glucose by releasing insulin into your blood from your pancreas.
This has two long-term effects – firstly it causes you to put on weight, as the excess sugar is converted to fat. But it can also cause you to become a diabetic, as your pancreas eventually gets overloaded and just gives up – this is why fat people are often also diabetic.
The rate at which a food releases glucose is referred to as its glycemic index (GI), and is the single most important factor in determining whether the food is fattening or not.
Now, this results in some weird outcomes. For example, look on the label for Nutella and you’ll see it’s loaded with sugar and fat – but it’s low in GI. And the reason is simply that the fat slows down the rate at which the sugar breaks down.
And this is a pattern – often the fibre in something slows down the sugar absorption rate, so you are far better, for example, eating whole fruits rather than fruit juices.
For further reading, have a look at Eat Yourself Slim, which explains all this in fine detail