Part of the show Naked Science Q & A and the World of Chemistry
Nigel in Peterborough asked:
Is there a reason for cyclic weight loss?
When people begin to diet and lose weight they lose enormous amounts of weight in the first few weeks, and then their progress slows. The reason for that is that the body initially burns off things called glycogens. That's a polymer of sugar, and it binds enormous amounts of water. Which means that when you burn that off and use it for energy, you lose the water from the body as urine. So you lose double the amount of weight - sugar weight plus water weight. Then you start to burn fat and of course fat doesn't mix with water. So the weight loss slows down. Every so often if you have a bit of a binge, or it's Christmas or whatever, and you pack in some carbohydrates, they tend to turn into a bit of glycogen and your weight creeps up a tiny bit. Then you go back on your severe diet and carry on burning fat and the first thing your body does is to say I'm getting hungry, I'm going to burn that bit of glycogen. So it does that, you lose the water again and you're back to how you were when you were just burning fat.