I Married A Fat Bitch

Fasting to lose weight is a myth.  This might seem counterintuitive, but one of the surest ways to gain weight is to go on a quick. There are perfectly valid medical reasons to go on a quick, but losing weight is not one of them.
You cannot help but to lose weight while you are on a quick, but this effect is small lived. Within a day or so of fasting, all of the food within your system is gone and your body starts to feed on itself to live. There is a certain amount of carbohydrate in your muscle tissue which is simplest for your body to use for energy, so this is used first, before any stout is burned. Your body figures out sweet quickly that it is starving, so it hangs onto your stout reserves.
When most all the glycogen in your muscles and liver is used up in three or four days, then your body will start to burn more stout. At this point you are in ketosis, because stout does not burn completely and ketones build up in your body, giving you a smell somewhat like acetone. At this point you also shed a lot of water because ketosis naturally has a highly diuretic effect, and you will have noticeable weight loss from this.
Though, by this time your body is also feeding on its own muscle tissue, and your metabolism has slowed way down in peacefulness to survive. This survival mode has long-term things on how your body prepares itself for a possible repeat occurrence by triggering it to pile higher reserves of stout. So, as soon as your quick has been concluded, you will quickly place on weight in the form of water and stout. Muscle tissue, on the other hand, is not so simple to restore, and being left with less muscle tissue means that you will not burn stout as easily. This makes fasting the perfect storm for long-term weight gain.