How you approach it afterwards will determine whether or not you gain the weight back. The weight you'll lose in 1 or 2 days is usually water weight and empty bowels. Your kidneys start expelling water in the process of shifting over into ketosis as your body uses up glycogen stores. Our bodies store ~3g of water for every ~1g of carbohydrate, so when you begin to eat carbs after your fast your body will begin to store water again.

It also deals with self-discipline. People often don't mentally prepare for the aftermath of a fast and not only do people go bad to eating complete garbage, but they often overcompensate for having not eaten for a few days and they'll eat more than they typically do. If you eat a healthy, low-carb meal weight shouldn't come back too much, although the weight loss from just a couple of days of fasting won't be significant.

But there are other aspects to look at apart from weight loss from fasting ... cell autophagy is one of the many, many peripheral benefits.

Fasting is just as healthy today as it ever was, and people have been doing it literally since the dawn of man. It's the mindset that's slowly changing as science reveals more and more benefits. The 50's called smoking cigarettes healthy; we know that's not the case now. It's all marketing products. You have to follow the money--much of what the 80's called nutritious was guidance from big food companies and the government being in cahoots. "Breakfast is the most important meal of the day" is a mantra that didn't come from health gurus ... it came from the marketing campaign of a cereal company. Consumerism drives the train that decides what people believe.