The reason this doesn't work is because your body becomes used to needing so many calories and carbs every day. Although cutting down to a healthy amount is the goal, it's best to step down gradually. If you don't, your body thinks you're starving and holds on to your fat for storage in fear that it's a famine. It's a natural body response for survival. This is what messes up your metabolism really bad is going on fad diets that drop your calorie intake.

For example, if you're eating 3000 calories a day, then cut out 500 calories for 2 weeks. From 2500 cut down to 2000. Now, everyone should calculate their "ideal" calorie intake by body size. Men need more than women. That "2000" caloriie diet is too "general." A short woman doesn't require as much as a very tall man.

You should never drop under 1000 calories a day.

Also if you cut carbs too fast your brain will crave them. Carbs are the primary fuel source of the body and brain. They are not bad. You just need to learn to eat healthier carbs rather than over processed refined carbs. 45 carbs per meal is about right for a healthy diet. But, most people are used to eating way more than that. If you cut all of them out at once you go nuts. Yet another reason fad diets don't work.

The final stage is you have to add exercise. Think of your body as a bank in reverse. Instead of trying to "save" calories, you want to burn them. Getting up and moving around is the #1 way to increase weight loss and prevent type 2 diabetes. If you diet and don't exercise you won't see a big change. Movement is key!