Knowing what you know about "calories in versus calories out" you will start to eat one piece of dried toast for breakfast, a small bowl of fruit for lunch, and a protein shake for supper. This total is going to be between 1/4 to 1/2 of a 2000 calorie diet - for your entire day! If you are averaging greater than 2000 calories per day, that reduction is even more.
You are starting to notice the pounds come off, which is making you happy. However, your appetite is not happy, after a few days of a diet that is drastically different than what you were accustomed to eating.
If it has been around the week mark and you go back to your normal way of eating, you have probably not altered your metabolic rate much. You will probably see a gradual weight gain if you haven't increased any in your exercise levels.
However, let's presume that you have begun this regimen and it has continued for a few months, and you are at or near your target weight. You can now fit into the clothes you wanted to wear or look the way you wanted to look. So you are going to return to that previous way of eating or at least increasing what you are currently eating significantly.
You have been training your body to be accustomed to a certain number of calories and have caused your metabolism to be reduced. Now that you are going back to the pre-diet diet, you are going to end up gaining the weight back and you are at a greater risk for being heavier than you were, depending on your calorie intake and your exercise regimen.
Metabolic rate will not alter much in a short amount of time; however, given a greater span of time, we can reduce that rate. Thus, when your diet is now given greater calories to burn, your body will have a high chance of not being able to function in a way to burn all of calories you are giving it. This will result in a likely weight gain. After buying your new wardrobe, you may not be able to fit into that wardrobe.
I have seen this exact scenario in people. Their bodies are trained in one lifestyle, they drastically reduce their calories, resulting in weight loss. Then they return to some or all of their previous calorie intake and gain the weight they were happy to lose.
There is a lot which goes into keeping weight off. Part of it is consistency, determination, commonsense, exercise, and proper diet.
For the normal lay person, 1-2 pounds per week is healthy to lose. If you have a high body mass index (BMI), you may see those pounds be higher in the first few months. But with everyone there will be a plateau period where you will not see the pounds come off like you would prefer. Be patient and look at what may be inhibiting the lack of weight loss. Outside factors, aside from "calories" do impact your body. Stress, for example, can inhibit the body from losing weight. For women, the menstrual portion of the month may also have an impact. Don't become too frustrated, just be patient and sensible.
Next time we will focus on weight loss inhibitors...such as stress.