I'm sure you've heard firsthand, or even experienced yourself, a diet “failing." Unable to stick to the regime, the weight came back. An all-too-common story. The reason this failed was the reason it was started: it was a diet.
Dieting is to make a temporary change in the food you consume to reach a goal. The word “temporary” comes into play heavily here. They are inherently designed to give quick results. After all, the quicker the results, the more likely you’ll recommend the diet to the next person looking to drop 15 pounds before swimsuit season.
There are, however, a few downsides to this approach. The first is psychological performance. Completely restricting what you eat to reach a caloric deficit is mentally exhausting. This is called “Ego Depletion," and several studies describe its effects on the body. It boils down to this: The more conscious decisions you force yourself to make against your impulse, the less of this finite resource you have to spend. If you completely cut out all of your favorite foods, you’ll be running low on willpower quickly, and when you run out, you binge. Moderation is the key in this situation. Instead of cutting it out altogether -- eat the ice cream, but only one scoop instead of two.
The second harmful effect is the physiological changes. When you begin withholding specific macronutrients like carbohydrates, your body has to go to your energy stores to power the machine. The first energy source it looks for is glycogen stores. This is a chemical stored in your muscle that is quick energy -- unfortunately, it’s not very efficient. It's depleted quickly, and the body is forced to go elsewhere. Fat stores are next, but it’s not pulled from exclusively. The fat cannot be transformed into that glycogen that powers your muscles, but "ATP," the chemical version of protein, can. It's found in excess in your muscles, and will be processed for energy. Breaking muscle down to fuel your body is the worst possible scenario -- and it's bound to happen when weight loss occurs, but the faster the loss is, the more muscle you’ll be consuming.
Finally, general performance is harmed when rapid weight loss occurs. A study of 16 amateur boxers found a decrease in performance of every boxer as they reached their target weight for competition. A combination of both psychological and physiological effects. Limiting the body’s intake significantly, for instance, when attempting to make a weight class, significantly impacts your entire body. When doing it for sustained weight loss, the effects are almost certainly a rebound in weight.