Fasting, or even skipping meals, sets you up to become extremely hungry at some point. You could possibly feel the effects of a fast (feeling overly hungry) for hours or even days after resuming eating. When this happens, foods you may have otherwise avoided may become even more tempting (and you're much more likely to overeat if you do consume them).
Fasting, and extreme dieting of any kind, can lead to a number of problems, including:
A slowed metabolism. Your body thinks it is starving and goes into starvation mode where it conserves energy and stores fat.
Energy dip. The body lacks fuel by being deprived of needed nutrients, leading to an energy level dip.
Mood changes. such as irritability are often a problem with any type of extreme dieting. Lacking particular types of foods (and nutrients) can lead to mood swings in a short time.
Health issues. Not regularly consuming balanced amounts of protein, carbohydrates, and fat can actually be harmful to normal regulation of body processes and functions. In very extreme cases, fasting can contribute to the development of kidney damage.
Muscle loss. Losing weight too quickly deprives your body of needed fuel and typically leads to loss of both water and muscle rather than body fat. True weight loss is fat loss; if you're only losing water (or worse, muscle), you're not really losing weight. When you return to normal eating habits, the weight you've lost -- and, often, then some -- will quickly return.
