Q & A: Is it true that fasting to lose weight can ultimately make you heavier than before you began dieting?
While it seems to make intuitive sense that fasting is the quick way to lose weight, it actually makes it a bigger challenge to keep weight off. Under normal circumstances the body uses glucose, the breakdown product of carbohydrates and fats, for fuel. The liver only stores enough glucose for one day. When you fast or severely restrict your caloric intake for an extended period of time, your body goes into "starvation mode." This means that your body begins to store fat instead of utilizing it for energy, and as an alternative begins to break down muscle mass to produce glucose for energy. The amount of muscle in your body impacts the rate at which you burn calories (your metabolic rate). The more muscle mass you lose, the more ineffective your body becomes at utilizing calories from fat, and instead starts to store them as body fat.
The longer the fast, the slower you will burn calories. As soon as your fast is over, you will gain weight faster than you would have previously – this is your body’s attempt to protect against another "famine." Those extra pounds will likely come back as stored body fat!