When the body is starved of food, it starts burning fat in order to replenish the energy levels in the body. This leads to faster weight loss. However, if the fast is extended for too long, the body will eventually start burning down the muscle protein.
Most medical professionals agree that fasting is not the healthiest of all weight loss programs. It results in fluid weight loss, yes, but it does not result in a substantial weight loss. Also, when one stops fasting and resumes normal eating habits, one puts on weight as swiftly as he/she had lost it.
Fasting for a day or two is acceptable provided you have been consuming a wholesome diet and that you have not been suffering from any medical conditions. Fasting can lead to exertion of the immune system and is especially not recommended to people suffering from liver and/or kidney ailments.
It is in fact true that one requires more energy when on is fasting. The transitions that the body goes through during a fast depends upon the length of the fast and its severity.
Technically, the physiological processes experienced during a fast starts eight hours after having the last meal. When the gut has completed its job of absorbing nutrients from ingested food, the body understands that further consumption of food would be an excess.
Glucose is the main source of energy for the body. Thus, once the body recognizes the need for more glucose to burn, it extracts the stored glucose to replenish the body with energy. When the body runs out of stored glucose, it starts burning fat and then eventually, muscle protein.
When the fast is extended over days, weeks and months it is clearly a sign of danger to the body. The phase in which the body starts burning the muscle protein is known as starvation and not fasting. Without doubt this is an extremely unhealthy state for the body and makes the person frail and weak. Also, when one fasts, one should eat nutritious food and a lot of fluids during breaks between fasts.
FROM AWARD-WINNING AUTHOR DR GOVIND SHUKLA, NUTRITION EXPERT

Govind Shukla, Specializes in Pharmacology, Toxicology, Nutraceuticals & Herbal Drugs has published More than 100 research papers in National & International Journals. He is also a reviewer of International Journal of Pharmacology & pharmacotherapeutics, Chief editor of IJPNR Journal & Freelance Medical Writer for Different publication Groups including Lambert Academic Publishing Saarbrucken, Germany.