The theory behind most of these diets are based on the principal that Carbohydrates are the elements responsible for weight gain – thus by eliminating or restricting the Carbohydrate intake, you loose and maintain weight. You are thus allowed no or little starches (bread, cereals, etc.) 

Will you loose weight: Yes, but it is not fat that you loose. Initially the weight loss is attributed to severe water loss. This is followed by the loss of lean muscle weight, which again would show up as weight loss. Remember that muscle weighs more than fat, so the weight loss looks impressive, but you are not loosing what you are aiming to loose, or should loose.

Effects on the body: Carbohydrates, as mentioned before play an essential role in providing the body with essential energy for normal operation. The energy carbohydrates provide us with, is called glucose, which is the only energy the brain, heart, liver and kidneys functions on.