High Protein / Low Carbohydrate: 

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. 

Glucose is stored in the liver as glycogen and when the body does not receive sufficient carbohydrates to produce glucose, the glucogen store is utilized. Unfortunately this glycogen store is quickly depleted and the body then sources its glucose from muscles-tissue in an attempt to sustain the vital organs.  The body can not use fat for this function since fat can not be converted to glucose. If this diet is maintained, it will inevitably lead to severe muscle tissue loss and damage of organs.

Upon termination of this diet, the body will eventually replenish all the lost fluids and the dieter gets very discouraged, since they will note a substantial weight gain.