The many diets out there, one known for the famous Atkins diet is a diet created in 1972 by Robert Atkins and which some people have used to lose weight quickly (but not say anything if it becomes to recover once it has left the diet).
This diet is only feasible to do for 30 days and always under strict control of a physician. Diet is dangerous for the body because it does is replace carbohydrates by proteins. In particular, the diet is based on protein intake for the body to convert them into energy (which converts to carbohydrates when consumed) once they are no longer in the body.
Foods that are allowed on this diet: red meat, eggs, cheeses, seafood, mayonnaise, sweets. But we have to eat very few carbohydrates commonly found in: vegetables, pasta, flour, rice, bread, sugar, milk, etc..
As you see, is a diet very different from normal diets can actually eat it eat everything, or at least as much as you like. Yes, the diet can not be made for more than 30 days because the body suffers a lot in the process.