You need a good amount of protein in your diet, otherwise the weight you lose will be muscle mass, and that begins a vicious cycle because the less muscle you have the slower your metabolism will work. That's why people on strict diets end up gaining the weight back quickly.
Also, cutting back on calories will slow your metabolism, you're better off doing a "zig zag" type of diet where you eat a reasonable amount of food every day and once every 3 or 4 days you eat slightly more calories to fire up your metabolism and trick your body. If you just eat the same low amount every day your metabolism will adjust and slow right down.