How quickly will I lose 50 pounds?
That is quite impossible. You would most likely die after 3 weeks on that diet-the average woman needs 2000 calories at least and younger people need more. I recommend a 1500 calorie diet with plenty exercise. That way you will be able to maintain your loss. Eat loads of green vegetables and fruits(not bananas) as they are filling and have very few calories. Avoid chocolate, cakes, cookies, fried foods, butter, margarine, oil etc. Even walking is good exercise although you might prefer cycling, swimming or something like that. Also remember that by increasing muscle you will help reduce fat. Every 1kg of muscle increases a persons metabolism by 50 calories.
