About the more than 2 lb loss a week... Your body, unless you are super morbidly obese, will not let go of more than 2 lbs or so of fat a week. It starts to panic and then it will give up its muscle in place of its fat. That is why someone who has 100 lbs or less to lose, should aim at 1-2 lbs a week loss.
About Grapefruit - Grapefruit is a natural appetite suppresant. If you want to go on a "grapefruit" diet, aim at eating a low fat/low processed carbs diet with eating an actual grapefruit before or as part of some of your meals.
Also this website has some humor in allowing you Mayonnaise and saying "hence the name, the Mayo diet". Just a side note, the Mayo clinic diet is some diet someone came up with and attributed it to the mayo clinic, even though it didn't come from the Mayo clinic.
You can't out-exercise poor eating habits.