The purpose of a 12-day grapefruit diet is to consume grapefruit prior to eating other foods in order to lose weight. Supporters of this diet claim that grapefruit contains enzymes that assist with weight loss when eaten before other foods.
General guidelines include increasing the intake of foods high in protein, fat and cholesterol while cutting back on sugar and carbohydrates, explains WebMD. Avoiding foods like celery and white onions is also suggested. Dieters are encouraged to drink eight glasses of water and one cup of coffee daily. Counting calories and eating grapefruit or drinking grapefruit juice prior to or with each meal are the foundations of the grapefruit diet.