The Grapefruit Diet claims that eating grapefruit can help raise metabolism and cause rapid weight loss of up to 10 pounds in 12 days. It works by eating half a grapefruit before each meal, as grapefruit enzymes are thought to be fat-burning. However, it is considered a fad diet due to extreme calorie restriction, lack of nutrition, short duration, and unrealistic promises. While some studies found weight loss from simply adding grapefruit to an existing diet, the diet is not considered a healthy or sustainable way to lose weight long-term.