I' ve been asked this question many times! And personally I wasn't believing in such a thing!
High consumption of grapefruit can increase your metabolism and decrease the fat absorption but it has calories!
But one of my clients has been drinking grapefruit juice every morning before water and even before breakfast!
I noticed a big difference in his body fat mass, more than how much I was expected, although he hasn't been increasing his physical activity. And while asking him about his exact food intake, I knew that he is adding one glass of grapefruit juice in the morning. (without asking me 
)
How does this work???
Grapefruit can block enzymes responsible of fat and carbohydrates storage. This is the whole point and it boosts your metabolism up to 30 percent when consuming it in the morning.
Grapefruit is low in calories, low in sodium, high in potassium and high in fat burning enzymes, increasing weight loss in different ways. The low calories means you can eat grapefruit without consuming too many calories. A low sodium intake helps flush away excess water weight caused from high sodium foods. And, the enzymes present in grapefruit help increase fat burning.
But be careful!!!
1- If you are under medications, grapefruit interferes a lot with your pills!
Read this warning
http://eatlikemira.com/2010/03/01/the-garpefruit-warning/
2 -Grapefruit is very sensitive to the stomach especially in the morning, so if you have any stomach problem, including reflux or acidity, it is advisable not to have it 
So grapefruit can be part of a healthy diet plan but be careful about its intake.
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