Question: What is the blood type diet, how does it work, and how effective is it in reducing weight?
Answer: The blood type diet is a book that tells a certain dietary approach based on your blood type. It's that simple. The diets that are prescribed as a result of your blood type are different in ways in terms of the meats that you should eat, whether it should be more meat or vegetarian-based. It's based on some theories about the absorption of foods into the body.
I'm not aware of any studies specifically studying the blood type diet. I looked at the books and looked at this one in particular and it, at the end of the day, restricts the sugars and refined carbohydrate for all of the different diets.
So I think it's a reasonable way to go because in general you'll be healthier if you restrict the junk food in the diets. But I don't know that the specific prescription based on your blood type for this diet has been adequately studied.