This is the indispensable handbook that lays the foundation for each of the four blood types, including clear, simple plans for each type.

From the book jacket: "If you've ever suspected that not everyone should eat the same thing or do the same exercise, you're right. Your blood type reflects your internal chemistry. It actually determines the way you absorb nutrients. What foods you absorb well and how your body handles stress differ with each blood type. Eat Right 4 Your Type will help you design a total health program that's right for your blood type."

"Within the diet itself are generally good diet recommendations," says David W. Grotto, RD, LD, a spokesman for the American Dietetic Association. "D'Adamo doesn't say avoid vegetables and fruit, for example -- but his specific recommendations based on blood type -- the science is not there to support it. I'm not aware that anyone has duplicated his research."

As for why so many have embraced the Eat Right for Your Type diet, Wiley has a theory: " Dieting is one of those things people feel desperate about. ... The blood-type diet sounds more scientific [than some others]."

But it's the very lack of a solid scientific background that rankles most experts.

"If this diet wasn't coming from the whole blood-type approach, I could recommend this," says Grotto, "but the philosophy of blood type is very obscure and lacking in science."