06/03/2013 11:16 am ET Updated Aug 03, 2013

Study Explores the Scientific Support for the Blood-Type Diet

Why are so many of us struggling with our weight and suffering illness? According to naturopath Dr. Peter D'Adamo, knowing your blood type, and eating and exercising according to it, is the key to avoiding health and weight problems. His bestseller, Eat Right For Your Type, which was published in 1996, has been translated to more than 50 languages, and has 7 million copies in print.

I know my blood type only too well. The Red Cross is extra-excited with my O-negative donations (O-negative blood is considered a universal donor), but as a pregnant woman I had the pleasure of receiving plenty of shots to prevent hemolytic disease of the newborn.

So how should I be eating? "Type Os thrive on intense physical exercise and animal protein," according to Eat Right For Your Type. Uh-oh! Although I exercise regularly, I've never run a marathon and I'm a vegetarian; I eat lots of grains and legumes, which, according to D'Adamo, contribute to weight gain in my "type." According to his diet, I should eat a lot of lean meat, which I don't, and avoid gluten, oranges, mushrooms, cauliflower, coffee and tea, black pepper, all vinegars and beans, which I certainly don't. I thought I was doing fine, but imagine how much better I could be feeling.

The Science Behind Blood-Type Diets

Blood types are perfectly scientific. There are about 30 different antigens than can be present or absent on the surface of a red blood cell. The ABO system for typing blood, which deals with just two of these antigens (A and B), is the most well-known and useful, since ABO compatibility is the most important for blood transfusion -- getting incompatible blood can be fatal. Most people have never heard of other blood types such as M, N and Kell.

But do blood types really inform about our disease susceptibility and gastrointestinal fitness?

The book offers plenty of anecdotal "evidence" of people whose symptoms improved when they learned to eat for their blood type. Stories of success sound very convincing, and nothing makes as memorable an impression as personal testimony. But anecdotes are not proof.

A new study in the American Journal of Clinical Nutrition assessed the evidence for blood-type diets in the scientific literature. The researchers looked for studies that grouped people according to blood type and studied whether adherence to a certain diet made a difference. Combing the entire life science and biomedical databases, the researchers could not find even one study showing an association between ABO blood type diets and health-related outcomes.

Not even one!

Is it possible that blood type affects susceptibility to disease and should predict your optimal diet? It is also perfectly possible that eye color and the curliness of hair could be associated with certain dietary outcomes, and that these traits are inherited together with traits that affect gastrointestinal differences. Much like the belief that blood type is predictive of personality, the notion that blood type affects metabolism and should determine your optimal diet is a belief -- in this case, apparently based on no facts whatsoever.

I have not given birth to any children of my blood type -- a bit of good news: according to D'Adamo, my kids can thrive as vegetarians -- so after proving I'm too lazy for "intense physical exercise," you won't be surprised I'm also too lazy to cook seperate, individual meals for my husband and three kids.

Dr. Ayala

For more by Ayala Laufer-Cahana, M.D., click here.

For more on diet and nutrition, click here.