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The 'Blood Type Diet'

The great thing about America is freedom of speech. We all have opinions and are able to share them even if they have no basis in fact. P.J. D'Adamo does this in his book of quackery, Eat Right 4 Your Type. The book has sold over 7 million copies in over 60 languages and has appeared on the New York Times bestseller list. All of this without a shred of evidence about what has to be the silliest diet ever thought up. That is our constitution: make something up, publish it, make money if people buy it.

When an author says things like, "The characterizations and suggestions I will make about your 'blood type personality' are based on the pooled impressions made from empirical observations of thousands of people over many years," run the other way. This is not science.

The problem with so many fad diets is that authors work so hard to make them sound very plausible. Even the most educated people, with degrees in statistics and backgrounds in biology, read these books and think, "This makes sense." And so it is with D'Adamo's "Blood Type Diet." He argues that our ABO blood group is linked to the dietary habits of our ancestors and that following the dietary habits associated with our blood type will improve our health and reduce our risk of chronic diseases, including cardiovascular diseases and cancers. Briefly:

Blood Type O is considered the "ancestral" blood group, so those with blood type O should follow a diet high in protein, similar to the hunter-gatherers.
Blood Type A supposedly evolved when humans settled into agrarian societies, so those with blood type A should follow a more vegetarian diet.
Blood Type B, on the other hand, were nomads, so those with blood type B should consume more dairy products.
Blood Type AB, then, is a combination of blood type A and B, so those with blood type AB should follow a diet in-between the Blood Type A diet and the Blood Type B diet.

This is just stupid. Quackery at its finest.

Mind you, this is D'Adamo's theory and is based on the evidence he presents in his book. Yet the one big piece of evidence he lacks is any actual evidence that adhering to one's blood-type-specific diet is better for you than adhering to one of the other blood-type-specific diets (following a Blood Type A diet if you are type O, for example).

Fortunately a team of researchers at the University of Toronto in Canada sought to remedy that oversight (PLOS One 2014;9(1):e84749). Making use of data gathered through the Toronto Nutrigenomics and Health Study, over 1,600 men and women in their twenties responded to a general health and lifestyle questionnaire as well as a detailed dietary questionnaire. Health information such as height, weight, blood pressure, glucose, and cholesterol scores were also recorded.

The scientists used Dr. D'Adamo's guidelines for each blood type diet to assign four different diet scores to each participant, based on how well their reported diet adhered to the four different blood type diets. They were then sorted into one of three levels of increasing adherence to each diet.

After comparing each participants' health scores with the blood type diet they most closely adhered to, the researchers found that:

• Those most closely following a Type A diet had lower Body Mass Index, total cholesterol, triglycerides, and insulin than those whose adherence to the Type A diet was very low;

• Those with the highest AB diet score had lower blood pressure, total cholesterol, and triglycerides than those whose AB diet score was low;

• A high Type O diet score was only associated with lower triglycerides; and

• Those following any level of a Type B diet tended to have lower HDL cholesterol.

Up until this point the researchers had not looked at the actual blood type of the individuals. When they revealed the actual blood type of each participant, they found that all four of the blood groups were equally represented among the four diets, showing that following the corresponding diet for one's actual blood type was no better (or worse) for you than following one of the other diets.

What this means for you

You might be wondering why the people following the Type A diet had so much better scores than those following the other blood type diets. The Type A diet is based on a high intake of grains, fruit, and vegetables, with lower intake of meats, not unlike a Mediterranean diet. The rest of the diet plans in this book are OK and for most people healthier than what they normally eat, but D'Adamo's book is pseudoscience at its finest.

First posted: January 29, 2014