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Some of my associates at work are talking about adjusting their diets based on their blood type. I was shown articles out of The Eat Right for Your Type Complete Blood Type Encyclopedia, but to me it seems like they’re concluding too much from too little. Is this nonsense, or does blood type tell about a person’s most suitable diet, disease resistance, etc?

If you think basing your diet on blood type is out there, you haven’t heard anything yet. In Japan, where this daft business started, some people organize company work teams, segregate school classrooms, and even choose their dream date by blood type. And if you get lucky on that date, not to worry—in Japan, vending machines sell condoms by blood type too.The most vocal American booster of blood-type theory is Peter D’Adamo, who has written or co-written dozens of works on the subject, his magnum opus being the one you reference. His major contentions:

  • The three most common blood types—A, B, and O—developed at different points in human evolution. (D’Adamo’s theory is relatively quiet about type AB blood.) Type O showed up first, allegedly when we were hominids with a high-meat diet. Type A blood initially appeared among Neolithic farmers, supposedly because it increased their ability to tolerate a high-grain diet. Finally, 10,000 to 15,000 years ago, type B blood emerged—the result, somehow, of climate change and increased availability of dairy products.

  • The logic of why this matters is a little intricate, but here goes: (a) red blood cells of each different blood type have a characteristic mix of antigens on their surfaces; (b) antigens provoke immune responses from corresponding antibodies in the blood; (c) among these antibodies are food proteins called lectins; (d) different foods contain different types of lectins; (e) if you eat the wrong foods and thus take in the wrong lectins, said lectins will stick to your blood cells and cause them to clump together; and (f) this is bad.

  • Therefore, you should eat more of certain foods and shun others based on your blood type. According to D’Adamo, O types do best eating lots of meat but few grains; type A individuals should emphasize grains and vegetables; and type B should avoid grains but can load up on meat, greens, and dairy.

  • D’Adamo downplays the blood type = personality school of thought. Nonetheless, he claims type O individuals are prone to bipolar disorder, whereas type A people are more likely to be obsessive-compulsive.

Holes might be picked in D’Adamo’s theory, the main one being that it doesn’t square with pretty much anything we know about evolutionary physiology. Geneticists think the blood types diverged millions, not thousands, of years ago. Type O is the most recent evolutionary mutation, not the oldest. Lectins having an affinity for a specific blood group exist but are rare in food. Type B blood supposedly evolved due to the influence of dairy in the diet, yet dairy intolerance is common in regions where type B is prevalent.

And crucially, D’Adamo’s evidence that his diet works consists of random anonymous anecdotes.

Which brings us to the question: Will D’Adamo’s diet recommendations do you any good? The bulk of Eat Right for Your Type is a collection of every ailment you ever heard of and a recommended dietary treatment for each based on blood type. In fairness, researchers have found links between certain diseases and blood types.

For example, people with type B blood may have a lower incidence of Type 2 diabetes than the A, O, and AB groups; type B folks, in fact, may have less than half the risk of type O. One study found a weak connection between gallstone formation and blood type, with type O folks being slightly less susceptible.

And A and B types are much less likely to develop peptic ulcers than O types, who are also more prone to norovirus infections. A recent study of a million Scandinavian blood donors found type AB folks were 26 percent more likely, and type B folks 8 percent less likely, to develop gastric cancers than O types.

Still, concluding from the above that a diet based on blood type will improve your health is a long leap. A 2012 review of 1,415 articles about blood-type diets found exactly one that met all the criteria for sound research, and it didn’t examine the ABO blood typing D’Adamo focuses on, but rather a different system of blood classification.

So it’s not out of the question you may someday get legitimate diet advice based on some form of blood typing. But avoiding poultry to cure AIDS, as D’Adamo claims? I’m sorry, that’s nuts.

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