Every couple of months I get asked this same question.

“Doc, what’s my blood type?”

I load my “why you don’t need to know your blood type” speech from my cerebrum and press replay, trying to add a little spontaneous variation for authenticity.

“Actually, I’ve never checked it.”

“I thought you check everything.”

“Nobody checks everything. There are thousands of different available blood tests. Most of them would be completely useless to you.”

“Well doc, could you check my blood type?”

“I’m happy to, but unless a surgeon asked you to have it checked in anticipation of a surgery, it’s really not a handy thing to know.”

“Really? What if I get hit by a truck and need a transfusion?”

“If you ever need a transfusion and tell them your blood type, they won’t believe you. They’ll check it again.”

“But I need to know it because I heard about this great diet…”

Oh, no! Another trusting soul nearly lost in the sticky swamp of quackery. In 1996 a naturopathic doctor published “Eat Right 4 Your Type” a book claiming that your blood type determines your ideal diet. If you want the physiologic details of why this connection between diet and blood type is complete nonsense, check out the links below, especially the detailed and scathing book review from Quackwatch. (By the way, Quackwatch is a terrific resource for sorting the scams from the truth in medicine.)

But forget the physiology. This link between optimal diet and blood type would be very easy to prove without knowing any physiology. You would just take a large group of overweight adults, check their blood types and randomize them into two groups. Group 1 would be assigned the diet appropriate for their blood type according to the book. Group 2 would be assigned a diet suggested by the book for some other blood type. If group 1 lost significantly more weight than group 2 you would have convincing proof that the book isn’t a pile of rubbish. That trial has never been done, though the book has been republished several times. So it makes claims that make no physiologic sense and are unproven.

So why do people fall for it? Because the diets suggested for each blood type are fairly reasonable. Any one of the four diets is fairly healthy and could lead to weight loss, but you can pick one of the four at random; they have nothing to do with blood type. So (just like with bogus cold remedies) people try it, get good results, and spread the word.

“Well, thanks, doc! Sounds like I should just eat less and maybe crank up the exercise a couple of notches.”

“You’re very welcome. By the way, there is a beautiful way to find out your blood type and save a life – donate blood.”

Thus, quackery is foiled again, and there is much rejoicing!

Learn more:

Quackwatch book review: Eat Right 4 Your Type

Wikipedia article: Blood type diet

WebMD article: The Eat Right for Your Blood Type Diet

The Cedars-Sinai blood donor facility

Public Service Announcement: Starve a Vampire, Donate Blood