Diet not working? Maybe it’s not your type

Robert H. Shmerling, MD

Senior Faculty Editor, Harvard Health Publishing

Follow me on Twitter @RobShmerling

Have you heard of the blood type diet? I thought it had been debunked long ago but patients keep asking about it, so I figured I should learn more.

What’s the Blood Type Diet?

In 1996 Peter D’Adamo, a naturopathic physician, published a book in which he described how people could be healthier, live longer, and achieve their ideal weight by eating according to their blood type. One’s choice of condiments, spices, and even exercise should depend on one’s blood type. Soon, the book was a best seller and people everywhere were finding out their blood type, revising their grocery lists, and changing how they ate, exercised, and thought about their health.

Here are some of the recommendations according to the “Eat Right for Your Type” diet:

  • Those with type O blood should choose high-protein foods and eat lots of meat, vegetables, fish, and fruit but limit grains, beans, and legumes. To lose weight, seafood, kelp, red meat, broccoli, spinach, and olive oil are best; wheat, corn, and dairy are to be avoided.
  • Those with type A blood should choose fruit, vegetables, tofu, seafood, turkey, and whole grains but avoid meat. For weight loss, seafood, vegetables, pineapple, olive oil, and soy are best; dairy, wheat, corn, and kidney beans should be avoided.
  • Those with type B blood should pick a diverse diet including meat, fruit, dairy, seafood, and grains. To lose weight, type B individuals should choose green vegetables, eggs, liver, and licorice tea but avoid chicken, corn, peanuts, and wheat.
  • Those with type AB blood should eat dairy, tofu, lamb, fish, grains, fruit, and vegetables. For weight loss, tofu, seafood, green vegetables, and kelp are best but chicken, corn, buckwheat, and kidney beans should be avoided.

As mentioned, the recommendations for the blood type diets extend well beyond food choices. For example, people with type O blood are advised to choose high-intensity aerobic exercise and take supplements for their sensitive stomachs, while those with type A blood should choose low-intensity activities and include meditation as part of their routine.

But does eating for your blood type work?

High-quality studies about the blood type diet had not been published in peer-reviewed medical literature. Even now, a search in the medical literature for the author’s name reveals no research pertaining to this diet. Studies published in 2013 and 2014 about the blood type diets are worth noting. The 2013 study analyzed the world’s medical literature and found no studies demonstrating benefit from a blood type diet. The 2014 study found that while people following any of the blood type diets had some improvement in certain cardiometabolic risk factors (such as cholesterol or blood pressure), those improvements were unrelated to blood type.

Does it make any sense?

The theory behind this diet is that blood type is closely tied to our ability to digest certain types of foods, so that the proper diet will improve digestion, help maintain ideal body weight, increase energy levels, and prevent disease, including cancer and cardiovascular disease.

Type O was said to be the original “ancestral” blood type of the earliest humans who were hunter-gatherers, with diets that were high in animal protein. Group A was said to evolve when humans began to farm and had more vegetarian diets. Group B blood types were said to arise among nomadic tribes who consumed a lot of dairy products. And since Group AB blood was supposed to have evolved from the intermingling of people with types A and B blood, type AB recommendations were intermediate between those for people with types A and B blood.

Each of these theories has been challenged. For example, there is evidence that type A was actually the first blood group to evolve in humans, not type O. In addition, there is no proven connection between blood type and digestion. So, in addition to a lack of evidence that the diet works, serious questions remain about why it should work in the first place.

So, what’s the downside?

It’s a fair question, especially since some improvements were seen in people who adopted certain blood type diets (see link above). Eating based on your blood type requires you to know your blood type and then follow a restrictive diet. Personal preferences might be a problem: a vegetarian with type O blood may struggle to stay on the assigned diet, and people who love red meat may be disappointed to learn they have type A blood. Recommended supplements are not cheap; neither are the recommended organic foods. And if you have certain health conditions, such as high cholesterol or diabetes, a nutritionist can make better evidence-based recommendations for you than those determined by your blood type.

Now what?

Advocates of blood type diets may say that while the ideal study has not yet been performed, the absence of evidence doesn’t prove they’re ineffective. And there’s also no proof that these diets are harmful. So, my guess is that interest in the blood type diets will not disappear any time soon. But there’s a reason that bookstores have rows and rows of books on diet, each claiming to be highly effective if not the best. We simply don’t know which diet is best for each individual person. And even if we did, sticking to any single diet is often challenging.

Stand by — it’s likely you’ll soon be hearing about yet another best diet. And my guess is that it won’t have anything to do with your blood type.


  1. Suzy Ballo

  2. Ariel Eshel

    The interesting question is “Does it make any sense?” – and in this context Peter D’Adamo suggested that there is a correlation between blood types and recommended personal diets.

    Although this specific hypothesis is wrong, still it is interesting since it suggested a correlation between a “kind of” genetic profile and a recommended diet.

    Nowadays we know that there is a correlation between the genetic profile and a recommended diet or other well-being recommended behavior. Just to make it clear – even when we know your relevant genetic profile, it gives just your “potential” and not the actual recommendation.

    Personal recommendations should take into account 3 more elements beyond your genetic profile – your current physical status, habits and the availability of what is recommended.

    Unfortunately, the genetic test and its interpretation is still not a cheap product (130$ and up) and so far, there is no approved (by FDA) application that will take into account all mentioned above factors.

    Most of providers supply just the relevant genetic profile and it’s interpretation, and leave the outline of the overall picture to nutritionists or fitness trainers. I doubt if they are equipped with the needed tools to give a comprehensive personal picture.

  3. Irene

    I agree with Eva. I fully believe that people have different optimal diets, but why go to a discredited fraudster to illustrate the point. I read several of his books and came to realize I wasted my money.
    They only thing helpful for me was cutting out wheat, because I have O blood. It was not helpful because of my blood, but because wheat is not healthy for other reasons.
    If Harvard wants me to invest my money in their health publications, they should limit themselves to hard science and not use urban myths in the hope they might prove correct in a future study.

  4. Bonnie West

    I totally support the Blood type diet. I have eaten based on the O plan & lost weight by avoiding the foods that it said to avoid. Now I have gone back to tons of cruciferous veggies and eating little protein, etc. and gaining weight while eating 1400 healthy calories a day and walking up to 2 miles per day & I am still gaining. I believe less protein is my problem for the O blood type which I have. It was amazing which foods that I should eat were the foods that I gravitated to as a child and growing up to adulthood. Always wondered why I liked soda water, etc. I would encourage people to read about it because I believe that they will be surprised! Bonnie West

  5. David Wilder

    I am type O, and am doing very well on a whole foods plant based diet. No eggs, meat, poultry, fish, or dairy, and very little refined carbs and oil. The kernel of truth in the blood type diet may be that different diets work for different people (it’s just not related to blood type). If what you are doing isn’t working, then try something else.

  6. Eva

    Why did you even bother posting this article? The author says that there is no science to support the blood type diet. If a few individuals had luck with it losing weight, I have heard time and again that no matter what you eliminate, narrowing your food choices will always result in some weight gain in the short term because we all crave variety, and not having variety means we get bored with our food and eat less. Come on, Harvard – I expect more from you than recycling a diet that has been discredited for decades.

  7. Marco Bellacci

    “Those with type O blood should choose high-protein foods and eat lots of meat, ”

    No proof that these diets are harmful?? Hm…

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