In the world of nutrition, there are dozens of different kinds of diets. Cutting out sugar, cutting out processed foods, cutting out gluten, eating a Mediterranean diet, eating a low-carb diet, a ketogenic diet — these are all examples of how different people attempt to tailor their diets to their needs.
Diets are also notoriously hard to stick to, especially if you live in America, where it's both harder, more expensive, and more time-consuming to do so than to just eat whatever comes your way. This means that diets are often viewed as ineffective. The diet itself may be fine, but the friction involved in sticking to it is high enough that you're more likely to "relapse" and lose the benefits of the diet.
Throughout the last few decades, more and more attention has been paid to this problem. Some people have worked to make healthier foods more readily available. Some people have packaged meals to sell to focus on a diet. Some people, though, have decided that the reason diets don't work is because they aren't quite right for you.
Enter the Blood Type Diet.
The blood type diet is a concept introduced by Peter D'Adamo in 1996 in his book titled Eat Right 4 Your Type. The book lays out the basics of blood types, sample meal plans for each blood type, and reasons why D'Adamo believes the blood type diet can work.
The science on this idea has now been examined in multiple observational studies, meta-analyses, and controlled trials. The conclusions are fairly consistent — and this review covers all of it.
Table of Contents
What Are Blood Types, Anyway?
The blood type diet is the theory that your body benefits more from some nutrients and less from others and that one of the key determining factors of what nutritional profile you need comes down to your blood type.
Most people are aware of the fact that blood types are a thing. A smaller but still large group of people know what their blood type is. Relatively few people know what blood types actually are and why they matter. So, let's start here: what are blood types, and why are they important?
Blood is not just a simple fluid. It carries a vast array of nutrients, minerals, molecules, and cells throughout the body. Different bodily processes use blood either to send nutrients and energy to cells, carry away waste products, or help mobilize responses to injury or illness.
It's that last bit that's important here. Blood contains antigens, which are molecules that trigger a reaction from the immune system. Immune reactions are critical to keep you healthy; they mobilize the immune system against viral and bacterial infections, for example.
The body also produces antigens of its own, as a marker for its own cells. The presence or absence of specific antigens is what determines blood type. This is critical, because if you need a transfusion of blood, receiving blood that your immune system treats as an invader can cause widespread systemic damage and can even be life-threatening.
The major blood types are determined by the presence or absence of the A antigen and B antigen in your blood cells, as opposed to in your blood plasma, as well as the presence or absence of a protein called the Rh factor. The ABO blood type schema, which you're likely familiar with, can be determined through these three factors.
- Group A means you have A in your cells and B in your plasma.
- Group B means you have B in your cells and A in your plasma.
- Group AB means both A and B are in your cells and not your plasma.
- Group O means you have neither A nor B in your cells but both in your plasma.
The + or – (positive or negative) is the presence or absence of the Rh protein. All told, this gives you 8 different blood types.
There's a lot more complexity to this, but this is enough to give you the main idea of why blood types are important. The question is, then, does this matter for your diet?
What is the Blood Type Diet?
It's undeniable that different people react to different things in different ways. Some people have higher pain tolerances than others or respond differently to pain medications. Some people have the gene that makes cilantro taste like soap, while others don't. Some people are allergic to pollen, and others aren't. When it comes to diet, some people lack the ability to digest lactose, have food sensitivities to alliums, or can't eat gluten without an unpleasant reaction.
It's natural to wonder if there is an underlying reason for these differences. Peter D'Adamo thought that maybe blood type is a key determining factor. His theory is that since blood is present in the gut (because it's present throughout the body), the antigens in your body react to different nutritional molecules in different ways.
More specifically, D'Adamo suggests that proteins called lectins, found in the food you eat, are either bound to antigens and carried away, or if they don't match your blood type, clump up and cause problems in your body.
D'Adamo based his diet on evolutionary changes in diet. According to him, early humans were still Type O blood back when they primarily ate a paleo diet, so people with Type O blood will process that diet the best. Later on, when agrarian society developed and farming was commonplace, Type A blood evolved with the greater degree of plant-based foods. Thus, he recommends a more plant-based diet to A-types. B-types, D'Adamo believes, evolved in nomadic people who ate a more balanced diet and thus should have a more balanced diet now. AB types, then, are the "modern" evolution of blood, and he suggests a mixed and modern diet for those people.
His book goes into recipes, foods to eat and avoid, and tips for the diet for each blood type. The full details can be found online if you're interested.
Is the Blood Type Diet Effective?
Not really. Let's look at the science and the circumstances.
First, who is Peter D'Adamo, and what are his credentials? He's a naturopath and author — not a historian, geneticist, or medical doctor. He's best known for his dietary advice centered around blood types and, more recently, genotypes, in which he divides people into six classifications: Hunter, Gatherer, Teacher, Explorer, Warrior, and Nomad. None of these frameworks are backed by peer-reviewed evidence, and none of this inspires much confidence in the underlying science.
Second, what about his theories? Well, right away, we run into a problem. D'Adamo theorizes that Type O blood — the type that is more "clean" of antigens and is considered the universal donor because of it — was the earliest blood type. His theory is that adulterations to blood evolved later.
The biggest issue with this is that it's not true. To the best understanding of modern evolutionary genetics, the A blood type is the oldest. Moreover, the divergence likely occurred some 400 million years ago. It's likely true that all of these blood types existed before humans evolved. Moreover, despite being "empty," the Type O blood is an abnormal gene mutation, whereas A and B are unmutated versions. To quote the above, "Although the O blood type is common in all populations around the world, there is no evidence that the O gene represents the ancestral gene at the ABO locus, nor is it reasonable to suppose that a defective gene would arise spontaneously and then evolve into normal genes."
The lectin theory, as well, may not be quite what D'Adamo represents it to be. Lectins are real, and they are often toxic to greater or lesser degrees. Some do little more than cause minor inflammation, while others are deadly; the toxic Ricin is a lectin. However, there's very little evidence and no human studies to prove that they interact with the immune system in the ways D'Adamo describes. Moreover, D'Adamo recommends paleo diets, which avoid a lot of cooking, but cooking is the primary way to remove lectins from foods.
Putting all of this aside, the blood type diet has been examined in human trials. What do those trials say?
An observational study in 2014 used a survey to assess adherence to blood type diet rules in a pool of 1,455 people. It found that the closer a diet matched to one of the blood type diet recommendations, the better the individual's health and biomarkers, like lower BMI, lower triglyceride levels, and better insulin resistance. While that sounds promising, here's the kicker: it didn't associate with the individual's blood type. People with Type O would benefit from a type A diet, people with AB would benefit from type B, and so on.
A meta-analysis of articles on the blood type diet was performed in 2013, and all but one of them had reasons to disqualify them, such as lacking control groups, not adequately recording data, or other common faults with studies. That one remaining study showed no real evidence to support the blood type diet specifically.
A 2018 study looked at 973 overweight adults and found that, while the recommendations in the blood type diet improved health outlooks, it was not correlated or associated with blood type.
A 2020 study, which examined 68 people, looked at the effects of a vegan diet across blood types and didn't observe tangible differences from type to type.
Finally, there's another nail in the coffin for the blood type diet. Not only does it ignore the Rh protein (by claiming it doesn't impact how the body handles lectins), it ignores the fact that the ABO paradigm isn't even the entire picture.
There are over 600 antigens that can be part of blood, and there are a variety of other blood groups, including the Duffy blood group, the Kell group, the Lutheran group, and the Kidd group. In fact, in 2022, researchers discovered a new blood group involving the Era, Erb, Er3, Er4, and Er5 antigens. As if that's not enough, more recently, another new group called the MAL group was discovered.
As it turns out, blood groups are much more nuanced than a reductive "this is what hunters eat, this is what gatherers eat" schema can account for. Even in D'Adamo's revision to genotypes, it's still intensely reductive.
Does the Blood Type Diet Work?
After all of this, you'd likely expect the answer to be no. But that's not the full picture. The truth is that the blood type diet can work, but not for the reasons D'Adamo cites.
When you ignore everything related to blood types and all of the little details like whether or not you're allowed to drink coconut juice or citrus, and just look at the core diets, you notice something. These diets are essentially reflective of the major modern diets, including a vegetarian diet, a Mediterranean diet, and a more keto-focused diet.
All of these are, broadly speaking, healthier and better than the average American's diet, which is full of heavily processed foods, trans fats, excessive amounts of sodium and preservatives, and other nutritional garbage. Switching from eating potato chips and candy to eating vegetables and fish is going to improve your health, no matter what your blood type is.
At the end of the day, that's what it all comes down to. Taking steps to improve diet quality, exercise consistency, and sleep are the evidence-backed foundations of good health. Your blood type matters for surgery, transfusions, and blood donation — but the research consistently shows it has no bearing on which diet pattern will work best for you. The blood type diet, when it works, works because it replaces poor dietary habits with structured, whole-food eating. That benefit belongs to the diet itself, not to any blood-type-specific mechanism.
Sources:
- Red Cross Blood Services – Blood Types Explained: https://www.redcrossblood.org/donate-blood/blood-types.html
- Cleveland Clinic – Blood Types: What They Are and Mean for Your Health: https://my.clevelandclinic.org/health/treatments/21213-blood-types
- Medline Plus – Hemolytic transfusion reaction: https://medlineplus.gov/ency/article/001303.htm
- Evolution of primate ABO blood group genes and their homologous genes: https://academic.oup.com/mbe/article/14/4/399/1051639?login=false
- Genetic of the ABO blood system and its link with the immune system: https://www.scielo.br/j/rbhh/a/QxNj553rg7PDrkxJChkjBgQ/?lang=en
- Food poisoning from raw red kidney beans: https://pubmed.ncbi.nlm.nih.gov/7407532/
- ABO Genotype, 'Blood-Type' Diet and Cardiometabolic Risk Factors: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0084749
- Blood type diets lack supporting evidence: a systematic review: https://www.sciencedirect.com/science/article/pii/S0002916523051377?via%3Dihub
- ABO Genotype Does Not Modify the Association between the "Blood-Type" Diet and Biomarkers of Cardiometabolic Disease in Overweight Adults: https://www.sciencedirect.com/science/article/pii/S0022316622107984?via%3Dihub
- Blood Type Is Not Associated with Changes in Cardiometabolic Outcomes in Response to a Plant-Based Dietary Intervention: https://www.jandonline.org/article/S2212-2672(20)31197-7/fulltext
- NHS scientists discover new blood group system: https://www.nhsbt.nhs.uk/news/nhs-scientists-discover-new-blood-group-system/
- New blood group solves half a century of mystery: https://b-s-h.org.uk/about-us/news/new-blood-group-solves-half-a-century-of-mystery
