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These Parts of the Blood Type Diet Haven’t Been Debunked

Article at a Glance

The blood type diet has been largely debunked, however, the main science backed diet issue for people to consider based on their blood type is the level of hydrochloric acid produced in the stomach.

Hydrochloric acid is essential for properly digesting food, especially animal protein, and there is good evidence demonstrating differences in hydrochloric acid levels based on people’s blood type, with O blood type having the highest levels.

There is also a link between risk for certain types of ulcers and blood type.

Yes, the blood type diet is largely thin on evidence, especially after this new study by plant based doctor Neil Barnard showing changes in weight, fasting blood sugar and cholesterol didn’t have anything to do with blood type in a group placed on a low-fat vegan diet. Although, as I will discuss in this post, there are a few narrow redeeming qualities to this much maligned fad diet.

The D’Adamo Blood Type Diet

In the early 2000’s, diets based on your ABO blood type were all the rage, with the D’Adamo diet being one of the most popular versions. Whilst a lot of claims were made about the diet, clinical studies were lacking. Issues like this frequently occur in medical research when the underlying effect is multifactorial; i.e. when all sorts of other factors such as individuals genetics or their environment influence outcome. Studies of 10 people might show an effect, but another study on a different 10 people might not. In instances like this we typically say the study is under-powered – don’t worry I won’t go into statistical analysis here, it makes my blood run cold as well… To investigate definitively, researchers need to make sure their study is sufficiently powered, which means they need lots of people to perform their comparison. While it’s relatively straightforward to perform a study with 10 or 100 people, it becomes a completely different ball game when you reach 1,000 people or more. Studies of this size are expensive and very time consuming and so are unlikely to be performed unless they are covering a major health risk. Which means a study with “sufficient power” is unlikely to be performed for a dietary intervention. Luckily there is a way around this. Scientists can perform a type of study known as a systematic review, where they pool the data from all relevant studies (not just those looking at the specific effect, but also other studies where appropriate data is recorded) and then analyse the effect.

Scientific review of blood type diets

In 2013, a group of researchers undertook just such a study into blood type diets.1 After screening 1,415 studies they identified 16 possible targets, which the further refined down to a single study of interest.2 The paper itself is quite difficult to follow so I’ll just take this quote from the authors:
None of the studies showed an association between ABO blood type diets and health-related outcomes.
Pretty conclusive stuff.

Follow-up study

This systematic review was followed up by a direct study investigating blood type diet and its impact on cardiometabolic risk factors in 1,455 individuals.3 Again the paper has a lot of detail and is perhaps again summed up best by the authors:
Our findings show that adherence to certain ‘Blood-Type’ diets is associated with a favorable profile for certain cardiometabolic risk factors in young adults, but these associations were not related to an individual’s ABO blood group.
My emphasis in bold. So, while a positive effect from following the low meat high fruit and vegetable diet was observed, this was not associated with any particular blood group. Rather, it is just an effect of following a traditional “healthy” diet. This study was considered the nail in the coffin of the blood type diet. Dr D’Adamo refutes this study on his site but once again, in a non peer review format.

Dr. Neil Barnard’s blood type diet study

Dr. Barnard’s recent study follows in the same vein as the 2013 analysis I discuss above. Dr. Barnard is a staunch advocate of plant based diets, and his blood type diet study shows no enhanced benefit for blood type A on a low-fat vegan diet vs. other blood types. Most participants saw improvements, it’s just that no one group saw greater results for cholesterol, BMI, fasting blood sugar, HbA1c, or weight loss than another due to blood type. The idea with this study was that, in theory, the low fat vegan diet well suited to type A blood types and not as well suited for those with type O blood, who under the rules of Dr. D’Adamo’s protocols would be healthier on a diet with more animal protein. Based on the cardiometabolic markers studied, this turned out not to be the case, which may have put the final nail in the coffin for the blood type diet, at least within the scientific community.

The end of blood type and diet?

So is that the end? Are blood type and diet definitively not linked? Well in the sense that you should follow a particular diet based on your blood type, yes. In that respect, the blood type diet has been debunked. However, there are some dietary risk factors which do seem to be associated with blood type, which can be influenced by certain dietary choices.

ABO Blood Classification

But before we look at that it’s worthwhile to go over the ABO blood classification as this can give us some idea where the initial blood type diet hypothesis came from, and how it might be relevant for other studies. The ABO blood system was first described at the start of the 20th century by the Austrian physician Karl Landsteiner who demonstrated that our red blood cells could carry one of two antigens (a protein which can cause immune cells to generate antibodies) on their surface; A or B or none at all. We now know that this expression is determined by our genetics, meaning each individual will have two types of antigen present, which are summarized in the table below.
Blood TypeABO
Three outcomes per gene means a possible 9 combinations. However, AA is the same as AO meaning that we actually end up with four groups. A, B, AB and O. People with O blood should only receive donations from other O individuals, but can donate to anyone. Whereas AB can receive blood from anyone but only donate amongst themselves. There are many other factors such as the Rhesus (Rh) system which tags a + or – to the blood type (+ can receive – but not vice-a-versa) which further complicate matters but ABO is the main typing group.

Diets for each blood type

O is Ancestral

The blood type diet was based around the idea that O is the ancestral blood group and so their optimal diet should match that of our early ancestors, being high in animal protein and low in grains, a version of the paleo diet.

A Vegetarian

A evolved next and so these individuals should benefit from a vegetarian diet as this was when we first settled into villages and began cultivating our food.

B can handle dairy

Finally, B was thought to be the last evolving group and so individuals are thought to deal well with dairy products.

AB Can handle variety

AB individuals are able to enjoy a wider diet type.


Lectins, a type of small molecule in different foods said to the defense mechanism of plants to deter predators, were hypothesized to promote red blood cells to clump in a process called agglutination resulting in poor health outcomes. As discussed above, the science backing this is rather poor, however there is some emerging evidence about other dietary impacts.

Blood type, stomach acid and ulcers

As far back as the 1960s, researchers were describing an association between blood group, the secretion of hydrochloric acid in the gut and the associated development of ulcers.4 Proper levels of hydrochloric acid are vital as it promotes the digestion of food, maintains correct gut flora and optimizes the environment for digestive enzymes. Blood Type and the Stomach Gene Food O blood type individuals were both more likely to secrete increased levels of hydrochloric acid and with increased risk of developing gastric ulcers. An interesting finding showed that A blood type individuals secreted less hydrochloric acid and that this resulted in an increase in a different type of ulcer. Bringing this research up to date, a huge study including over 1,000,000 patients from Sweden and Denmark confirmed the increased risk of developing ulcers for O blood type individuals. They didn’t replicate the same findings for A type individuals but did show that these patients were of increased gastric cancer risk.5 In this table taken from (R) the adjusted rate ratio shows the relative risk of developing gastric or duodenal ulcers, a higher number means a higher risk. As you can see O type individuals are at the highest risk. Conversely, when looking at cancer risk a different effect was observed. With the A antigen (A alone or AB) being associated with an increased cancer risk.

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How to lower ulcer risk

As we’ve discussed previously, Hpylori infection is commonly associated with the development of gastric ulcers, and several groups have demonstrated an association based on blood type; with A and O types thought to be at more risk although the exact mechanism linking the two remains unknown, and some studies are in disagreement.678 So if your blood type can predict gastric ulcer and cancer risk, and this seems to be driven by changes in susceptibility to Hpylori infection what can you do about it?

Potential dietary changes based on blood type

From a dietary perspective, high levels of fibre have been shown to be protective, other factors such as smoking, high alcohol intake and excessive red meat intake have also been associated with increased risk, although the relative effect is unclear. Therefore, the sort of mixed vegetable rich diet which is widely accepted as healthy should prove most beneficial to those with type O blood (but this type of diet is also beneficial in general).910 Those with blood type A secrete less hydrochloric acid into the gut and therefore may suffer from impaired digestion, particularly of protein rich and difficult to digest foods such as many red meats.11 This lack of digestion can limit the movement of food through the gut and promote bacterial colonization, especially from H. pylori.7However, no studies directly investigating type A associated lack of digestion with H. pylori overgrowth have been performed. See also: Probiotics aren’t the only way to restore gut health

Specific nutrients

The major nutrient or supplement to recommend is Saccharomyces boulardii which John has written an excellent post on. The section dealing with H. pylori is of obvious interest, with S. boulardii exerting a protective effect in association with antibiotics.12 Use of S. boulardii will likely be beneficial for those with both type A or type O blood who are at greater risk of H. pylori colonization. Vitamin A intake has also been associated with a reduction in ulcer risk overall,9 vitamin C and E were also linked but the results were less clear. However, no studies investigating these nutrients in relation to blood type and ulcer risk have been performed. From a practical standpoint, those with an impaired ability to digest proteins due to a lack of stomach acid may benefit from experimenting with betaine HCL and digestive enzymes.

Genetic markers

Aside from blood group, which can be determined genetically but is more accurately determined using simple lab assays, the SNP rs4073 in the IL8 gene might also be of interest. Carriers of the risk allele ‘A’ are of significantly elevated risk of developing H. pylori infections,13 and so this SNP might be of particular interest for those with A or O blood type. However no direct studies have been performed linking the two.

Take-home message

To summarize, the blood type diet has been largely discredited, although the healthy vegetable rich diet it proposes would likely be beneficial for most individuals. However, blood type can impact on the risk of developing gastric ulcers, with type O being the highest risk group. There are several general and specific dietary changes that can be made to reduce the risk of developing ulcers, but as of yet no studies linking everything together have been performed.

Dr. Aaron Gardner, BSc, MRes, PhD

Dr. Aaron Gardner, BSc, MRes, PhD is a life-scientist with a strong background in genetics and medical research, and the developing fields of personalized medicine and nutrition. Read his full bio here.

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