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The blood type diet has been debunked, or has it?

Article at a Glance
  • While there may not be much in the way of science to back up most of the claims made by the famous blood type diet, the diet is founded on some sound scientific principles.
  • It turns out that stomach acid levels, as well as the risk for certain types of ulcers, vary by blood type.
  • Variations in stomach acid can explain why one individual might thrive on a higher protein diet, while another wouldn’t be as healthy.

Top-level summary: Hope the headline didn’t get you too excited. Yes, the blood type diet has largely been debunked. Although, as I will discuss in this post, there are a few narrow redeeming qualities to this much maligned fad diet.

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.

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.

Systematic review of blood type diets

In 2013, a group of researchers undertook just such a study into blood type diets (R). After screening 1,415 studies they identified 16 possible targets, which the further refined down to a single study of interest (R). 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 (R). 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.

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 summarised in the table below.

Blood TypeABO
AAAABAO
BBABBBO
OAOBOOO

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.

Alleged basis of the blood type diet

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

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

Science Score: 

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 (R). Proper levels of hydrochloric acid are vital as it promotes the digestion of food, maintains correct gut flora and optimises 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 of 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 (R).

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.

What causes the difference and what can I do about it?

Science Score: 

At this point we step away from the really well defined science hence the drop in science score.

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 (R,R,R); 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.

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?

Dietary changes

Science Score: 

From a dietary perspective, high levels of fibre have been shown to be protective (R), 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 (R,R). 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).

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 (R). This lack of digestion can limit the movement of food through the gut and promote bacterial colonisation, especially from H. pylori (R)However 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

Science Score: 

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 (R)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 colonisation.

Vitamin A intake has also been associated with a reduction in ulcer risk overall (R), 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.

Genetic markers

Science Score: 

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 (R), 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 summarise, 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.

See also: Is a high protein diet healthy? and Candida overgrowth: what you need to know

Aaron Gardner

Dr Aaron Gardner is a life-scientist with a strong background in genetics and medical research, and a particular interest in the developing fields of personalised medicine and nutrition.

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10 Comments

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  1. Aaron, great post. As I read this, I can’t help but think there are potential, if not yet completely proven, health implications for the variations in stomach acid based on blood type.

    For example, I’ve written recently about Candida, and how it needs a more alkaline PH to become pathogenic. It would seem based on your findings that those with type A blood, who have lower levels of hydrochloric acid, would be more prone to Candida, especially if they find themselves on a diet that is higher in animal protein?

    Seems also that some of the issues I list in my post “Is a high protein diet making you sick?” Could be more relevant for type A, especially is they have urea cycle deficiencies, but based on our offline conversations, you’re sticking firm that these associations are as yet not established by credible science?

    • Thanks John. I think you’re definitely right that there are connections out there related to pH changes in the gut. The whole system is really quite sensitive and set up to work under optimal conditions so slight changes can really throw things out of balance. The problem is there are so many factors feeding into it (genetics, nutritional intake, gut microbiome and probably lots of other environmental factors as well) that it’s going to take a while to unravel.

      I also think it will be a highly personal and likely fluctuate overtime as well, so what’s best for you now might change in a year or 10 years.

      The candida and high protein issues are very relevant, and all the pieces of the story seem to be there, they’ve just not been all linked together yet… but we’ll keep an eye on the scientific literature and see what comes out.

      • bokus says:

        What does ‘ph changes in the gut’ mean? A change in ph of stomach acid? A change in the small intestinal ph? A change in the cecum? a change in the colon?

        What does that even mean?

        How can you think you’re debunking anything with such nonsensical overarching statements?

        • Hi Bokus, I used gut as a catch-all term as although a lot of the changes in pH are measured in the stomach, the ulcers often form in different regions. For example the table in the post is looking specifically at duodenal ulcers.

  2. Avishek Saha says:

    Interesting, but I disagree that it’s a fad diet. It was marketed as such but the research it’s based on seems to be largely proprietary. It’s not indexed on pubmed or published anywhere. It’s empirical, and the only way of knowing what that research is to contact Dr. D’Adamo himself.

    • Avishek, I will let Aaron weigh in, but major journals, such as the American Journal of Clinical Nutrition, have done analysis of the diet and have found, at least comprehensively, it lacks evidence. See: Blood type diet lacks supporting evidence: a systematic review. Having said that, and as you can tell from my question for Aaron above, I am not necessarily a blood type diet “hater.” I think there are dietary implications for people based on their levels of hydrochloric acid, it’s just that the diet’s creators make larger claims than they can prove.

    • Thanks for your comment Avishek, as John said we’re not completely down on the influence of blood on dietary intake. From my reading of the published data (and that’s all I can base my posts on) I would say that the highly specific diet plans aren’t really based on any firm science, but there are some broad brush statements that can be made, such as those relating to pH or H. pylori infection.

  3. bokus says:

    Just looking over this article fast I see some flaws in your assessment.

    First you claim that the blood type diet was debunked based solely on a lack of cardiometric associations. Who ever claimed that cardiometrics were a metric of success? Could it be possible that any one of hundreds of other metrics could be positively associated with the blood type diet?

    Also I saw the phrase ‘blood type diets’. What does this mean? Does it refer to the D’adamo diet?

    Third, and I’ve seen this so many times on the webz, is the false claim that Blood type O should ‘eat a lot of meat’. Did you ever read D’adamo’s book(s). I did. The portions of meat, even for O, is very small, even in comparison to a SAD diet.

    There are a lot of traps people fall into when supposedly debunking good information like D’adamo has presented. I doubt people were eating grass fed meats in the test samples, and probably a lot were eating processed meats.

    • Hi Bokus,

      To answer your first question the systematic review found only the following paper worthy of inclusion based on their criteria:

      https://www.ncbi.nlm.nih.gov/pubmed/9212175?dopt=Abstract

      Which had a focus on serum LDL cholesterol as an output. While they didn’t include the other papers in their analysis they did make the following statement

      “None of the studies showed an association between ABO blood type diets and health-related outcomes.”

      and after reading the references it is possible to come to the same conclusion. I guess it is possible that there are metrics that are positively associated with blood type diets but these have never been shown. I know Dr D’Adamo has stated that trials for blood type diet were expected in 1998 and 2004, but these haven’t materialised.

      2) There are several variations of blood type diets but yes Dr D’Adamo’s is the most famous and probably most widely followed. But the above statements apply to all such diets.

      3) Taken from Dr D’Adamo’s material (with my emphasis):
      “Hi Bokus,

      To answer your first question the systematic review found only the following paper worthy of inclusion based on their criteria:

      https://www.ncbi.nlm.nih.gov/pubmed/9212175?dopt=Abstract

      Which had a focus on serum LDL cholesterol as an output. While they didn’t include the other papers in their analysis they did make the following statement

      “None of the studies showed an association between ABO blood type diets and health-related outcomes.”

      and after reading the references it is possible to come to the same conclusion. I guess it is possible that there are metrics that are positively associated with blood type diets but these have never been shown. I know Dr D’Adamo has stated that trials for blood type diet were expected in 1998 and 2004, but these haven’t materialised.

      2) There are several variations of blood type diets but yes Dr D’Adamo’s is the most famous and probably most widely followed. But the above statements apply to all such diets.

      3) Taken from Dr D’Adamo’s material (https://www.4yourtype.com/blood-type-o/ – my emphasis below):
      “DIET PROFILE High Protein: Meat, Fish, Vegetables, Fruit, Limited Grains, Beans, Legumes
      WEIGHT LOSS KEY: Avoid: Wheat, Corn, Lentils, Kidney Beans, Dairy Beneficial: Kelp, Seafood, Red meat, Kale, Spinach, Broccoli, Olive Oil”

      Which is quite a strong push towards a paleo diet rich in meat, and yes I have read his books and online material.

      4) As far as I’m aware Dr D’Adamo hasn’t presented any data, although I’m happy to be corrected about this.

  4. Terri Nopp says:

    You contradicted yourself so many times in this article just by using logic. So you’re saying blood types have different acidic content in their bodies. That blood types are prone to diseases. And yet you can’t figure out how the different acids that digest our food wouldn’t have different effects on our bodies?

    Sometimes in their quest to condemn scientists become blinded by their ego’s. Instead of conducting research to DISPROVE a theory, start to conduct your own research based on the chemistry alone.

    Type A blood types have an elevated risk of pancreatic cancer for example.

    Different acids in the blood types = different digestion issues and interactions with food. Food = health.

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