Does the blood type diet work?

The objection

“My blood type is O, the hunter type, so my body is built for meat. And my ancestors thrived on it, so it’s in my biology. Blood groups clearly matter for health, so why wouldn’t they matter for diet?”

The answer

The blood type diet comes from Peter D’Adamo’s 1996 book Eat Right 4 Your Type, which by 2014 had sold more than 7 million copies in over 60 languages, according to the diet’s official website as cited by Toronto researchers that year [2]. It holds that type O is the original human blood group and does best on a hunter-gatherer diet high in animal protein, that type A arose when people settled into farming and should thrive on a vegetarian diet, and that type B suits dairy, with each plan listing foods to eat and foods to avoid [2]. Those are testable claims, and they have been tested.

What does the research on blood type diets show?

The studies so far have found no benefit from matching a diet to your blood type. A 2013 systematic review led from the Belgian Red Cross-Flanders screened 1,415 references turned up by searches of three medical databases [1]. Sixteen were picked out for a closer look and only one met its criteria, and that one compared LDL cholesterol responses to a low-fat diet across a different blood group system, MNS, so it could not answer the question [1]. The review found no study showing health effects of ABO blood type diets [1].

Researchers in Toronto then checked the theory’s central prediction in their own data. In their 2014 study of 1,455 adults aged 20 to 29, people whose eating most resembled the Type A plan, which is heavy on grains, fruit and vegetables, had lower body mass index, blood pressure, cholesterol, triglycerides and insulin, while the meat-heavy Type O plan went with lower triglycerides only [2]. These links were the same whether or not a person’s blood group matched the diet they were eating [2]. A few blood sugar measures showed statistical interactions that the authors thought may be due to chance, and even taken at face value they showed no extra benefit from matching, and perhaps some adverse effects [2]. This was a snapshot study based on food questionnaires, so it shows associations rather than cause, and its authors suggest that these links may explain the anecdotal support for the plans, which are generally prudent diets [2]. One of them declared shares in Nutrigenomix, a genetic testing company for personalised nutrition [2].

Their 2018 study followed 973 overweight adults through a six-month diet trial [3]. People whose eating moved closer to the Type A, B or O plans saw bigger falls in BMI or waist size, the meat-heavy Type O plan included, but matching the plan to a person’s ABO genotype did not change any of these associations [3].

The test closest to this objection is a 2021 analysis of 68 overweight participants who followed a low-fat vegan diet for 16 weeks as part of a randomised trial [4]. People with type A, the group the theory says suits plant food, lost 5.7 kg on average against 7.0 kg for everyone else, and people with type O lost 7.1 kg against 6.2 kg [4]. Neither gap was statistically significant, and changes in body fat, cholesterol and blood sugar did not differ by blood type either [4]. It is a small secondary analysis that could miss a modest difference, and it was run by the Physicians Committee for Responsible Medicine, which promotes plant-based diets [14].

Study	Design	People	Did blood group matter?
Cusack et al. (2013) [1]	Systematic review	1,415 references screened	No study had tested it
Wang et al. (2014) [2]	Snapshot study	1,455 adults aged 20 to 29	No
Wang et al. (2018) [3]	Within a six-month diet trial	973 overweight adults	No
Barnard et al. (2021) [4]	Within a 16-week vegan diet trial	68 overweight participants	No

None of these was the trial the 2013 review called for, comparing people of one blood type who follow its plan with people of the same type who stay on a standard diet [1]. Each of the later three did look for the matching effect the whole theory depends on, and none found it.

Is type O really the original hunter’s blood group?

The genetics does not support it, because the A and B blood groups are far older than our species. A 2012 study in PNAS found that the A and B versions of the ABO gene go back at least 20 million years and are shared by descent between humans and gibbons, and among Old World monkeys [5]. O versions are loss-of-function copies of the gene that appear to be specific to each species [5], and the 2014 Toronto authors note that analyses of the human gene suggest A may be the ancestral human type [2]. Type A was around long before farming, and before humans, so the hunter-and-farmer story behind the diet falls apart. Its proposed mechanism, that lectins in some foods clump the blood cells of the wrong blood group, is unproven too: the Toronto authors say studies to date suggest food lectins cause no blood-group-specific clumping [2].

Does blood type affect health at all?

Blood group does affect some health risks, blood clots among them, but no diet matched to blood type has been shown to change them. It has been studied for links with cancers, malaria, cholera and heart and metabolic disease [2]. People with type O have lower levels of two clotting proteins, von Willebrand factor and factor VIII, and a 2012 meta-analysis of 38 studies, covering 10,305 people who had had a clot in a vein, found that a blood group other than O went with about twice the odds of such a clot (pooled odds ratio 2.09, 95% confidence interval 1.83 to 2.38) [6]. That figure comes from comparing people who had had a clot with people who had not [6]. ABO antigens sit on cells lining the gut as well as on red blood cells [5], and in a 2022 study of 7,738 people in the Dutch Microbiome Project, the ABO gene was one of only two places in the genome with a robust link to gut bacteria [15]. That link could be explained by people’s secretor status, which is set by a different gene, FUT2, while the other robust link, near the lactase gene, seemed to vary with how much lactose people ate [15]. So it was reasonable to ask whether blood group changes how people respond to food, and the studies above asked exactly that.

Does your ancestry mean you need meat?

Ancestry changes how well people digest or make some nutrients, and none of the differences found so far makes meat necessary: the NHS says its healthy eating guide applies to vegans and to people of all ethnic origins, though not to children under 2, who have different needs [11]. Lactase persistence, the ability to digest milk sugar as an adult, varies widely between populations and is an adaptation to keeping dairy animals [7]. People from populations with starch-rich diets carry, on average, more copies of the gene for the starch-digesting enzyme in saliva than people from populations that traditionally ate little starch, such as rainforest and Arctic hunter-gatherers [8]. Ancestry also affects how the body makes long-chain fats. In a 2015 genome scan of Greenlandic Inuit, whose traditional diet was rich in protein and omega-3 fats, the strongest sign of adaptation was in a cluster of fatty acid desaturase genes [16]. A 2016 study found, in global genome data, that 70% of South Asians carry two copies of a FADS2 variant linked to more of the body’s own production of long-chain omega-6 fats, against 17% of Europeans, and its authors suggest the variant may have been favoured by a traditional plant-based diet [17]. Where ancestry does change the advice, it changes how a nutrient is best obtained: the NHS advises people with dark skin, for example from an African, African-Caribbean or south Asian background, to consider 10 micrograms of vitamin D a day all year round, because they may not make enough from sunlight [9].

With good planning, the NHS says, a vegan diet can provide all the nutrients the body needs, and without it people can miss out on calcium, iron, B12, iodine and selenium [11]. That planning includes B12 from fortified foods or a supplement, a reliable iodine source, and vitamin D in autumn and winter, the last of which the NHS advises everyone to consider, meat-eaters included [9][10][11]. The British Dietetic Association’s 2021 fact sheet says “carefully planned plant-based diets can support healthy living at every age and life stage” [10], and the US Academy of Nutrition and Dietetics’ 2025 position says that in adults who are not pregnant or breastfeeding, appropriately planned vegan diets can be nutritionally adequate [12]. The NHS adds that vegans who are pregnant or breastfeeding need to make sure they get enough vitamins and minerals for their child to develop healthily [11]. The broader claim that humans as a species evolved to need meat has its own answer: are humans designed to eat meat?

People really do respond differently to the same food. In the PREDICT 1 study (2020) of 1,002 UK adults, blood sugar and blood fat responses to identical meals varied widely from person to person, and genetic variants had only a modest effect on the predictions [13]. The study was supported by the nutrition company Zoe, which employed ten of its 25 authors and retained seven more as consultants [13]. Personalised nutrition is a real field, and the blood type studies above show that sorting people into four blood groups is not how it works.

If you take medicine for blood pressure, cholesterol or diabetes, talk to your GP before making a big change to what you eat, and don’t change or stop any medicine without their advice. Blood type plans also come with lists of foods to avoid, and if cutting out foods or worrying about weight starts to take over, eating-disorder charities can help: Beat in the UK, Bodywhys in Ireland, ANAD in the US, NEDIC in Canada, the Butterfly Foundation in Australia, and EDANZ, which supports families and carers, in New Zealand.