Has the blood type diet ever been tested?

Once, and indirectly. A 16-week trial of a low fat vegan diet did blood typing on 68 people in its diet group and compared results by blood group. Weight fell 5.7 kg in type A against 7.0 kg in everyone else, and 7.1 kg in type O against 6.2 kg in everyone else, both too close to call. Total cholesterol and every other result also showed no difference. Nobody has ever assigned people to their matching diet against a non-matching one, which is the test the idea actually needs.

Why do people say it worked for them?

Because three of the four diets are familiar healthy eating plans under different names, and the largest study that watched people is fairly clear about it. Across 1,455 people, following the Type A diet went with lower BMI, waist, blood pressure, cholesterol, triglycerides and insulin measures. The Type AB diet went with nearly the same, and the Type O diet with lower triglycerides. Matching the diet to the blood group did not change any of those links. The food was doing the work.

Does blood type matter for health at all?

Yes, and not for this. ABO type is truly linked to disease. In twelve long-term studies, the A1 gene variant carried 1.48 to 1.71 times the odds of pancreatic cancer compared with O/O, while the A2 variant carried no extra risk. In a Norwegian study, people in the top quarter for blood levels of the ABO enzyme had 2.12 times the risk of blood clots in the veins. Those links work through the enzyme that sets blood type, and no trial has tested whether any diet changes them.

What about the idea that blood groups evolved at different stages of human diet?

The genetics runs against the timeline. The ABO system is shared with the great apes and has been kept going for a very long time by natural selection, which seems to act on the immune response to germs. That places its origin long before farming. It does not disprove the diet claim, which would need its own test. It does remove the history story usually told to support it.

Is there any harm in trying it?

It depends which of the four you are given. The Type A and Type AB diets are plant-forward, and went with better markers in the study that watched people. The Type O diet cuts grains and dairy, which raises the same fibre and calcium question as paleo. The Type B diet cuts chicken, corn, lentils, peanuts, sesame and wheat, went with no heart or metabolism benefit in that same study, and is a big cut with nothing behind it.

Elimination6 studies in people cited

Blood type diet

Written by Aaron CuhaReviewed Sep 2026

Also called: Eat Right 4 Your Type, ABO diet, D'Adamo diet

One real test exists: a side analysis inside a 16-week vegan trial, with blood typing on 68 people. People with blood type A lost 5.7 kg and everyone else lost 7.0 kg, too close to call, and that ran opposite to the prediction. Nobody has ever assigned people to the diet that matches their blood type against one that does not.

What it actually is

Four eating plans assigned by ABO blood group, from a 1996 popular book rather than from a research program. Type O is steered toward a high protein, meat-forward diet that avoids grains and dairy. Type A is steered toward a mostly vegetarian one. Type B gets a diet that allows dairy but cuts chicken, corn and several other foods. Type AB gets a mix. The four differ from each other in ways that go far beyond blood type. The type A diet is close to a low fat vegetarian diet and the type O diet is close to paleo, which is why the data from studies that watch people is more interesting than it first looks.

How its fans say it works

Here is the idea at its strongest, before it is tested. It says lectins, proteins found in food, react with the blood type markers on gut and blood cells in a way that depends on blood group. It says the four blood groups appeared at different stages of human diet history, and that eating outside your type causes clumping of cells, inflammation and chronic disease. There is something real underneath this, which is why it cannot simply be brushed off. ABO blood type is a real, working genetic difference. The markers are sugars placed on cell surfaces by an enzyme, and ABO type is truly linked to disease. In twelve long-term studies with 1,533 pancreatic cancer cases and 1,582 people without it, people with the A1 gene variant had higher odds: 1.48 times for A1/O and 1.71 times for A1/A1, compared with O/O. The A2 variant carried no extra risk, which tracks how active that enzyme is. In a Norwegian study with 294 cases, people in the top quarter for blood levels of the ABO enzyme had 2.12 times the risk of blood clots in the veins. So ABO matters in the body. The history half of the claim runs into a different problem. Great apes share the ABO system too, kept going by natural selection acting on the immune system, which places its origin long before farming. The question the diet asks is narrower than any of this, and it is the only one that can be tested directly: does matching a diet to your blood group change what that diet does to you?

What happened when calories were controlled

Best kind of study available: tracked, not tested

Nobody was given a diet. People who choose a diet differ from people who do not in other ways too, so this shows a link, not a cause.

No calorie-controlled study of this diet exists, and neither does any trial built to test it. The only trial evidence is a side analysis inside a study run for another purpose, and it found blood group made no difference to any result it measured.

Wang 2014 is the large study that watched people, and its result is more interesting than its reputation. 1,455 people in the Toronto Nutrigenomics and Health study filled in a 196-item food questionnaire covering one month. Each person got a score for how closely they followed each of the four blood type diets, and their blood group was found from their genes. Following the Type A diet more closely went with lower BMI, waist size, blood pressure, cholesterol, triglycerides, insulin and two insulin resistance scores, all clear results. Following the Type AB diet went with the same list, except BMI and waist. Following the Type O diet went with lower triglycerides, a clear result. The Type B diet went with nothing.
Then comes the sentence the study was built to produce. Matching the blood type diets to the matching blood group did not change the size of any of those links. Three of the four diets look good on heart and metabolism markers, for everyone, whatever their blood group. That is what you would expect if the diets work through the food in them, not through matching blood type.
The only trial test, Wang 2021, is a side analysis of the diet group in a 16-week trial of a low fat vegan diet against making no diet change. 68 people in the diet group had their blood typed, and results were compared between type A and everyone else, and between type O and everyone else. Average weight lost was 5.7 kg for type A against 7.0 kg for everyone else, and 7.1 kg for type O against 6.2 kg for everyone else. Total cholesterol fell 17.2 mg/dL in type A against 18.3 in everyone else, and 17.4 against 18.4 for type O against everyone else. None of these differences was real, and neither were any others, including body fat, belly fat, blood fats, fasting blood sugar and long-term blood sugar (HbA1c).
Look at which way the numbers point before looking at whether they are real. The blood type idea predicts that type A should do best on a plant-based diet and type O worst. On weight, the numbers ran the other way in both comparisons, and neither was a clear result. 68 people in one group of one trial is a small test, and the uncertainty is wide. It is also the only trial evidence there is.
Cusack 2013 is the systematic review, which searched three big research databases. It screened 1,415 references and found 16 articles, of which one was eligible. That one looked at LDL cholesterol on a low fat diet by a different blood group system (MNS), not ABO, so it did not answer the question. No studies showing the health effects of ABO blood type diets were found. The review's own account of what would be needed is the design nobody has run: compare health results between people following their blood type diet and people staying on a standard diet, within the same blood type.

What happened when people just ate it

The Toronto study is the data from people eating on their own, and it is worth reading as a positive finding rather than a negative one. People who ate more like the Type A or Type AB diet had better heart and metabolism markers than people who did not, and their blood group made no difference to that. In practice, both of those diets are plant-forward and lower in processed food. The Type O diet, which is closest to paleo, went with lower triglycerides, the same direction the low carb research predicts. The Type B diet, the one that overlaps least with any standard healthy eating plan, went with nothing at all. That spread of results is what you would see if the food, not the blood type, explains it. Nobody has published data on sticking with this diet, dropping out, or long-term follow-up, in any blood group. There is a useful parallel from a different claim about personalised eating. The Food4Me trial split adults across seven European countries into groups. One got standard diet advice. The others got personal advice based on their diet alone, on their diet plus body measurements, or on their diet plus body measurements plus five diet-related gene variants. Personal advice led to bigger and better changes than standard advice: less red meat, salt and saturated fat, and higher healthy eating scores. But there was no sign that adding body measurements or genes made it work any better. Different genes and a different claim, and the same result: the personal framing did the motivating, and the gene layer added nothing you could measure.

Protein, and whether it muddies the result

The four diets differ hugely in protein, which is the biggest unchecked factor in the whole idea. Type O is steered toward a meat-forward, grain-free diet that in practice looks like paleo, and type A toward a mostly vegetarian one. No study of this diet has reported protein intake, and none has measured muscle in any blood group, so the difference in protein between the four diets has never even been described with numbers. The measured protein intakes, and what each did, are in the protein section of this site and on the high protein page.

The measured intakes behind the protein figures are on the protein page.

What reliably moves

Measure	Direction	From
Body weight on a plant-based diet, by blood group	No difference	5.7 kg lost in blood type A against 7.0 kg in everyone else, and 7.1 kg in type O against 6.2 kg in everyone else, among 68 typed people in one group of a 16-week trial. Neither gap was real, and both ran opposite to the prediction.
Total cholesterol on a plant-based diet, by blood group	No difference	Down 17.2 mg/dL in type A against 18.3 in everyone else, and 17.4 against 18.4 for type O against everyone else. Body fat, belly fat, fasting blood sugar and long-term blood sugar (HbA1c) also showed no difference.
Heart and metabolism markers by how closely people followed the Type A and Type AB diets	Better, in everyone	Lower BMI, waist, blood pressure, cholesterol, triglycerides, insulin and two insulin resistance scores for following Type A, and the same except BMI and waist for Type AB, across 1,455 people. Matching the diet to the right blood group did not change the size of any link.
Markers by how closely people followed the Type B diet	Nothing	No clear link with any heart or metabolism marker in the same study. The Type B diet is the one that overlaps least with any standard healthy eating plan.
Blood clots in the veins, by ABO	Up in non-O, and nothing to do with diet	People in the top quarter for blood levels of the ABO enzyme had 2.12 times the risk of the bottom quarter, across 294 cases, rising to 3.71 for clots with no obvious trigger. Accounting for two clotting factors (von Willebrand factor and factor VIII) weakened this only a little.
Pancreatic cancer by ABO gene variant	Up for A1, not for A2, and nothing to do with diet	Odds of 1.48 times for A1/O and 1.71 times for A1/A1 compared with O/O, with the A2 variant carrying no extra risk (A1 against A2, a clear difference), across twelve long-term studies. It tracks how active the blood type enzyme is, which is the biology the diet borrows and does not test.

Long term, and serious health events

Nothing. No long-term study has followed people eating a diet matched to their blood type. No trial has run longer than the 16 weeks of the study the side analysis sits inside. And no outcome of any kind, disease or death, has been measured in any blood group on any of these four diets. The 2013 review found one eligible article out of 1,415 screened, and it did not look at ABO. Since then, the evidence has grown by one side analysis in 68 people. That is the complete record.

Sources

Study in people2021
Blood Type Is Not Associated with Changes in Cardiometabolic Outcomes in Response to a Plant-Based Dietary Intervention
Journal of the Academy of Nutrition and Dietetics
This is the only evidence where people were assigned a diet. 68 people with known blood types ate a low fat vegan diet for 16 weeks. Type A people lost 5.7 kg against 7.0 kg for everyone else, and type O lost 7.1 kg against 6.2 kg. Blood type made no real difference to weight, cholesterol or anything else measured.
The only randomised evidence. Secondary analysis of 68 typed participants in the intervention arm of a 16-week randomised low-fat vegan trial. Weight minus 5.7 kg in type A against minus 7.0 in all others (p = 0.09); minus 7.1 in type O against minus 6.2 (p = 0.33). Total cholesterol p = 0.90 and p = 0.89. No significant difference in any outcome.
Study in people2014
ABO genotype, 'blood-type' diet and cardiometabolic risk factors
PLoS One
1,455 people filled in a food questionnaire and had their blood type genes tested. Eating the Type A diet went with lower weight, waist, blood pressure, cholesterol, blood fats and insulin measures. The Type AB and O diets had some links, and Type B had none. But the links were the same whether or not the diet matched the person's own blood type.
1,455 participants, 196-item food frequency questionnaire, ABO genotype from rs8176719 and rs8176746. Adherence to the Type A diet associated with lower BMI, waist, blood pressure, cholesterol, triglycerides, insulin, HOMA-IR and HOMA-Beta; Type AB with the same except BMI and waist; Type O with lower triglycerides; Type B with nothing. Matching the diet to the blood group did not change any effect size.
Review of studies2013
Blood type diets lack supporting evidence: a systematic review
American Journal of Clinical Nutrition
A review searched 1,415 papers and found 16 articles. Only one fit, and it looked at cholesterol response to a low fat diet by a different blood group system, not ABO. No study showed health effects from eating for your ABO blood type.
1,415 references screened across the Cochrane Library, MEDLINE and Embase, 16 articles identified, one eligible, and that one examined LDL response to a low-fat diet by MNS blood type rather than ABO. No studies showing health effects of ABO blood type diets were identified. GRADE approach.
Study in people2017
Effect of personalized nutrition on health-related behaviour change: evidence from the Food4Me European randomized controlled trial
International Journal of Epidemiology
A different claim about personalized eating, with a similar result. 1,269 adults in seven countries got standard advice or advice tailored to their diet, their body measures, or their genes. Tailored advice cut red meat, salt and saturated fat and improved diet quality. Adding body measures or genes did not make it work any better.
A different personalisation claim with a structurally similar result. 1,269 adults across seven countries randomised to conventional advice or to personalised advice based on diet, diet plus phenotype, or diet plus phenotype plus five genetic variants. Personalised advice cut red meat, salt and saturated fat and raised diet quality, and there was no evidence that phenotypic or genotypic information enhanced the effect.
Study in people2025
Histo-blood group ABO system transferase plasma levels and risk of future venous thromboembolism: the HUNT study
Blood
A real blood type link to disease, with nothing to do with diet. Researchers looked at 294 people who got blood clots in the veins, against a sample of 1,066. Those with the highest blood levels of the ABO enzyme had a clearly higher risk than those with the lowest. The risk was higher still for clots with no clear trigger.
A real ABO disease association, unrelated to diet. 294 incident venous thromboembolism cases and a 1,066-person subcohort. Highest quartile of plasma ABO transferase carried a hazard ratio of 2.12 (1.39 to 3.22) against the lowest, 3.71 for unprovoked events, attenuating only moderately on adjustment for von Willebrand factor and factor VIII.
Study in people2010
Variant ABO blood group alleles, secretor status, and risk of pancreatic cancer: results from the pancreatic cancer cohort consortium
Cancer Epidemiology, Biomarkers and Prevention
Twelve studies of 1,533 people with pancreatic cancer and 1,582 without. People with the A1 blood type gene had a higher risk than type O, while the A2 version carried no extra risk. The risk tracked how active the blood group enzyme was. Whether people released blood type markers into body fluids made no difference.
Twelve prospective cohorts, 1,533 cases and 1,582 controls. Against O/O, odds ratios were 1.48 for A1/O and 1.71 for A1/A1, while A2 alleles carried no excess (A1 against A2, p = 0.004). Risk tracked glycosyltransferase activity. Secretor phenotype was not an effect modifier.
Study in people2009
Genome-wide association study identifies variants in the ABO locus associated with susceptibility to pancreatic cancer
Nature Genetics
The large gene scan that first linked the ABO blood type gene to pancreatic cancer. It is cited here to show the blood type biology is real and was found without any diet idea. It says nothing about diet.
The genome-wide scan that established the ABO locus association, cited here to show that the ABO biology the diet borrows from is real and was discovered independently of any dietary hypothesis. It says nothing about diet.
Review of studies2015
Trans-species polymorphism in humans and the great apes is generally maintained by balancing selection that modulates the host immune response
Human Genomics
This bears on the timing part of the blood type diet claim. The ABO blood groups are shared by humans and other primates, and have been kept over a very long time because they help immune defense. That puts their origin long before farming.
Bears on the chronology half of the claim. The histo-blood ABO group is one of a small number of well-characterised trans-species polymorphisms shared between humans and other primates and maintained by long-term balancing selection acting on immune function, which places its origin far earlier than agriculture.
Review of studies2001
ABH and Lewis histo-blood group antigens, a model for the meaning of oligosaccharide diversity in the face of a changing world
Biochimie
Background on what blood group markers are thought to be for. The paper argues that variety in these markers helps a species cope with many fast changing germs. It says their role in cancer and heart disease is a side effect, not the reason they evolved.
Argues that the polymorphism of the ABH and Lewis antigen gene families provides intraspecies diversity for coping with diverse and rapidly evolving pathogens, and that the role of these antigens in cancer and cardiovascular disease is incidental rather than of evolutionary significance. Background on what the antigens are believed to be for.

What people report

These are things people say about their own eating. Nobody checked them, so they are not evidence. They are here because they tell you what to expect and what to watch for, which the studies do not. They cannot tell you whether anything works.

Feeling better within weeks, the most common report. That is the expected result whenever anyone swaps a haphazard diet for any structured one.
Weight loss on the Type O and Type A diets, reported often. That fits those being familiar low carb and plant-forward diets under different names.
No effect at all, reported commonly and rarely written up. That is the finding the side analysis of the trial supports.
Symptoms coming back after eating a banned food again. Nobody was kept from knowing what they ate, so this cannot be told apart from expecting it.
Frustration with the Type B list in particular, which cuts chicken, corn, lentils, peanuts, sesame and wheat and is the hardest of the four to follow.
Confusion when family members with different blood groups are told to eat opposite diets, the most common practical reason for stopping.
The cost of the type-specific supplement lines sold alongside the plan.

Sources: Blood type diet forums, write-ups from practitioners in the plan's own network, and the popular books that carry it. Note the business setup: the diet is tied to a branded supplement and testing line, and that disclosure belongs next to these reports. Unchecked self-report throughout. For this diet, it is close to all of the supporting material, because the peer-reviewed record holds one review that found nothing eligible, one study that found the effects had nothing to do with blood group, and one side analysis that found blood group made no difference.

Who this is wrong for

Anyone with blood type O being steered away from grains and beans without a plan to replace the fibre, and anyone with blood type B cutting out chicken, corn, lentils, peanuts, sesame and wheat. Those are big cuts with no trial behind them, and the Type B diet was the one linked to no heart or metabolism benefit at all in the largest study that watched people.
Anyone who will accept their blood group as the reason for a result. In the only trial test, the numbers ran opposite to the prediction, and no comparison was a clear result.
Anyone paying for blood-group supplements, tests or meal plans. A blood type test tells you your blood group, which is worth knowing for transfusions and for the real links between ABO and disease. It does not tell you what to eat.
Anyone with a history of eating disorders. Four lists of banned foods, assigned by something about you that can never change, is a poor fit.
Anyone at high risk of the real ABO-linked conditions who takes up the diet because of that risk. The links with blood clots and pancreatic cancer work through the enzyme that sets blood type, and no trial has tested whether any diet changes them.
Anyone treating this as a personalised nutrition method. The one trial of gene-based personalisation, using a different set of genes, found that adding gene information on top of personal diet advice brought no extra benefit.

Questions

Has the blood type diet ever been tested?: Once, and indirectly. A 16-week trial of a low fat vegan diet did blood typing on 68 people in its diet group and compared results by blood group. Weight fell 5.7 kg in type A against 7.0 kg in everyone else, and 7.1 kg in type O against 6.2 kg in everyone else, both too close to call. Total cholesterol and every other result also showed no difference. Nobody has ever assigned people to their matching diet against a non-matching one, which is the test the idea actually needs.
Why do people say it worked for them?: Because three of the four diets are familiar healthy eating plans under different names, and the largest study that watched people is fairly clear about it. Across 1,455 people, following the Type A diet went with lower BMI, waist, blood pressure, cholesterol, triglycerides and insulin measures. The Type AB diet went with nearly the same, and the Type O diet with lower triglycerides. Matching the diet to the blood group did not change any of those links. The food was doing the work.
Does blood type matter for health at all?: Yes, and not for this. ABO type is truly linked to disease. In twelve long-term studies, the A1 gene variant carried 1.48 to 1.71 times the odds of pancreatic cancer compared with O/O, while the A2 variant carried no extra risk. In a Norwegian study, people in the top quarter for blood levels of the ABO enzyme had 2.12 times the risk of blood clots in the veins. Those links work through the enzyme that sets blood type, and no trial has tested whether any diet changes them.
What about the idea that blood groups evolved at different stages of human diet?: The genetics runs against the timeline. The ABO system is shared with the great apes and has been kept going for a very long time by natural selection, which seems to act on the immune response to germs. That places its origin long before farming. It does not disprove the diet claim, which would need its own test. It does remove the history story usually told to support it.
Is there any harm in trying it?: It depends which of the four you are given. The Type A and Type AB diets are plant-forward, and went with better markers in the study that watched people. The Type O diet cuts grains and dairy, which raises the same fibre and calcium question as paleo. The Type B diet cuts chicken, corn, lentils, peanuts, sesame and wheat, went with no heart or metabolism benefit in that same study, and is a big cut with nothing behind it.