The Blood Type Diet: What the Evidence Actually Shows

The core claim of the diet

Peter D'Adamo, a naturopathic doctor, published Eat Right 4 Your Type in 1996. The book argued that each ABO blood type evolved for a different ancestral diet, that food lectins bind preferentially to specific ABO antigens, and that eating foods compatible with your type would improve digestion, energy, weight management, and protection against chronic disease.

The four diet templates are: type O (high-protein, low-grain, low-dairy, said to suit hunter-gatherer ancestry), type A (largely vegetarian, said to suit early agricultural ancestry), type B (varied with dairy emphasis, said to suit nomadic herding ancestry), and type AB (a hybrid of A and B). Each template includes long lists of foods to favour, neutral foods, and foods to avoid.

The book has sold over seven million copies and spawned an industry of branded supplements, food rosters, and follow-up titles. The hypothesis is testable, and it has been tested. The results, summarised below, did not support the underlying claim.

What the systematic reviews found

In 2013, Cusack and colleagues published a systematic review in the American Journal of Clinical Nutrition that searched the published evidence for blood-type-matched dietary interventions. The conclusion: no evidence existed to validate the blood type diet as a dietary intervention. The reviewers found that no controlled trial of the diet had been published in any peer-reviewed venue.

In 2014, Wang and colleagues published a much larger empirical test in PLOS ONE. Using 1,455 adults from the Toronto Nutrigenomics study, they measured adherence to each of the four type-specific diets, then looked for the predicted interaction. The diet that the type-A template recommends (largely plant-based) did improve cardiovascular markers. So did the type-AB template. So did partial adherence in any direction. But the improvements were independent of the participant's actual blood type. People with type O on the type-A diet improved as much as people with type A. The signature prediction of the hypothesis (a diet-by-blood-type interaction) was absent.

A 2018 follow-up by the same group looked at insulin resistance and again found dietary effects independent of ABO type. Neither result is consistent with the lectin-and-antigen mechanism the diet relies on.

What the lectin claim actually implies

Lectins are carbohydrate-binding proteins found in many plant foods, particularly legumes and grains. Some lectins do bind to ABO-related sugar structures in laboratory dishes. The diet's mechanism takes that observation and projects it onto the digestive tract, claiming that incompatible food lectins cause cellular damage that explains everything from fatigue to cancer risk.

The Harvard T.H. Chan School of Public Health notes that the cooking process used for legumes and grains destroys the great majority of active lectins. Active lectins remain in raw kidney beans (which is why undercooked kidney beans cause acute toxicity) and in a few other raw foods, but the amounts in normal cooked food are too small to produce measurable health effects through this mechanism. The leap from in-vitro lectin binding to clinically meaningful in-vivo effect requires evidence that the diet has not produced.

The American Cancer Society dietary guidance reflects the broader nutritional consensus and does not single out blood type as a factor in food choice.

Why people report feeling better on it

Many people who follow the type-A diet do report increased energy, weight loss, and reduced gastrointestinal symptoms. The reason is not blood type. The type-A diet eliminates ultra-processed food, reduces added sugar, increases vegetable intake, and structures meal patterns. Any diet that does these four things tends to produce similar self-reported improvements.

The Wang 2014 study controlled for exactly this. By measuring whether people on the type-A diet improved more if they were actually type-A, the researchers separated the diet's effect from the blood-type-matching claim. The diet effect was real. The matching effect was not.

This is the same pattern seen in many food-elimination diets: the elimination produces structure and reduces processed food intake, the structure helps, and the underlying theory (gluten, lectins, nightshades, blood type) is not the cause of the benefit.

What evidence-based dietary advice actually looks like

The dietary patterns with the strongest cardiovascular and metabolic evidence base are the NHS Eatwell Guide, the Mediterranean diet, the DASH diet (for blood pressure), and the USDA MyPlate guidance. None of these are blood-type-specific. All four emphasise vegetables and fruit, whole grains, lean protein, and limited added sugar and saturated fat.

For people with specific clinical needs (diabetes, hypertension, chronic kidney disease, cardiovascular disease), the relevant guidance comes from Diabetes UK, the American Heart Association, or a consultation with a registered dietitian. Substituting the blood type diet for that guidance is the main clinical concern about the diet, not direct harm from the diet itself.

See also our blood type personality page for the parallel cultural belief tested by Cramer and Imaike, and our blood type myths roundup for the fuller list of claims with their evidence status.

If you want to try a diet change anyway

People often want to make a dietary change for reasons unconnected to evidence: the structure of the diet appeals, the social context appeals, the philosophy resonates. A few practical points if you choose to try one of the type-specific diets:

The type-A template is largely vegetarian. If you follow it strictly, plan vitamin B12 supplementation (B12 is found in animal foods and fermented soy in small amounts; vegetarians and vegans should follow NHS vitamin B guidance or supplement). Iron and zinc absorption from plant sources is lower than from meat; pair iron-rich plant foods with vitamin-C-rich foods to improve absorption.

The type-O template is high in animal protein and low in grain. If you have cardiovascular risk factors, this template may push saturated fat intake above guideline levels; choose lean meats and fish where possible. People with chronic kidney disease should not follow high-protein diets without renal-team input.

The type-B and type-AB templates contain most food groups and pose fewer specific nutritional risks, but they still involve restriction (avoiding chicken on type B, avoiding red meat on type AB) that has no evidence base. None of the four templates should replace clinical dietary advice for a diagnosed condition.

Frequently asked questions

Related pages