There has been an abrupt increase on social media in the number of professionals in my field who now call themselves experts on the blood type diet. Even though it was extremely popular in the 2000s, with celebrities including the recent Oscar Demi Moore crediting the diet for helping them maintain good physical shape, few of today’s young are familiar with this dietary “approach.”
Briefly, the diet assumes that lectins, proteins found in foods such as grains, legumes, and certain vegetables like tomatoes and zucchini, can interact negatively with the HLA antigens detected in your blood and characterize your blood type.
Based on these interactions, knowing your blood type and seeking guidance from a professional familiar with the topic will provide a diet offering benefits such as improved digestion and metabolism, reduced inflammation, increased energy, pain relief—especially for patients with rheumatic diseases—and, of course, weight loss.
However, there are very few rigorous studies on the blood type diet, and the available research contradicts its claims. To be charitable, these modern-day internet professionals are outdated or lack critical thinking.
The Birth of the Theory
Dr. James L. D’Adamo, an American naturopath, created the blood type diet. He played a key role in establishing guidelines for regulating and licensing naturopathy professionals in the United States, serving as the director of the New Hampshire Board of Naturopathy and vice president of the Naturopathy Association of North America.
His first publication on the subject was One Man’s Food... Is Someone Else’s Poison, released in 1980, in which the blood type diet hypothesis was presented. Let’s focus on Chapter 3, The D’Adamo Discovery. Here, Dr. D’Adamo describes how, before his findings, naturopaths fell into two groups: those advocating a vegetarian diet as the ideal approach to health and disease management and those believing the perfect diet for everyone consisted of four essential elements—meat, milk, vegetables, and fruit.
Despite their differences, both groups shared a common belief: a single dietary approach could work for all humanity. Unlike them, D’Adamo argued that just as no two people are alike, they should not consume the same foods.
After testing his theory on more than a hundred patients, he concluded that
- People with blood type A produce excess mucus and phlegm when consuming animal products, including dairy. This group, he claimed, achieved optimal health on a completely vegetarian diet.
- People with Group O, in turn, consisted of dynamic and vigorous individuals who struggled to meet their protein needs on a vegetarian diet. To compensate, D’Adamo recommended consuming meat and dairy products.
Additionally, he linked blood types to specific physiological traits and energy levels. He described type O individuals as strong and active, with high body vibration, while types A and AB were considered more fragile and low in energy. Based on this, he classified foods by vibrational level, with animal proteins and dairy as high-vibration foods and vegetables, fruits, and grains as low-vibration foods. He believed that foods conflicting with a person’s vibration could cause stress, organ dysfunction, and disease.
After 20 years of research, D’Adamo believed his hypothesis to be a universal truth. However, he made several errors, particularly in believing that he was "chosen" by God to revolutionize his field and in misinterpreting scientific results, often attributing positive outcomes to his interventions while overlooking factors such as chance or natural symptom regression.
Despite being the theory’s creator, Dr. James D’Adamo was not its most prominent figure. That role belonged to his son, Peter D’Adamo, a naturopath and author of Eat Right 4 Your Type.
Continuing the Legacy
In 1982, during his third year of naturopathy at John Bastyr College in Seattle, Peter dedicated himself to investigating his father’s theory and assessing its scientific validity. Although his father relied on subjective impressions of blood type rather than an objective evaluation, Peter found some scientific evidence linking blood type to certain diseases:
- Patients with blood type O had a higher risk of developing peptic ulcers, a condition associated with high stomach acid levels.
- Blood type A was linked to stomach cancer, a disease often associated with low gastric acid production.
Peter also examined how lectins interact with blood types. He theorized that lectins act like antigens. If a person has antibodies against a specific lectin, consuming it could cause blood cells to clump, leading to inflammation.
To detect this, he used the Indican Scale urine test, claiming that frequent intake of lectin-rich foods increases indican levels, a metabolite produced from the breakdown of tryptophan, that can indicate digestive and absorption issues when found at high levels that Peter believed signaled a rise in carcinogenic substances.
Additionally, he asserted that a blood type-specific diet offers numerous benefits, including reduced infection risk, weight loss, protection against diseases such as cancer, diabetes, and cardiovascular issues, and even slowed aging.
I am type AB. Based on Peter D’Adamo’s claims, I would be doomed to gain weight—regardless of consuming a hypocaloric or normocaloric diet—whenever eating corn and wheat. Conversely, I would lose weight by consuming dairy and seafood, again, without considering calorie intake.
Even stranger, while chicken and beef are deemed "harmful" to my blood type, ostrich meat is classified as "neutral," and lamb as "beneficial." As for dairy, the diet permits mozzarella cheese and yogurt (labeled "highly beneficial") and allows cheddar cheese and skim milk ("neutral") but forbids whole milk and parmesan cheese ("highly harmful").
Peter suggests that type AB individuals, prone to stress-induced irritability and hyperactivity, should engage in moderate activities such as swimming and cycling or relaxing practices like yoga.
He claims that type AB individuals avoid worrying about details, don’t hold grudges, and are charismatic, popular, and kind—flattering, I must admit. However, due to our "promiscuous" immune system, they cannot be considered loyal.
Ultimately, version 2.0 of the blood type diet suffers from the same fundamental flaws as the original:
- Overreliance on anecdotal cases as proof of efficacy
- Lack of scientific methodology.
Despite these efforts, we now know that the blood type diet is a pseudoscientific practice with no credible supporting evidence.
“Every time you come across a new diet that claims to be suitable for any type of person, be suspicious.”
- Peter D’Adamo
A fitting piece of advice—if only he had applied it to his work.
The Real Scientific Evidence
Although scientific literature on the blood type diet remains limited, some researchers have explored its plausibility and potential health outcomes. In 2014, an article published in PLoS One examined the relationship between blood type diets and biomarkers of cardiometabolic health while also assessing whether an individual’s ABO genotype influences these associations.
The study recruited 1,639 young adults aged 20 to 29 over six years. Participants completed a general health and lifestyle questionnaire covering personal characteristics.
The researchers assessed food intake monthly using a semi-quantitative dietary questionnaire and analyzed each blood type diet [1] across the entire population without considering blood groups.
The results showed that adherence to blood-type diets was associated with favorable cardiometabolic risk profiles, particularly in the Type A diet, where adherence correlated with lower BMI, blood pressure, waist circumference, total cholesterol, triglycerides, and insulin levels, regardless of blood group. This is unsurprising, as the Type A diet emphasizes high consumption of fruits and vegetables while limiting meat.
However, these benefits likely result from the general dietary patterns rather than any connection to blood type, which may explain the anecdotal evidence supporting these diets.
It is important to note the potential inaccuracies of using a monthly dietary questionnaire to assess food intake, making such tools useful but not precise in measuring actual nutritional consumption.
That said, this wasn't the only study examining the 'blood type diet' theory. The American Journal of Clinical Nutrition published a systematic review a year earlier.
Researchers conducted a systematic review focusing exclusively on health-related outcomes from diet adherence. They identified 1,415 articles, but using the inclusion criteria of an experimental or observational study of humans grouped by blood type, with adherence to a diet designed to improve health measured by any quantified health measure, only one study qualified. Even that study had significant flaws with a small sample size and concerns about the distribution of participants to each dietary group, leaving its conclusion, dismissing a link between blood type and diet, technically unresolved.
The review authors concluded that despite the diet’s popularity and numerous health claims, no studies have rigorously tested its benefits, and no evidence supports the idea that blood-type-based diets improve health.
Since this review is 11 years old, debunking something as peculiar as the blood type diet feels almost tragic. Hopefully, in the future, we’ll look back and see this mockery of naturopathy as akin to the humor theory of disease. Unfortunately, given the current information landscape, I suspect this is an overly optimistic stance.
[1] Type A diet recommends a high consumption of grains, fruits, and vegetables. Type B diet suggests high consumption of dairy products and moderate intake of other food groups. Type O diet promotes high meat consumption and avoidance of grain-based products, while Type AB is similar to Type B with more dietary restrictions.
Sources: ABO Genotype, ‘Blood-Type’ Diet and Cardiometabolic Risk Factors. PLoS One. DOI:10.1371/journal.pone.0084749
Blood type diets lack supporting evidence: a systematic review. The American Journal of Clinical Nutrition. DOI: 10.3945/ajcn.113.05869
