In 1996, naturopathic doctor Peter D’Adamo published the book Eat Right For Your Type, which presented the hypothesis that diet, exercise, and lifestyle changes tailored to one’s blood type are an effective tool for weight loss, increased energy, and the reduction of myriad other ailments. This has popularly become known as the "blood type diet", for which he is the primary proponent.
D’Adamo’s website provides this description:
A naturopathic physician who is also an author, researcher-educator, Ivesian, amateur horologist, budding software developer and air-cooled enthusiast. He is considered a world expert in glycobiology, principally the ABO (ABH) blood groups and the secretor (FUT2) polymorphisms.
Since its original publication, Eat Right For Your Type has sold over seven million copies, has been promoted by popular (but scientifically unreliable) health personalities like Dr. Oz and has created an entire industry built around the diet, including a series of follow up-books, cookbooks, a variety of blood tests for sale, and other supplements.
There are many different ways to classify blood types, but the most common (and the primary one D’Adamo employs) is the ABO system, which is determined by the presence or absence of two genetically regulated antigens – A and B – attached to the surface of red blood cells. The four blood type groups are A, B, AB, and O.
Superficially, there might appear to be a wealth of complex, technical, and well-documented support for the concept. In addition to his popular books, D’Adamo has written numerous detailed blog posts describing how blood types might affect the body.
Much of the rationale presented on his website fits the following basic structure: there’s a study that shows or hints at various conditions correlating with blood type, therefore a diet tailored to blood type would be more helpful than a general one.
To date, however, no peer-reviewed scientific literature has provided any evidence to support the latter class of arguments. This is a point D’Amado himself cedes. In response to a 2014 study in PLOS ONE that was highly critical of his work, he stated (17 years after first publishing his book):
That the [blood type diet] theory is currently unproven by rigorous scientific study is not argued. Hopefully in time this can be rectified by studies which accurately and comprehensively prove or disprove the hypothesis.
In lieu of accurate and comprehensive studies that prove or disprove his hypotheses, D’Adamo’s basic rationale is rooted primarily in four observations listed on his website:
First, that people with different blood types appear to have differing susceptibility to a variety of illnesses; second, that people with different blood types produce and degrade differing amounts of the stress related hormones cortisol or adrenaline; third, that the antigens that determine your blood type are found in other places besides your blood; and fourth, that that blood type has a strong influence on an individual's gut bacteria.
There are indeed varying levels of scientific basis for the above claims. That blood antigens are found in the tissue of multiple systems is not controversial. A 2013 review paper on the relationship between disease and blood type published in the journal Blood Transfusion concluded:
There is accumulating evidence that the ABO blood group also plays a key role in various human diseases such as cardiovascular, neoplastic and infectious disorders. In the near future, probably many of the statistical associations observed between ABO and diseases will be re-assessed by [genome-wide association studies]. These studies have already confirmed some of the relations detected by targeted researches carried out long before the current era of genomics, such as those with [venous thromboembolism], [coronary heart disease], and pancreatic cancer.
With respect to blood type and stress hormones (which play a big role in his lifestyle and exercise recommendations), D’Adamo cites a limited 1992 study of 15 white male veterans published in the journal Psychosomatic Medicine that showed differences in the production and removal of cortisone in A and O blood types.
The gut bacteria connection is one of the most prominent aspects of his theory. Studies have indeed observed differences in the relative contribution of gut bacteria based on blood type. A 2012 study published in BMC Microbiology that analyzed the gut flora and blood type of 79 Finnish volunteers found:
That the ABO blood group is one of the genetically determined host factors modulating the composition of the human intestinal microbiota, thus enabling new applications in the field of personalized nutrition and medicine.
Much of the dietary recommendations for the blood type diet are based on the concept of eating foods your body is naturally better at handling, as stated on D’Adamo’s website:
For example, the microbiome of certain people developed to break down carbohydrates much more efficiently (blood type A). People lacking this ability (blood type O) tend to store carbs as fat.
Another key diet/blood connection promoted by D’Adamo stems from lectins — a group of proteins found in many foods — which he argues can mimic different blood-type antigens, and “interact” with your kidneys, liver, gut, stomach, or other organ systems:
For the most part our immune systems protect us from lectins. Ninety-five percent of the lectins we absorb from our typical diets are sloughed off by the body. But at least 5 percent of the lectins we eat are filtered into the bloodstream, where they react with and destroy red and white blood cells. The actions of lectins in the digestive tract can be even more powerful. There they often create a violent inflammation of the sensitive mucous of the intestines, and this agglutinative action may mimic food allergies. Even a minute quantity of a lectin is capable of agglutinating a huge number of cells if the particular blood type is reactive.
There are independent scientific reports of potential ways that lectins might interact with different bodily systems to cause disease as described above, but no peer-reviewed research directly tests a potential link between those phenomena and the claim that a blood type based diet would ameliorate any of those potential mechanisms.
A final aspect of D’Adamo’s blood type diet is the additional classification of a person’s “secretor/non-secretor status”. This is a classification independent of ABO blood type and refers to a person's genetically-determined ability to secrete blood type antigens into your saliva or mucus. In a 2001 paper in Alternative Medicine Reviews, he argued that this classification could have sweeping implications for human health:
Understanding the clinical significance of ABH secretor status and the Lewis blood groups can provide insight into seemingly unrelated aspects of physiology, including variations in intestinal alkaline phosphatase activity, propensities toward blood clotting, reliability of some tumor markers, the composition of breast milk, and several generalized aspects of the immune function.
The information summarized above provides a broad outline of the most commonly promoted scientific rationale for the blood-type diet, though there are pages and pages more written by D’Adamo on the topic. In essence there are a number of “seemingly unrelated” facts about blood that could, maybe, respond to different diet and exercise regimens.
A followup series of books published by D’Adamo included a different book for each ailment that he believed his diet might improve. Included in this series: arthritis, diabetes, cancer, cardiovascular disease, allergies, fatigue, menopause, and aging.
The only problem is that there is literally no evidence demonstrating his ideas work, and D’Adamo himself doesn’t appear to be super interested in producing that evidence. In 1996, in the first edition of Eat Right For Your Type, D’Amato wrote:
Even now, as I write this, I am beginning the eighth year of a ten-year trial on reproductive cancers, using the Blood Type Diets. My results are encouraging. So far, the women in my trial have double the survival rate published by the American Cancer Society. By the time I release the results in another two years, I expect to make it scientifically demonstrable that the Blood Type Diet plays a role in cancer remission.
Such a study has yet to be released. On his website, D’Adamo has a section for scientific papers that support the science behind the diet. Nearly all of these self-authored papers are from the late 80s and early 90s, or are more recent blog posts written by D’Adamo.
With the exception of that 2001 paper mentioned earlier, the only scientific papers he has published are either in the long defunct (but allegedly peer-reviewed) journal the Townsend Letter for Doctors (which frequently published studies on the efficacy of homeopathy), and the Journal of Naturopathic Medicine, which no longer exists and is not catalogued in any major scientific or medical journal index.
There have, however, been a few peer-reviewed studies that sought to test some aspects of the blood-type diet.
The first was a 2013 literature review published in the American Journal of Clinical Nutrition that sought to find any existing, statistically rigorous research showing that tailoring diet to blood type has a beneficial effect. The effort yielded only 16 studies, and of these only one met their standards for inclusion. Further, that study didn’t actually address the question of whether or not blood-type diets themselves were efficacious. The authors of this literature review conclude:
No evidence currently exists to validate the purported health benefits of blood type diets. To validate these claims, studies are required that compare the health outcomes between participants adhering to a particular blood type diet (experimental group) and participants continuing a standard diet (control group) within a particular blood type population.
D’Adamo argued in a blog post that the absence of papers should not be surprising, given the cutting-edge and novel nature of his then 17-year-old hypothesis:
Not surprisingly, they didn’t find any. Had they contacted me prior to the study I could have saved them a lot of extra work. I’ve looked high and low and also never found one. That’s how original this theory is.
A 2014 study published in the journal PLOS ONE attempted to provide novel research into the claims of the blood-type diet by analyzing the results of a larger scale nutrition study that utilized 1,455 subjects who “were participants of the Toronto Nutrigenomics and Health study” and whose “dietary intake was assessed using a one-month, 196-item food frequency questionnaire and a diet score was calculated to determine relative adherence to each of the four ‘Blood-Type’ diets.’”
That study concluded that, while individuals who adhered to any one of the four different blood-type diets did have favorable health benefits, there was no relationship between blood type and those favorable effects. According to a University of Toronto press release:
“Based on the data of 1,455 study participants, we found no evidence to support the 'blood-type' diet theory," said the senior author of the study, Dr. Ahmed El-Sohemy, Associate Professor and Canada Research Chair in Nutrigenomics at the U of T.
"The way an individual responds to any one of these diets has absolutely nothing to do with their blood type and has everything to do with their ability to stick to a sensible vegetarian or low-carbohydrate diet," said El-Sohemy.
D’Adamo has subsequently published a number of blog posts criticizing the study for not accurately following the prescribed blood type diet while simultaneously celebrating the fact that the diet (in general) helped people lose weight and improve health. Ahmed El-Sohemy, the lead researcher behind the PLOS ONE study, has composed detailed responses to each of these posts in the comments section, if you want an illuminating read.
D’Adamo is certainly prolific, both in his numerous blogs and in his library of best selling books. Lost within that sea of complex jargon, speculative observations, and anecdotal tales, however, is the fact that there is no widely accepted or peer-reviewed evidence that modifying one’s diet to match blood type has any effect on one's health.