Written by Michael Klaper, M.D.

According to the blood type diet theory, people with Type A blood (39% of the population) are best off as vegetarians, while people with Type O (46% of the population) are natural born meat eaters. Type B’s (11% of the population) are somewhere in the middle. Simple and clear cut? The problem is the theory is based on shaky science.

Update April 2008: Colleen and John discuss this during a recent podcast. Colleen responds to the common concerns about the Blood Type Diet theory by pointing out that she, herself, is Type O, the blood type of those who are “genetically required to eat meat” according to this theory. Colleen reports that she has felt so much better as a vegan for the past four years. And a past member in the Resource Centre, Kate, has been a very successful Type O vegan for over 10 years. Colleen has also learned of several more Type O vegetarians and vegans among her friends.

The following article originally appeared in the January 1999 issue of Lifelines. A longer more indepth version is available at EarthSave’s website.

The “blood type diet” theory has gained widespread public attention since the release of Eat Right For Your Type by Peter J. D’Adamo, N. D. The book’s basic premise is that Type O’s are the dominant, hunter-type, genetically required to eat meat, while Type A’s are docile vegetarians, and Type B’s are dairy-eating omnivores. However, the “blood type diet” theory and the book which promotes it, present many scientific and nutritional problems which have raised the concern of numerous scientists and health professionals.

Adverse effects of lectins? Where is the evidence?

For example, author D’Adamo states that, “…certain beans and legumes, especially lentils and kidney beans, contain lectins that deposit in your muscle tissues, making them more alkaline and less charged for physical activity.” (p.53) This is quite a serious scientific charge, and an alarming thought if you are blood Type O. Worrisome statements like these should be backed up with solid scientific evidence – complete with “footnoted” references – which the book repeatedly fails to do.

To begin to convince me of the adverse effects of lectins, D’Adamo would have to publish photographs, taken through a microscope, of muscle fibers sampled from people with Type O, Type A, Type B, and Type AB blood after they have eaten kidney beans and/or lentils. (Sampling of muscle fibers, fat tissue, etc., is a common, safe, and virtually painless technique, known as “skinny needle” biopsy, and is routinely performed on paid volunteers by researchers in nutrition, exercise physiology, pharmacology, aging, and other sciences.) The tissue photographs should clearly show the lectin deposits appearing in the muscles of people with Type O blood – and not in the muscle samples from the people with Type A blood. Eat Right For Your Type presents neither photos nor corroborating studies to support its speculations, which for me, severely curbs this book’s credibility.

Beyond the limits of believability

For me what really pushes the “blood type” theory beyond the limits of believability is D’Adamo’s postulation that lectin proteins on some foods cause blood agglutination in people of certain blood types who are “not genetically/evolutionarily suited” to eat those foods. Agglutination is a very serious, and potentially life-threatening, phenomenon, whereby the red cells in the bloodstream stick together, forming irreversible clumps.

What is so bad about little clumps of red blood cells sailing through the bloodstream? Red blood cells deliver oxygen to the cells of vital tissues like the brain, heart and kidneys. To accomplish this delivery, red blood cells must flow through capillaries so narrow that they must line up single file. If the red cells are being agglutinated by lectins or anything else, clumps of red cells will clog up the capillaries and block the blood flow. Thus, the blood stream will be preventedfrom delivering its life-sustaining cargo of oxygen to the tissues served by those capillaries. Cells deprived of oxygen become damaged, and eventually die.

A blood type astrology that imposes limiting stereotypes on very complex human beings

Since most people are unaware of their blood type, it is reasonable to assume that many of us regularly eat the “wrong foods” for our blood type (e.g., Type O eating wheat, Type A eating meat, etc.). Thus, according to D’Adamo’s theory, everyone experiences repeated showers of agglutinated red cells throughout their bloodstream, day after day, month after month, year after year. If the capillary beds in your heart, lungs, kidneys, brain, eyes, and other essential organs are subjected to a repeated barrage of agglutinated red cells, they will eventually begin to clog up, resulting in tissue damage. The brain, heart, lungs, kidneys and adrenals would soon be irreparably damaged by these processes, which could prove fatal for millions of people.

Pathologists and other medical scientists would be very familiar with a syndrome of organ failure due to lectin-induced micro-infarctions (cell death). The existence and intricacies of such a widespread disease would be as well-known as is atherosclerosis. Yet, I am aware of no such description in the pathologic literature and no pathologist I know has ever mentioned this as a cause of any disease in humans.

A wheat allergy is a more likely explanation

When I read a generalized statement, like, “Type O’s do not tolerate whole wheat products at all,” (p.63) I have to wonder what he means by ‘at all’. Do Type O’s eat a whole wheat cracker and fall on the ground holding their abdomen while vomiting? Or worse yet, do they suffer immediate brain damage due to clogging blood cells? How much wheat can Type O’s eat before their blood clogs? One hamburger bun? One noodle?

I’m not denying that many people experience problems when they eat wheat. However, I believe they do so because they have a true wheat allergy, gluten intolerance, or some other verifiable mechanism. Like D’Adamo, I grant that wheat can be a problematic food for people with colitis, and I often recommend eliminating it from the diet. Lectins may even play a role in the inflammatory process for some people. However, to convince me that blood type is the key determining factor in the diet-colitis link, D’Adamo would have to present solid evidence that wheat-induced colon dysfunction is a condition peculiar to (or significantly more common in) people with Type O blood. Yet, the text of Eat Right For Your Type is strangely devoid of any scientific citations, and consequently, I find no convincing proof of the lectin-colitis connection in this book.

Unsafe advice for Type B’s of Asian descent

A statement that causes me great concern regarding the safety of this book’s dietary advice appears on page 37. Despite widespread knowledge that many non-Caucasians are intolerant of dairy products, the book recommends that “Type B’s of Asian descent may need to incorporate them (dairy products) more slowly into their diets as they adjust their systems to them.” I fear that the consequences of this for many lactase-deficient readers will be severe bouts of abdominal cramps, diarrhea, and possibly more serious conditions, like colitis.

Iodine and Type O’s

Another troubling assertion in Eat Right For Your Type is, “This condition, called hypothyroidism, occurs because Type O’s tend not to produce enough iodine.” (p.53) The reality is that the body does not “produce” iodine, rather we get it by eating iodine-containing foods. To worry tens of millions of Type O readers that they “may not be producing enough iodine” (which no one does) and are thus at risk for hypothyroidism, is unfounded and, unnecessarily worrying. (People should be encouraged to eat sufficient iodine-containing foods, especially if they are vegetarians or vegans.)

The ABO blood typing system is very limited

D’Adamo’s system seems to create a “blood type astrology” (“What’s your type? O Positive? knew it! So am I!”) that imposes strange, limiting stereotypes on very complex human beings.

Remember, there is nothing sacrosanct about the ABO blood typing system devised by Dr. Landsteiner in the 1920’s. It is only one system classifying more than thirty proteins on the surface of cells that determine other blood groups, with names like Auberger, Diego, Duffy, Kell, Kidd, Lewis, Lutheran, MNSs, P, Rh, Sutter, and Xg. This means that food selections that may be “right” for the ABO blood group system might be “dead wrong” for someone’s Kell or idd antigens. Why are we deifying the D- galactosamine-fucose molecules on the red cell surfaces that determine ABO Type?

We all have different nutritional needs, and different people are suited to different types and amounts of food. However, I do not believe that these differences are significantly determined by blood type. I certainly haven’t come across any research or scientific evidence in Eat Right For Your Type which proves to me otherwise. My (admittedly over-simplified) suggestions for optimal nutrition are: pay attention to what your body tells you, and focus your diet on fresh, whole foods – grown as organically as possible, and served with ample portions of love and laughter.

longer more indepth version of this article with references is available at EarthSave’s website.

Michael Klaper, M.D. is the director of the Institute of Nutrition Education and Research. His video, ‘Diet for all Reasons’, and his book ‘Vegan Nutrition: Pure & Simple’, are available for loan from Toronto Vegetarian Association’s Resource Centre.

The Blood Type Diet: Latest Diet Scam

A critique of the blood type diet theory by two naturopathic physicians. “The foundation for the blood type diet theory is comprised of three ‘legs’. The individual legs contain inaccuracies which render each one scientifically and logically flawed.”