The Blood Type Diet, popularized by Dr Peter D’Adamo, was all the rage for awhile—and it seems to be enjoying a resurgence, thanks to Dr Gundry’s Low Lectin Diet, based on a similar idea. Both diets revolve around the concept of lectins, or proteins that protrude from a cell’s surface and react with red blood cells, producing clotting (among other things).

The difference is, Dr Gundry’s diet recommends avoiding all foods high in lectins for all adherents, while Dr D’Adamo’s diet recommends that individuals only avoid certain foods whose lectins are known to cross-react with their blood type. I addressed my opinion of Dr Gundry’s diet here. But is there anything to the idea that your blood type affects your digestion?

The Blood Type Diet Concept

Our red blood cells carry one of two antigens, or peptide sequences: A, or B. These make up two of the four possible blood types. The other possibilities are AB (you get both of them, one from each parent), or O (you get neither). Which combination you get is genetically determined.

According to the Blood Type Diet, O types are descended from our “hunter-gatherer” ancestors. As such, they do best with high protein, low grain diets. A blood types are descended from our early agrarian ancestors, and therefore do best with a vegetarian diet. B types are the type that can best handle dairy products. Since AB individuals have antigens from both A and B, they can consume a more diverse diet.

This is an extreme over-simplification, though: the recommended diet for each blood type is quite specific, sorting foods you’ve probably never heard of into “highly beneficial, neutral, and avoid” categories, according to the reaction of those foods’ lectins with red blood cell antigens.

The Data Behind the Blood Type Diet 

In 2013, this systematic review created a lot of buzz with the categorical statement that there were no studies available to support the health claims of the Blood Type Diet. Of all the articles considered for inclusion, only one met their search criteria—and even it did not directly address the Blood Type Diet. In other words, the question had never been subjected to a peer-reviewed study up until that point. 

In 2014, the question was raised again in this study, which did specifically investigate the effect of the Blood Type Diet on risk factors for heart disease. It tested 1455 individuals for blood type, and and subsequently tracked participants’ BMI, waist circumference, blood pressure, serum cholesterol levels, triglycerides, insulin, and beta cell function. At the same time, participants’ adherence to their recommended blood type diet was assessed via a food frequency questionnaire. The conclusion? Those who adhered to any of the blood type diets experienced improvement in the tracked risk factors, whether or not they strictly followed the diet recommended for their own blood type. All four diets are considered “healthy”, and can all therefore be expected to confer health benefits—but independent of blood type.

Dr D’Adamo’s response to this study states that the study was poorly designed—namely, their questionnaire was far too general, ignoring many foods that should have been sorted into their respective blood types, and combining foods (such as “pizza” and “hamburger” and “sandwich”) that should have been separated out into their constituent parts in order to clarify their true effect upon blood type. He therefore maintains that at this point, no conclusive studies exist on the diet, for good or ill.

Hydrochloric Acid and Blood Type

That may be true of the minutiae of Dr D’Adamo’s diet, but we can draw a few broad strokes correlations between blood type and digestion, especially as it relates to stomach acid. 

Specifically, Type O’s tend to have higher stomach acid than other types, while Type A’s tend to have lower stomach acid than other types. This does mean that Type O’s are better equipped to break down dense protein (like meat), while Type A’s are less well equipped, and will therefore do better with vegetarian proteins.

Adequate stomach acid is necessary for not just proper digestion of dense protein, though; it also helps to maintain proper balance in the microbiome. Aside from merely helping with intestinal digestion, the microbiome helps protect the body against foreign invaders, and also helps to educate your immune system about the difference between friend and foe. This helps to minimize allergies and autoimmunity. Perhaps low stomach acid can help explain why Type A’s are at greater risk for gastric cancer than other types.

On the other hand, too much stomach acid can set you up for ulcers—and indeed, Type Os are apparently at greater risk for those.

Does eating a protein-dense diet for Type Os, and a primarily vegetarian diet for Type As, affect these propensities? We don’t have the peer-reviewed studies to tell us definitively. But in the case of Type A’s, it stands to reason that poorly digested animal protein may ferment, increasing the imbalance in the microbiome. This could adversely affect micronutrient status, as well as immune function. It also stands to reason that the higher hydrochloric acid levels in Type Os would better equip them for eating meat, though I’m not sure how doing so would affect their propensity toward ulcers.

The Upshot

There may be some truth behind the Blood Type Diet’s more specific claims, but we cannot yet verify those with large-scale studies. In my clinical experience, though, I can certainly attest that there’s no such thing as a “one size fits all” diet. Some people do great vegetarian; some really don’t. Some people thrive on the ketogenic diet or other low-carb diets; others, not so much.

In some cases I do genetic testing in order to determine which diet is best. But if you know your blood type, you’ve already got a pretty solid clue.