Last week I wrote here on the association between blood type and gastric acid secretion. The conclusion was that Blood Type O produces the highest amount of gastric acid—especially secretors (people with the FUT2 gene that means they secrete their blood type into other types of bodily fluids besides just blood—such as digestive juices, for instance. About 80% of the population are secretors.) By contrast, Type A produces the lowest amount of gastric acid.

Blood Types and the Microbiome

There is also growing evidence that your blood type plays a profound role in the composition of your gut microbiome, but there is a greater difference between secretors and non-secretors than there is between the individual secretor blood types (A, B, AB, or O).

But why?

It’s possible that this might be in part due to the alteration of gastric secretions—lower stomach acid can render the terrain more favorable to bacterial overgrowth. I haven’t been able to find specific studies to demonstrate higher risks of certain blood types for SIBO, but this study does say that “blood group A antigens in the secretor subjects are associated with an expansion families of bacteria within the gut.” This was my theory, too: you’d think those with lower gastric acid production would be at higher risk, and we’ve established that those individuals are Blood Type A (or, presumably, anybody who has been taking acid-blocking medications for any length of time).

According to Dr D’Adamo, there is another reason why blood type influences the microbiome composition, at least in the case of secretors. Certain bacteria are able to metabolize the antigens expressed that indicate blood type and use it as food–different bacteria for different blood types. And as we all know, what you feed, grows.

It’s also quite clear that your blood type influences susceptibility to various different types of infectious disease. This makes sense, since your microbiome plays a key role in protecting you against foreign invaders.

Evidence for the Blood Type Diet?

As a reminder, the Blood Type Diet is based on the idea that lectins (or tiny proteins) on certain foods cross-react with the antigens produced by certain blood types, producing agglutination (sticking together) of red blood cells, and therefore cellular damage and inflammation.

On one hand, we know that the process of digestion and cooking destroys most lectins in our foodsmost, but not all. But in the case of secretors, some of those lectins will come in contact with blood antigens in the digestive tract itself. And we do have evidence that lectins from food interact with blood antigens—potentially causing agglutination. If this is the case, it does stand to reason that the resulting inflammation might serve as a trigger for leaky gut syndrome. Once this occurs, some of those lectins that might otherwise have been destroyed in digestion could wander far and wide throughout the body (along with lipopolysaccharides, or LPS, which would certainly also contribute to inflammation.)

The Upshot

Your microbiome is a big deal. It’s critically important to your immune system, helping to defend you against foreign invaders, and also teaching it to recognize friend versus foe. This helps to protect you against allergies and autoimmunity.

While the jury is still out on whether following the blood type diet can improve your health, the number of recent studies indicating an effect of blood type on microflora certainly suggest that the need for further large-scale studies.