Diet Review: The blood type diet: Bad Bloody Science?

I generally try to avoid things that annoy me, but too many of you have asked me to review the Blood Type Diet, so I had to bite the bullet. The Blood Type Diet has been around since 1996 (aka far too long) and for some reason, people are still thinking that it has some scientific merit.

SIGH. Is it because a doctor invented it?

Just so you know, Dr. D’Adamo is not a medical doctor, he’s an naturopathic doctor. You’ve got to watch those semantics, although when it comes to writing nutrition books, both professions have about the same track record for writing garbage.

Whatever D’Adamo may be licensed in, the first lesson for everyone is that just because a (medical or otherwise) doctor writes a nutrition book, doesn’t mean they know anything about nutrition.

Keep in mind that every healthcare profession has its own shortfalls – but if you’re looking for solid evidence-based medicine, remember that NDs tend to focus more on alternative therapies, many of which are disproven or not based in science.

Hey, if that’s what you’re into, that’s great. Just take it at face value.

A special thanks to the super-intelligent Nanci Guest, RD, MSc, CSSC, and PhD candidate at the University of Toronto – who knows a heck of a lot more than I do about blood types and genetics – for answering all my questions for this post and making sure you readers got the correct info about this diet.

What is the Blood Type Diet?

You probably know what blood type you are – A, B, AB, or O. These blood types are called the ‘ABO’ blood group or genotypes, which is the classification system for human blood.

According to the Blood Type diet, our blood type can predict how our bodies react to food, our susceptibility to disease, and our natural reaction to stress. There is some truth to that, as studies suggest that certain blood types are associated with an increased risk of some illnesses.

But that’s where the truth ends – because while diet can affect components of your blood – like cholesterol, triglycerides, and iron; your blood type does not have proven significant impact on the way food is metabolized by your body. So like many ‘alternative’ therapies and diets, the author is taking one true scientific fact and running with it….like, off a cliff.

I think because blood types and genetics are so complicated, people automatically defer to anyone who seems like they know stuff about them. You’ve got to watch that, because in case of this and many other ‘scientific’ diets (ahem, Hormone Reset Diet, I’m talking about you), the author is full of BS.

My favorite claim from the Blood Type Diet is that if you’re a blood type O, you can’t digest certain carbohydrates and are more likely to have temper tantrums. I’m sorry, what? Absurd.

Let’s take a closer look at the Blood Type Diet.

According to the diet, each ABO blood group has their own list of foods to avoid, depending on the point in history when that particular blood group evolved. The quick rundown:

Type A’s should be vegetarian, avoiding all animal products.

Type B’s should be eating lots of dairy products.

Type AB’s should be eating lots of dairy, but also eggs and fish and no meat.

Type O’s should be eating a lot of animal products, since D’Amato claims that this blood type is the ‘ancestral’, or first, blood type that existed – during the hunter/gatherer period.

I’m type A, so according to this diet, this is how I should be eating:

The key factor in the development of Type A can be traced to the struggle for survival long ago, when there was a rapidly dwindling supply of hunting game stock. Having exhausted the great game herds of Africa, humans pushed farther out from their ancestral home into Europe and Asia. The cultivation of grains and livestock changed everything.

For the first time, people were able to forego the hand to mouth lifestyle of the hunter/gatherer and establish stable communities. Over time the adaptations that produced Blood Type A were based on the need to fully utilize nutrients from carbohydrate sources. These biological adaptations can still be observed today in Type A’s digestive structure.

Low levels of hydrochloric acid in the stomach and high intestinal disaccharide digestive enzyme levels permit the more efficient digestion of carbohydrates. According to Dr. D’Adamo, these are also the very factors, along with low levels of intestinal alkaline phosphatase, that make it difficult for Type As to digest and metabolize animal protein and fat.

Let me summarize that in normal-person language:

Type O’s:

Went from Africa to Europe and Asia and began cultivating food instead of hunting and gathering it. This led to the creation of type A blood, and the resulting blood adaptations, leading to:

Low levels of stomach acid.

High levels of an enzyme required to break down carbohydrate.

Low levels of intestinal alkaline phosphatase, the function of which is to help absorb fat and help to attenuate inflammation, among other things.

The Blood Type diet goes on to say, “Type A’s flourish on a vegetarian diet – if you are accustomed to eating meat, you will lose weight and have more energy once you eliminate the toxic foods from your diet.”

Aaaand here we go with the ‘toxin’ thing. RED FLAG!

It’s sort of all over for me once anyone uses the word ‘toxin’ related to food. It reeks of fear mongering and, frankly, it’s a dead giveaway that someone doesn’t science. I truly believe that if you have to scare your readers into believing what you’re saying, then you must have something to hide, and/or something to sell.

In this case, it’s both. D’Adamo’s assertions about this diet, and his historical facts, aren’t proven by science. In fact, there was zero research actually done until 2014 to test the validity of his diet, and that research wasn’t done by D’Amato.

I mean, if you’re going to make big claims about a diet you’ve invented, why wouldn’t you test it first?

I don’t want to sound jaded or anything, but it makes me want to poke my eyes out when I see outrageous diet books and plans being sold by doctors who ride on their MD credentials to sell absolute malarkey. And again, D’Adamo isn’t even a medical doctor.

D’Adamo also has something to sell besides the book: a whole line of very expen$ive supplements for each blood type! WOW! What a SHOCKER!!! Because god forbid type A’s should take supplements meant for type AB’s, they might grow another head or something!! SIGH.

The Science behind the Blood Type Diet.

The study that’s been done to test the validity of the Blood Type diet was conducted by researchers from the University of Toronto in Canada, and published in 2014 in the journal PLoS One. I asked Nanci Guest to explain the findings, and this is what she said:

Their findings showed that adherence to certain ‘blood type’ diets is associated with favorable effects to blood lipids, lower BMI and blood pressure factors, but these associations were not related to an individual’s ABO genotype (meaning, blood type had nothing to do with it).

The researcher’s findings that an association between the blood type A diet adherence and favorable cardiometabolic risk profile was not surprising considering this diet’s emphasis on high consumption of fruits and vegetables, and low consumption of meat products, which is similar to a plant-focused or Mediterranean dietary pattern that has been recommended by various health agencies because of its association with a lower risk of cardiovascular diseases.

Adherence to the blood type AB diet was also associated with favorable levels of several risk factors, despite its recommendation for certain dairy and meat products.

Such benefits may be attributed to the list of certain food items considered healthy, which are recommended. For example, individuals with blood group AB are advised to avoid butter and to consume eggs and fish as their main animal-protein source.

This is in contrast to the blood type B diet, which would be viewed as less healthy and had fewer restrictions on many animal products. These differences between the two diets may partially explain why a favorable cardiometabolic profile was associated with adherence to the blood type AB diet, but not for the blood type B diet.

The blood type O diet is similar to low-carbohydrate diets, which may explain why adherence to this type of diet was associated with lower serum triglycerides, as previously observed for other low-carbohydrate diets (and here).

Guest also notes that D’Amato might need a history lesson:

Blood group A has been suggested to be the ancestral human blood group rather than group O as postulated by D’Adamo.

I wouldn’t be a dietitian if I didn’t pause here to comment on D’Adamo’s claim that lectins have a negative effect when consumed by certain blood types.

Lectins – the proteins in some foods – are the ‘new gluten’ in that questionable and disreputable ‘health gurus’ and doctors looking to make a buck are warning people to stay away from lectin-containing foods because they make us sick, weak, and whatever else.

There is zero research to prove that dietary lectins have a significant effect on one blood type over another, or that we shouldn’t be eating them. While it’s true that we shouldn’t be eating some lectin-containing foods without cooking them first, but if you’re not eating raw kidney beans, you don’t have to worry.

For more about lectins, I reviewed the Plant Paradox and lectin kerfuffle here. But bottom line is that the lectin and blood type claim that D’Adamo makes is completely nonsensical.

In Short:

There’s nothing magical about the blood type diet. Any positive effects you may experience on the diet are most likely related to the fact that you’re cutting foods out of your diet, not because anything is related to your blood type.

Plus, the fact that humans with all blood types have been consuming mixed diets for centuries, and our lifespans and quality of life continue to improve, should be enough to steer you away from a diet like this.

There’s nothing wrong or dangerous for healthy people who want to try the Blood Type Diet. Just don’t believe that the ‘science’ behind the diet is actual science. More like pseudoscience. And, do not buy his supplements. Please. Do. Not.

Be wary of anyone who tries to scare you into buying something, and/or with something to sell. Common sense dictates that.

Just because a doctor writes a book, doesn’t mean they’re selling credible information. Many, many doctors (naturopathic, chiropractic, and medical etc.) ride on their credentials – and the trust of laypeople – to sell useless diets.