Beware of the blood-type diet

Almost 20 years ago, a naturopath made a big name for himself with his now-famous “blood-type” diet. He claims your blood type should determine how much meat, vegetables, and dairy you eat. And he wrote a book about the diet that sold millions of copies. A few years back, one snazzy TV doctor even claimed to lose weight by following the diet.

Now, I’ll admit, the name of the diet may sound scientific to some. But it’s actually just another fad diet…with no actual scientific evidence to back it up. In fact, Canadian researchers just published a great study debunking the diet completely. I’ll tell you more about what they found in a moment.

But first, let’s talk about blood typing…

Your blood type depends on whether you have certain proteins, called antigens, on the surface of your red blood cells. This determines whether you’re Type A, B, AB, or O.

The physician who developed the blood-type diet took this basic biology and went about 1,000 steps beyond any real science. He argues that Type O is the oldest blood type. And he calls them the “hunter” types. So, accordingly, anyone with Type O should eat a high-protein diet with plenty of meat. Type A is agrarian. So they should follow a mainly vegetarian diet. Type B is the “nomad” and thrives on dairy. But he didn’t know what to make of Type AB. He just calls it the “enigma.” And he says it’s a mixture of A and B.

Now I’m a medical anthropologist. So I can soundly say, with some measure of authority, that this theory always struck me as complete nonsense.

No matter which “type” of blood you have, all red blood cells do the same thing. They carry oxygen in hemoglobin molecules for respiration. In fact, this is their one and only function. They don’t have nuclei. So they can’t direct metabolic functions. Or play a part in metabolic functions. They don’t even have anything to do with digestion.

So, you may wonder, why would someone develop a diet based on something that has nothing to do with metabolism? Or even digestion?

Well, the scientists at University of Toronto wondered too.

In January, they published results from a study that involved almost 1,500 healthy, young adults. They asked the participants to fill out questionnaires about their diet. They also sampled the participants’ blood to determine blood type. And they measured different health markers, such as blood sugar levels and BMI. Then they analyzed the data to see if men and women improved their health markers by following a diet “correctly” matched to their blood type, according to this theory.

In other words, were the men and women who followed their correct blood-type diet indeed healthier.

Of course they weren’t.

In fact, according to lead researcher Dr. Ahmed El-Sohemy, “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.” He also said, “It was an intriguing hypothesis so we felt we should put it to the test. We can now be confident in saying that the blood type diet hypothesis is false.”

I’m glad we finally have the evidence to put this absurd diet out to pasture. But even before this study came out, the diet never made much sense to me.

Digestion and metabolism are highly complex processes. They involve hundreds of enzymes and genes. So I could never see how the presence or absence of three protein antigens on the surface of your red blood cells could determine anything about your diet or nutrition. And any reductionist theory that tries to explain everything about your metabolism based upon a few simple, arbitrary markers is…well…simple-minded. And it’s bound to miss the complexities of your individual nutritional needs.

What does work for everyone, regardless of blood type, is this…

Eat right to feel right. Follow a diet filled with green, leafy vegetables. And limit your carbohydrates. It’s that simple. Save your blood type for when you donate blood. In fact, donating blood regularly can help you lower any excess iron and reduce your risk of chronic diseases. So do it often!


1. “ABO Genotype, ‘Blood-Type’ Diet and Cardiometabolic Risk Factors,” PloS One, published online: 1/15/2014

2. “Theory behind popular blood-type diet debunked,” Science Daily ( 1/15/2014