© Elena Boils

In 1996 a naturopath named Peter D’Adamo published a book called Eat Right 4 Your Type. D’Adamo argued that we must eat according to our blood type, in order to harmonise with our evolutionary heritage.

Blood types, he claimed, “appear to have arrived at critical junctures of human development.” According to D’Adamo, type O blood arose in our hunter-gatherer ancestors in Africa, type A at the dawn of agriculture, and type B developed between 10,000 and 15,000 years ago in the Himalayan highlands. Type AB, he argued, is a modern blending of A and B.

From these suppositions D’Adamo then claimed that our blood type determines what food we should eat. With my agriculture-based type A blood, for example, I should be a vegetarian. People with the ancient hunter type O should have a meat-rich diet and avoid grains and dairy. According to the book, foods that aren’t suited to our blood type contain antigens that can cause all sorts of illness. D’Adamo recommended his diet as a way to reduce infections, lose weight, fight cancer and diabetes, and slow the ageing process.

D’Adamo’s book has sold 7 million copies and has been translated into 60 languages. It’s been followed by a string of other blood type diet books; D’Adamo also sells a line of blood-type-tailored diet supplements on his website. As a result, doctors often get asked by their patients if blood type diets actually work.

The best way to answer that question is to run an experiment. In Eat Right 4 Your Type D’Adamo wrote that he was in the eighth year of a decade-long trial of blood type diets on women with cancer. Eighteen years later, however, the data from this trial have not yet been published.

Recently, researchers at the Red Cross in Belgium decided to see if there was any other evidence in the diet’s favour. They hunted through the scientific literature for experiments that measured the benefits of diets based on blood types. Although they examined over 1,000 studies, their efforts were futile. “There is no direct evidence supporting the health effects of the ABO blood type diet,” says Emmy De Buck of the Belgian Red Cross-Flanders.

After De Buck and her colleagues published their review in the American Journal of Clinical Nutrition, D’Adamo responded on his blog. In spite of the lack of published evidence supporting his Blood Type Diet, he claimed that the science behind it is right. “There is good science behind the blood type diets, just like there was good science behind Einstein’s mathmatical [sic] calculations that led to the Theory of Relativity,” he wrote.

Comparisons to Einstein notwithstanding, the scientists who actually do research on blood types categorically reject such a claim. “The promotion of these diets is wrong,” a group of researchers flatly declared in Transfusion Medicine Reviews.

Nevertheless, some people who follow the Blood Type Diet see positive results. According to Ahmed El-Sohemy, a nutritional scientist at the University of Toronto, that’s no reason to think that blood types have anything to do with the diet’s success.

El-Sohemy is an expert in the emerging field of nutrigenomics. He and his colleagues have brought together 1,500 volunteers to study, tracking the foods they eat and their health. They are analysing the DNA of their subjects to see how their genes may influence how food affects them. Two people may respond very differently to the same diet based on their genes.

“Almost every time I give talks about this, someone at the end asks me, ‘Oh, is this like the Blood Type Diet?’” says El-Sohemy. As a scientist, he found Eat Right 4 Your Type lacking. “None of the stuff in the book is backed by science,” he says. But El-Sohemy realised that since he knew the blood types of his 1,500 volunteers, he could see if the Blood Type Diet actually did people any good.

El-Sohemy and his colleagues divided up their subjects by their diets. Some ate the meat-based diets D’Adamo recommended for type O, some ate a mostly vegetarian diet as recommended for type A, and so on. The scientists gave each person in the study a score for how well they adhered to each blood type diet.

The researchers did find, in fact, that some of the diets could do people some good. People who stuck to the type A diet, for example, had lower body mass index scores, smaller waists and lower blood pressure. People on the type O diet had lower triglycerides. The type B diet – rich in dairy products – provided no benefits.

“The catch,” says El-Sohemy, “is that it has nothing to do with people’s blood type.” In other words, if you have type O blood, you can still benefit from a so-called type A diet just as much as someone with type A blood – probably because the benefits of a mostly vegetarian diet can be enjoyed by anyone. Anyone on a type O diet cuts out lots of carbohydrates, with the attending benefits of this being available to virtually everyone. Likewise, a diet rich in dairy products isn’t healthy for anyone – no matter their blood type.