For someone who had hoped to go vegetarian — and who is among the 45 percent of people in the U.S. with Type O blood — the news was surprising.
The idea of eating according to your blood type became popular with the 1996 book "Eat Right 4 Your Type" (Putnam Adult) by Peter D'Adamo, a naturopath physician. The theory is that the genes behind blood type also are behind the expression of other proteins in our body, which relate to how we digest foods.
Type O's, having the oldest blood type, draw on our hunter roots and perform best on lean meats (including fish), fruits and vegetables, as well as with intense exercise, while gluten, some beans and dairy lead to weight gain, sickness and sluggishness, the theory posits. The proteins in legumes and dairy tend to cause inflammation in Type O's, goes the theory, so without meat it's difficult to fulfill their protein needs.
Rather than banning Type O's from being vegetarians, D'Adamo said, "a more helpful way of thinking is that people who are blood Type O may want to explore a more high-protein, lower-carbohydrate lifestyle in lieu of their current dietary choices, should they suffer from signs that they are not getting enough protein in their diets: digestive disorders, fatigue, low immunity or slow metabolism."
Meantime, people with blood Type A do best as vegetarians, B's are hardier omnivores and ABs are a combination.
Most nutritionists do not subscribe to eating by blood type because it is not backed by hard scientific data, said Marjorie Nolan, a dietitian and spokeswoman for the American Dietetic Association.
The evidence supporting the blood-type diet is based largely on anecdotes and research showing links between blood type and certain illnesses (for example, Type O's are more likely to have stomach ulcers). There have been no peer-reviewed studies published showing that different blood types perform better on certain foods.
D'Adamo says small-scale studies his team has conducted looking at improvements in digestive malabsorption may serve as templates for larger studies, but those will be expensive, complicated and time-consuming.
Dr. Michael Greger, founder of NutritionFacts.org, said the premise of the blood-type diet is wrong: The blood-type system, which predates humans, is far more complicated than just ABO, he said.
"People crave individualized, personalized science, but this is pseudoscience," said Greger, a general practitioner specializing in clinical nutrition.
People who lose weight or feel better after starting the blood-type diet could actually be uncovering an allergy, or may just be eliminating junk food, Greger said.
While she applauds D'Adamo's marketing, Dr. Ingrid Kohlstadt, editor of the medical textbook "Food and Nutrients in Disease Management" (CRC Press) and an associate at Johns Hopkins Bloomberg School of Public Health, said the connection between blood type and susceptibilities is not strong enough to be meaningful from a medical or public health perspective. She said she would rather prescribe a diet according to a person's gum health, which is strongly associated with heart disease, than his or her blood type.
On the plus side, any of the four blood-type diets is healthier than how most Americans eat, Kohlstadt said.