Updated 18 June 2014

The blood type diet

Though the blood type diet remains controversial among mainstream medical doctors, there are millions of people who swear by it.


Some claim that eating specific foods based on your blood type can make you thin, energetic and revitalised. But is it just another diet myth? Olga Khazan takes a closer look.

As a health researcher, Sue Visser usually tries to discredit fad diets and nutrition quacks. But when it came to Peter D'Adamo's blood-type diet, she said, "he caught me".

Read: Give blood lose weight

The karate black-belt decided to give the diet a try 12 years ago, and she's been true to her type-O regimen ever since. She said she lost 9 kilograms and feels healthier and more energetic than before.

"The advantages are really, really profound," Visser said. "Because you don't have a disruptive bowel, you're not putting on weight and you're not disrupting your thyroid. You have a different mental space if you begin to understand your blood type."

Most people don't give much thought to their blood types until it's time for surgery, but for the past 15 years, naturopathic doctor Peter D'Adamo has sold over 5 million books that give a breakdown of what an A, B, AB, or O type should eat.

Read: Blood type linked to stroke

But while a few other (mostly naturopathic) nutritionists second Visser's rave reviews, the diet has raised controversy among mainstream medical doctors because there have been no controlled scientific studies to test D'Adamo's claims.

Meals by blood type

D'Adamo says that three decades of intensive medical research led him to the idea that lectins, or sugar-binding proteins, found in foods impact different blood types in different ways. Eat the wrong type, D'Adamo says, and the lectins will coagulate in the veins, causing bodily tissues to stick together. The result is "inflammation, digestive disturbances and immunological problems", according to his website.

"Using genes like blood type just allow an extra level of discernment, which is often the difference between a healthy diet and a therapeutic one," D'Adamo said in an e-mail interview.

His books trace the roots of the blood types to their ancestry and argue that the lifestyles of our forefathers form the basis of our nutritional needs today.

  • Type O was the first blood type - the "canny, aggressive predator" for whom D'Adamo recommends a low-carb diet centred on lean, organic meats, vegetables and fruit. He also suggests avoiding wheat, dairy, caffeine and alcohol, since Type O's tend to have higher stomach acid that can lead to ulcers.
  • Type A's have agrarian roots, D'Adamo says, so they will flourish on a vegetarian diet. Type A's should stick to fruit and vegetables in a pure, organic state, and avoid meat and heavy starches.
  • Type B should eat a balanced diet, but they should avoid corn, wheat, buckwheat, lentils, tomatoes, peanuts and sesame seeds in order to prevent weight gain.
  • Type AB shares some properties with both the A and B types. D'Adamo recommends eating tofu, seafood, dairy and green vegetables, but avoiding meat.

Feeling good all over

He even breaks down the blood types further into categories like "secretors" and "non-secretors", which he says determine whether the person would effectively metabolise specific foods, like lentils or cucumbers.

Its adherents claim that the blood type diet can do more than melt away fat. Virginia Oram, a naturopath from Oregon, wrote that after following the diet for just six weeks, "I noticed a 'brain fog' lifted that I had not even realised was there; I felt more mentally sharp". According to Oram, most people who try the diet for four to six weeks stick with it because of how good they feel.

Visser said she noticed the biggest difference when she stopped eating wheat.

"Type O immune systems are hyperactive and most likely to overreact to certain carbohydrates," she said. "Whereas type A immune systems are sluggish. They can invite in lecthins that cause cancer and the immune system will shake hands with them."

Finding harmony between the types

The diet can at first seem difficult because so many foods are forbidden for each type, which can make on-the-go meals or family dinners a nightmare.

"Initially I found it very cumbersome and time consuming," Visser said. Finding overlapping meals for her Type O self and Type A husband and son was challenging until she realised it might be easier to find foods that are suitable for all types.

Over time, Visser has developed a series of lists and menus using the "acceptable for all types" foods, which she markets through books and through her Cape Town-based Nature Fresh health store.

"Everyone can eat rice and broccoli," she said. "I've changed the paradigm to say, "what can all people eat?"

Diet divination

But critics say that D'Adamo's findings rest on flimsy evidence, given that there have been no clinical trials showing that blood type has anything to do with diseases like hypothryroidism.

"People are prone to magical thinking, so they see no problem in connecting diet to blood type when a 'doctor' shows them this way of looking at things," said Dr Robert Carroll, an American philosophy professor and outspoken critic of alternative medicine. "This is one of those things that real scientists won't see any reason to investigate because of what is already known about blood type and its irrelevancy to diet and body chemistry."

Other doctors and nutritionists have also spoken out against the theory. Dr Victor Herbert, a former nutritionist at New York's Mt Sinai Medical Center, called it "pure horse manure" in one report. 

Just because something is specific to the body, sceptics argue, doesn't mean that it's related to digestion.

"The blood-type diet, like most other diets and detox plans, is attractive to the many people who mistrust real science and are attracted to the unscientific notion that anything "natural" is better than anything "synthetic", Carroll explained. "It is easy to find numerous enthusiastic testimonials for these products that are persuasive to lazy people ignorant of science."

The diet may not be in vain

In fact, Visser has no shortage of glowing testimonials. She claims to have helped hundreds of people with everything from arthritis to heartburn by turning them onto the programme.

"People can actually see it working," she said. "If they don't believe you, once they're on the blood type diet and they go back and eat a slice of bread or something, their condition will come back."

Regardless of whether his followers have been misled, the odd thing about D'Adamo's diet is that it works.

Oram said that as a Type O, her typical day's food is a protein smoothie for breakfast, a salad with fish for lunch, and quinoa with vegetables and free-range meat for dinner.

Each of D'Adamo's other recommended diet types are similarly healthy, emphasising lean meats and whole grains while limiting sugars and starches. Eating this way means that the diet is effective for up to 80 percent of people who try it, according to D'Adamo's metric. However, it could be that the key is not a blood-based breakthrough but the simple math of fewer empty calories.

"If the diet is followed properly, ensuring that there is a lot of variety and that suitable alternatives are found to replace the foods that are to be avoided, then I would say that it is a healthy and beneficial programme to be on," said Frances van Reenen, a Cape Town nutritionist, in an e-mail interview.

But she also cautions that blood type alone may not be specific enough to determine dietary needs.

"It does have its merits and many people do very well on the diet; however I do feel that we as individuals are even more unique than our blood types," she said.

Van Reenen suggests that instead of testing their blood for type, people should test it for allergies.

"This will then allow you to accurately cut out the foods that you react to without having to cut out a host of foods that you body can actually handle."

Read more:
The health benefits of donating blood
Blood type diets not backed by science
'Sow' your blood so that others may 'harvest'
- (Olga Khazan, Health24, July 2010)


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