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Many people only see a person's blood type as important when it comes to giving blood or receiving a blood transfusion. However, some argue that eating a diet specific to a particular blood type may offer health benefits.

A naturopath called Peter D'Adamo popularized the idea that a diet based on blood type could help a person achieve good overall health and reduce the risk of developing certain diseases.

However, research on the effects of a blood type diet is scarce, and the studies available have not proven its effectiveness. For example, the authors of a 2014 study concluded that their findings did not support the claims that a blood type diet provides specific benefits.

A 2013 review of data relating to the blood type diet also found that no evidence is currently available to verify the benefits of this eating plan.

In this article, we explain why some people believe that their blood type should influence their diet. We also look in more detail at the proposed benefits of eating for the O positive and negative blood types and discuss which foods a person following this diet should eat and avoid.

a man shopping for vegetables to eat on his O blood type diet Share on Pinterest
A person on a O blood type diet can eat most types of vegetables.

Different blood types may make people more or less susceptible to certain conditions. A large genetic study from 2019, for example, found that people with type A and AB blood have an increased risk of gastric cancer.

The results of an earlier study suggested that people belonging to the A, B, or AB blood groups have an increased risk of coronary heart disease.

A 2019 study in Croatia also found an association between the AB blood type and the risk of psychiatric disorders. The researchers suggested that those in the AB blood group are three times as likely as those in other blood groups to have a psychiatric disorder.

According to D'Adamo, a chemical reaction occurs when the blood comes into contact with certain foods, specifically with a protein called lectin.

Lectins have a direct effect on the blood and the digestive tract. The protein binds to cells within the body, causing them to clump together, which can potentially lead to hormonal disruptions.

The effect that this produces on the body is similar to that of a foreign substance being present.

D'Adamo suggests that people with different blood types react differently to particular types of food and their lectins. As a result, he recommends specific diets for people with these different blood types.

However, no research to date suggests that lectins affect people in specific blood groups differently.

A 2014 study showed that adhering to the blood type O diet reduced levels of triglycerides in the blood, the storage form of fat. Despite this, the study authors concluded that the drop in triglyceride levels occurred due to factors unrelated to a person's blood group.

Read more about the lectin-free diet here.

According to proponents of the diet, people with blood type O should eat and avoid certain foods.

It is worth noting again that there is little reliable evidence to support the use of this diet.

Animal proteins

The diet plan dictates that people in the O blood group should consume "beneficial" animal products, including:

  • beef
  • lamb
  • mutton
  • veal
  • venison
  • cod
  • herring
  • mackerel

People with type O blood following the diet are allowed to consume most meat products, including cold-water fish.

Dairy

Although people looking to follow the blood type O diet should generally avoid dairy and eggs, they can occasionally consume the following products:

  • butter
  • cheeses, such as farmer, feta, mozzarella, and goat's cheese
  • soy milk

Nuts

Nuts are a great source of protein and healthful fats, and people following the diet can typically have most nuts, except for the ones in the "foods to avoid" list below.

Suitable nuts and seeds, including walnuts, hazelnuts, and almonds, are available to purchase online. Pumpkin seeds and walnuts supposedly provide the most benefits.

Beans

People with type O blood tolerate beans. Proponents of the diet believe that the most beneficial beans for these individuals are:

  • adzuki beans
  • pinto beans
  • black eyed peas

Grains

While most people on the O positive and negative diet should avoid grains, a few can remain in the diet, including:

  • amaranth
  • buckwheat
  • rice
  • kamut
  • kasha
  • millet

Some people may also be able to eat gluten-containing grains, while others will be unable to tolerate them. These grains include:

  • barley
  • rye
  • spelt
  • Essene bread and Ezekiel bread

Many of the less widely available grains are available to purchase online, including amaranth, kamut, and kasha, as well as Ezekiel bread.

Vegetables

With a few exceptions — which are on the list of foods to avoid — people with type O blood can eat most vegetables, including:

  • tomatoes
  • artichoke
  • chicory
  • dandelion
  • garlic
  • horseradish
  • kale
  • okra
  • onions
  • parsley
  • parsnips
  • red peppers
  • sweet potatoes
  • pumpkin
  • seaweed
  • turnips

Fruits

Some of the recommended fruits for people with type O blood include:

  • plums
  • prunes
  • figs
  • grapefruit
  • most berries

Spices and condiments

Spices that people following the diet can enjoy include:

  • kelp-based seasoning
  • iodized salt
  • parsley
  • curry
  • cayenne pepper
  • chocolate
  • cocoa
  • honey

People can purchase many of these ingredients online, including kelp-based seasoning and iodized salt.

Beverages

People following an O positive and negative diet can drink the following:

  • seltzer water
  • club soda
  • sparkling water
  • tea
  • wine

Supporters of the diet suggest that people with type O blood may experience unwanted weight gain when they eat particular food groups, such as dairy. Specific products, such as wheat-based foods that contain gluten, may also lead to an inflammatory and autoimmune response.

Animal proteins

The diet plan for people with type O blood excludes:

pork, including ham and bacon

  • goose
  • barracuda
  • pickled herring
  • catfish
  • smoked salmon
  • caviar
  • octopus
  • conch

Dairy

People following the diet should avoid dairy products not in the "foods to eat" list, including yogurt.

Nuts

Particular nuts also do not feature in the diet plan, including:

  • Brazil nuts
  • cashews
  • peanuts
  • pistachios
  • poppy seeds

Beans

Beans that proponents of the diet advise against include:

  • copper beans
  • kidney beans
  • tamarind beans
  • lentils, including domestic, green, and red lentils

Grains

The diet also excludes the following grains:

  • corn
  • gluten
  • graham
  • bulgur, durum, sprouted, white and whole wheat
  • wheat germ and bran
  • farina
  • oat
  • seven grain

Vegetables

Although people in blood group O tolerate many vegetables well, the advice is that they should avoid:

vegetables belonging to the Brassica family, including cabbage, Brussels sprouts, cauliflower, and mustard greens

  • alfalfa sprouts
  • shiitake mushrooms
  • fermented olives
  • vegetables from the nightshade family, including eggplant and potatoes
  • corn
  • leek

Fruits

People following the O positive and negative diet should also avoid several specific fruits:

  • melons, including cantaloupe and honeydew
  • oranges
  • tangerines
  • strawberries and blackberries
  • rhubarb
  • avocado
  • coconut and products that contain coconut

Spices and condiments

  • Spices and condiments to avoid in the diet include:
  • pepper, both white and black
  • vinegar
  • capers
  • cinnamon
  • cornstarch and corn syrup
  • nutmeg
  • vanilla
  • ketchup
  • pickles
  • mayonnaise
  • relish

Beverages

People with type O blood should avoid drinking the following, according to the diet plan:

  • beer
  • coffee
  • distilled liquor
  • black tea

Blood consists of many vital components, including plasma, red and white blood cells, and platelets.

The blood also contains antigens. These are substances on the surface of the cell that trigger the immune system to attack foreign compounds and organisms, such as bacteria. Antigens can also trigger the immune system to attack blood types that have different antigens.

Doctors classify blood types as A, B, AB, and O. They also label them as either rhesus (Rh) positive or Rh negative to denote the presence or absence of an antigen called Rh factor.

For example, a person with O+ blood does not have A or B antigens on the surface of the cells but does have Rh factor. In contrast, O- blood has no A or B antigens and no Rh factor. For this reason, medical experts consider type O- blood to be a universal red cell donor type.

There is currently no strong evidence to prove that this diet is effective or to support its use.

People following a specific blood type diet may report improved health, but this could occur due to them eating more healthful food in general.

It is always important for a person to pay attention to how their body reacts to the foods and beverages that they consume so that they can get to know what diet works best for them. It is best to maintain a well-balanced, varied diet that contains foods with high nutritional content.

As with any diet or exercise program, it is important to speak with a doctor before embarking on a specific diet for blood type.

Q:

I have heard that lectins, such as those in kidney beans, are poisonous. Should I avoid them even though no research supports the O blood type diet?

A:

Lectins are present in many foods other than kidney beans. They are in grains, nuts, seeds, and legumes.

As research has shown lectins to reduce mineral absorption and affect digestive function, reducing lectin exposure is a good idea whether or not you are following a blood type diet.

You can easily do this by soaking, sprouting, or cooking these foods before eating them.

Natalie Butler, R.D., L.D. Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.