The blood type diet was created by Dr. Peter J. D’Adamo, the naturopathic physician who wrote "Eat Right 4 Your Type," a New York Times' best-selling book, in 1996. The relatively popular diet is founded on the principle that your blood type strongly influences how your body assimilates or reacts to food, how your immune system behaves, how your body deals with stress, and your likelihood of developing certain diseases. After characterizing the nature of the four general blood types -- A, B, AB and O -- the diet offers food and exercise plans designed to meet the individual needs of each type. However, no scientific evidence supports the premise that certain blood types need specific diets.
O Blood Type Characteristics
Dr. D’Adamo characterizes O as the “hunter” blood type, which means that O types thrive on a diet rich in animal protein -- specifically, organic meat, poultry and fish. According to D’Adamo, O types have a higher than average stomach acid content, which makes them more efficient at digesting meat than other blood types, but also makes them more prone to stomach ulcers. People with type O blood are also more likely to have low levels of the thyroid hormone, according to D’Adamo, making them more prone to hypothyroidism, a condition that can cause weight gain and fatigue. Despite these two common O-type health issues, the average O type has a highly active immune system. People with type O blood also have a robust digestive system and respond best to stress through intense physical activity.
Major Dietary Restrictions
The blood type diet suggests that O types will lose most of their excess weight, at least initially, by avoiding grain products. Although certain whole grains, like brown rice and millet, are considered neither good nor bad for O types, the diet specifically calls for avoiding wheat products, stating that gluten lectins, the reactive proteins in wheat, are the leading cause of weight gain in O types because they hinder insulin metabolism, interfere with efficient calorie use and promote inflammation.
D’Adamo advises O types to avoid corn for similar reasons and further recommends that O types steer clear of simple carbohydrates, especially in the form of refined grains like pasta, because O types convert these foods into fat and triglycerides more readily.
Dairy products are another food that O types should severely limit. According to the diet, people with type O blood are generally intolerant to the lactose dairy products -- including milk, cheese and yogurt -- and don’t digest them very well. Although eggs aren’t a source of lactose, D’Adamo puts them in the same category, stating that they’re a poor protein source that O types should avoid.
O types should completely avoid certain vegetables, as well; lentils, kidney beans and navy beans contain lectins that deposit in muscle tissues, making it harder to burn calories efficiently. Cabbage, Brussels sprouts, cauliflower and mustard greens are also said to promote weight gain in O types, because they interfere with thyroid activity.
Other Foods to Restrict
Although the blood type diet recommends that O types eat lean beef, lamb and poultry and lean and fatty fish as often as desired, it does advise against consuming certain kinds of animal protein, including catfish, smoked salmon and pork in any form. Similarly, O types can generally include oils in their diet -- olive and flaxseed oils are recommended for this type -- but other oils, including corn, peanut, safflower and cottonseed oil, should be avoided. The diet suggests that, while O types can still eat certain nuts as long as they’re not a primary protein source, Brazil nuts, cashews, peanuts and pistachios should be avoided all together. The O type diet also recommends avoiding avocados, olives, mushrooms, potatoes, oranges, strawberries, blackberries, honeydew melon and cantaloupe, as well as pickles, coffee, black tea, regular or diet soda, apple juice, cinnamon, nutmeg, vanilla and vinegar.
A Lack of Scientific Evidence
It wouldn’t be surprising if adhering to the blood type diet helped you lose weight – any diet that strictly limits refined carbohydrates is likely to result in some kind of weight loss. While the diet does promote healthy foods, no research exists that supports the claim that eating for your blood type is directly correlated with improved health and a lower risk of disease.
A major systematic review published by the American Journal of Clinical Nutrition in 2013 found that, after researchers examined more than 1,000 studies on blood type and diet, absolutely no evidence exists that the blood type diet is a valid theory. While the review acknowledged that certain blood types are prone to certain diseases, the blood type diet has not been shown to play a role in prevention, and its purported health effects should be seen as theoretical until they are scientifically proven.