Diet not working? Maybe it’s not your type

Diet not working? Maybe it’s not your type

Harvard Health Blog

Have you heard of the blood type diet? I thought it had been debunked long ago but patients keep asking about it, so I figured I should learn more.

What’s the blood type diet?

In 1996 Peter D’Adamo, a naturopathic physician, published a book in which he described how people could be healthier, live longer, and achieve their ideal weight by eating according to their blood type. One’s choice of condiments, spices and even exercise should depend on one’s blood type. Soon, the book was a best seller and people everywhere were finding out their blood type, revising their grocery lists, and changing how they ate, exercised, and thought about their health.

Here are some of the recommendations according to the “Eat Right for Your Type” diet:

–Those with type O blood should choose high-protein foods and eat lots of meat, vegetables, fish and fruit but limit grains, beans and legumes. To lose weight, seafood, kelp, red meat, broccoli, spinach and olive oil are best; wheat, corn and dairy are to be avoided.

–Those with type A blood should choose fruit, vegetables, tofu, seafood, turkey and whole grains but avoid meat. For weight loss, seafood, vegetables, pineapple, olive oil and soy are best; dairy, wheat, corn and kidney beans should be avoided.

–Those with type B blood should pick a diverse diet including meat, fruit, dairy, seafood and grains. To lose weight, type B individuals should choose green vegetables, eggs, liver and licorice tea but avoid chicken, corn, peanuts and wheat.

–Those with type AB blood should eat dairy, tofu, lamb, fish, grains, fruit and vegetables. For weight loss, tofu, seafood, green vegetables and kelp are best but chicken, corn, buckwheat and kidney beans should be avoided.

As mentioned, the recommendations for the blood type diets extend well beyond food choices. For example, people with type O blood are advised to choose high-intensity aerobic exercise and take supplements for their sensitive stomachs, while those with type A blood should choose low-intensity activities and include meditation as part of their routine.

But does it work?

High-quality studies about the blood type diet had not been published in peer-reviewed medical literature. Even now, a search in the medical literature for the author’s name reveals no research pertaining to this diet. Studies published in 2013 and 2014 about the blood type diets are worth noting. The 2013 study analyzed the world’s medical literature and found no studies demonstrating benefit from a blood type diet. The 2014 study found that while people following any of the blood type diets had some improvement in certain cardiometabolic risk factors (such as cholesterol or blood pressure), those improvements were unrelated to blood type.

Does it make any sense?

The theory behind this diet is that blood type is closely tied to our ability to digest certain types of foods, so that the proper diet will improve digestion, help maintain ideal body weight, increase energy levels, and prevent disease, including cancer and cardiovascular disease.

Type O was said to be the original “ancestral” blood type of the earliest humans who were hunter-gatherers, with diets that were high in animal protein. Group A was said to evolve when humans began to farm and had more vegetarian diets. Group B blood types were said to arise among nomadic tribes who consumed a lot of dairy products. And since Group AB blood was supposed to have evolved from the intermingling of people with types A and B blood, type AB recommendations were intermediate between those for people with types A and B blood.

Each of these theories has been challenged. For example, there is evidence that type A was actually the first blood group to evolve in humans, not type O. In addition, there is no proven connection between blood type and digestion. So, in addition to a lack of evidence that the diet works, serious questions remain about why it should work in the first place.

So, what’s the downside?

It’s a fair question, especially since some improvements were seen in people who adopted certain blood type diets (see link above). Eating based on your blood type requires you to know your blood type and then follow a restrictive diet. Personal preferences might be a problem: A vegetarian with type O blood may struggle to stay on the assigned diet, and people who love red meat may be disappointed to learn they have type A blood. Recommended supplements are not cheap; neither are the recommended organic foods. And if you have certain health conditions, such as high cholesterol or diabetes, a nutritionist can make better evidence-based recommendations for you than those determined by your blood type.

Now what?

Advocates of blood type diets may say that while the ideal study has not yet been performed, the absence of evidence doesn’t prove they’re ineffective. And there’s also no proof that these diets are harmful. So, my guess is that interest in the blood type diets will not disappear any time soon. But there’s a reason that bookstores have rows and rows of books on diet, each claiming to be highly effective if not the best. We simply don’t know which diet is best for each individual person. And even if we did, sticking to any single diet is often challenging.

Stand by — it’s likely you’ll soon be hearing about yet another best diet. And my guess is that it won’t have anything to do with your blood type.

(Robert H. Shmerling, M.D., is faculty editor of Harvard Health Publications.)

 

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