On Nutrition

I remember when I first heard of the blood type diet. It was 1998, the thought of becoming a dietitian wasn’t even a glimmer in my mind, and I was fully entrenched in diet culture. A coworker was raving about the diet, which had debuted a few years earlier. The claim is that the foods you eat react chemically with your blood type, and that eating the “right” foods for your blood type will help you lose weight and digest your food better. Is there a more enticing siren song than that?

I checked out a book about the diet from the library, and tried it for a hot minute before I heeded the alarm bells in my head. I have type A blood, which according to the book, meant should eat lots of grains, fruits and vegetables and avoid meat. Even though I already ate many meatless meals, the diet didn’t make intuitive sense, and felt unnecessarily restrictive. I wonder how someone with type O blood who is happily vegan or vegetarian would react to the idea that they “should” eat lots of meat, poultry and fish and avoid grains and beans?

Almost 25 years since publication of the original blood type diet book, people still ask me if they should try it. My answer is no, because there’s no research supporting blood type diets, with the latest nail in the coffin coming from an unexpected source.

I didn’t take much note of the results from a randomized clinical trial published Nov. 30 in the Journal of the American Medical Association showing that 16 weeks on a low-fat vegan diet — lots of fruits, vegetables, grains and legumes — produced positive changes that could reduce the risk of type 2 diabetes. While a vegan diet isn’t the only diet that can help reduce disease risk, it is one of them, so I was not surprised by the results. What did spark my interest was a secondary study, this one published in the Journal of the Academy of Nutrition and Dietetics, which found that people with type A blood and those with type O blood benefitted equally from that vegan diet.

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This observation is reminiscent of a 2014 study of 1,455 young adults which showed that the “type A diet” improved several health markers, such as lower blood pressure and cholesterol levels, regardless of the participant’s blood type. A year before that, a systematic review published in The American Journal of Clinical Nutrition concluded that there was no evidence to support any benefits of blood type diets. Theories about which blood types emerged with hunter-gatherers, early farmers, and nomadic tribes — all of which were used as support for blood type diets — have also been debunked. So has the supposed connection between blood type and digestion.

When people do feel better on a blood type diet, it’s likely because their recommended foods happen to align with their preferences, and the diet prompted them to cook more at home and eat fewer highly processed convenience foods. Any weight loss can be explained by the restrictiveness of the diet, with weight regain as likely as on any other restrictive diet.

Personalized nutrition has become a holy grail since the mapping of the human genome was completed in 2003. Until we truly know which diet is best for each individual person — which won’t involve blood type — the right diet for you is one that makes you feel your best, which includes a wide variety of foods and nutrients, addresses any health issues, and fits into your life.