I was 15 the first time I ever went to a nutritionist. I’d already been dieting for years with no major weight loss (perhaps because I was just the average chubby adolescent, but that’s another can of worms). A nutritionist now seemed like a better option — someone with common sense and degrees, rather than a flashy fad program. And this woman seemed super legit and science-y. Her practice was located in Chappaqua, a very expensive suburb in Westchester. Her office was bright and clearly decorated by a professional. Furthermore, she began the session by asking my blood type. What could be more scientific than that?The blood-type diet — otherwise known as “Eat Right 4 Your Type” — has by now been soundly debunked. Yet, in the late 1990s, it held pride of place as the hottest concept in dieting, embraced by celebrities, self-proclaimed nutrition pros, and teenagers like me who believed they’d discovered the most ancient secret to looking cute in hip-huggers. What fools we’d been to chug SlimFast and count Weight Watchers Points, when all along the answer had been coursing through our very veins.My veins were filled with Type O, and therefore I was instructed to renounce my vegetarianism and embrace a meat- and fat-heavy lifestyle. Among other rules, my fancy nutritionist ordered me to eat plenty of beef, venison, and pineapple juice, while avoiding things like strawberries, lentils, and almost all dairy (butter was okay). The good, bad, and very bad food lists were long and followed no logic I could understand — but that’s why I had a professional, right? (Here, it’s worth noting that “nutritionist” is not a regulated term. Anyone can claim the title. The more you know!)Thus the blood-type diet became a strange and meat-heavy chapter in my life (it is quite literally a whole damn chapter of my memoir ). I can’t really blame my teenage self for buying the hype, omnipresent as it was. When a fad hits, we mostly read the headlines, not the gentle caveat five paragraphs down noting that “the most common criticism is the lack of scientific evidence…” Needless to say, the only thing the diet did for me in the long term was make me a meat eater again. Soon the blood-type buzz faded, leaving room for new popular diets and their shiny new promises of thinness.In the intervening years, I tried dozens, eventually recognizing the nonsense behind them all and quitting diets for good. Hindsight, research, and common sense make the blood-type diet almost laughable in its intricate illogic. Molasses is good, cabbage is bad, and never, ever eat yogurt — it’s as if the Mad Hatter designed this menu. Yet I can’t help but notice that in recent years the conversation around this diet has begun resurface . Let’s take a brief reality check before we fall down this rabbit hole again.In 1996, Peter J. D’Adamo published Eat Right 4 Your Type, the best-selling book that launched the blood-type diet into cultural consciousness. D’Adamo presented a scientific image — his headshot had him in a white coat, standing in what appears to be a lab. In fact, he is a naturopathic practitioner and, as he describes himself on his website, a “researcher-educator, Ivesian, amateur horologist, budding software developer, and air-cooled enthusiast.” As of 2007, he is also a second-degree black belt.D’Adamo was in the first graduating class of Bastyr University, a school of alternative medicine, which, like naturopathy itself, is the subject of constant criticism for promoting medical beliefs with no scientific basis (and in Bastyr’s case, for dubious academic practices as well). He was the son of another famous naturopath, James D’Adamo, who first posited the idea that a diet based on blood type might have health benefits. The senior D’Adamo prescribed a low-fat, vegetarian diet to all his patients, noting that some seemed to exhibit improved health, others remained the same, and some got worse. Could blood type be the cause? While his theory was based only on observations within his practice, his son sought to validate them with research. “What I had been taught about blood transfusions and other aspects of blood typing didn’t give me any information that supported my father’s ideas about how people should eat,” said Peter J. D’Adamo in an undated interview. “I was incredulous as well.”While at Bastyr, he began looking at connections between blood types and illnesses, thinking , “‘If my father’s right, the type A’s should have illnesses associated with eating meat, because he said they shouldn’t consume that.’ It was no surprise when I found that a lot of health problems associated with excessive animal protein consumption, like heart disease, cancer, and vascular disorders, were much more common in type A’s.” According to D’Adamo, each blood type has different abilities to process certain foods — as well as the lectins found in many of them. Lectins are a vast group of carbohydrate-binding macromolecules that serve numerous biological functions. D’Adamo’s claims that the different blood groups are unable to properly metabolize certain lectins, and therefore if you eat the wrong food, the lectin “settles” somewhere in your body, causing agglutination ( cell clumping ). This “dangerous glue” he continues , can cause everything from hormone disruption to liver cirrhosis, or even block blood flow to the kidneys, “to name just a few effects.”Today, D’Adamo sells a pill of “blocking sugars” called Deflect , designed to stop this clumping. Indeed, he now has several supplement lines intended to support each blood-specific diet, but back in the ’90s the diet alone was the cure. Based on his and his father’s observations, D’Adamo formulated four separate profiles based on each of the blood types. In summary:This, the oldest blood type, is well suited to metabolizing animal protein, fat, and cholesterol, but not grains or dairy. As this blood type is descended from hunters, the fight-or-flight response is strong and can translate to anger issues or manic episodes. Type O's are also vulnerable to destructive habits when bored; they should avoid caffeine and lentils, engage in vigorous exercise, and remember to chew slowly.This blood type emerged with the rise of community living, when, thanks to the dwindling supply of game to hunt , human digestion was forced to adapt to carbohydrate consumption. Type A’s, therefore, should eat mostly vegetables and soy proteins, being mindful of their highly sensitive immune systems and increased risk of life-threatening disease (as well as naturally higher stress levels). They should avoid crowds and corn, and practice tai chi B is for balance! ” While Type A and O are on opposite ends of the spectrum, B falls somewhere in the omnivorous middle . Meat, dairy, grains, legumes, fruit, vegetables — Type B’s really need them all (except chicken). When these are out of balance, the B’s can be prone to stress and illness, but when they are eating for their type, they are more physically and mentally fit than other blood types. D’Adamo notes that B’s may also have a “sixth sense,” as they are intuitors.The rarest and newest of the blood types is what D’Adamo calls “ the chameleon .” It is the only one that emerged not from environmental factors but from intermingling, and is somehow more mystical than the others. Lamb, dairy, tofu, and grains are all good for AB’s, while buckwheat and smoked meats can be problematic. They are charismatic, have low stomach acid, and should practice visualization techniques.“It’s just a really cool idea that has no substantive support.” This is the take of Dr. Ruth Kava , PhD, RD, the Senior Nutrition Fellow at the American Council on Science and Health. “There's no consistency, no logical rationale for this diet.” It’s a common-sense conclusion when you look closely at D’Adamo’s plans — both their broad generalizations about billions of people and highly specific instructions about how they should eat (and exercise and manage their mental health). But Dr. Kava isn’t as swayed by that part. The idea that blood types emerged with the milestones of human societal development also seems like logic on the surface. “There's no real scientific connection between [these events],” she adds. “But it does sound very impressive.”