10/11/2014 04:33 GMT | Updated 09/01/2015 05:59 GMT

When Will the Blood Type Diet Finally Go Away?

For those of us who are passionate about nutritional science, the blood type diet is a persistent source of frustration and bewilderment. Several years ago we were asking the question, "How can a theoretical diet with no scientific basis be so wildly popular?" Today we are asking, "How can a diet proven to have absolutely no scientific basis be so wildly popular?" Since 2013, two important studies have thoroughly debunked the blood type diet, and yet, its popularity endures. During the past two weeks alone, three prominent celebrities have been singing its praises.

First, Victoria's Secret model Elyse Taylor told the Daily Mail she's lost 30 kilos gained during pregnancy, thanks in part to the blood type diet. Next, Australian boxer Lauryn Eagle declared herself "a heavy believer in the blood type diet," crediting it with much of her fitness success. Finally, superstar singer Usher, a man who's cycled through numerous diet trends, told Billboard that he's currently following the blood type diet.

In case you're unfamiliar, the blood type diet debuted in 1997 with the publication of naturopath Peter D'Adamo's book, Eat Right For Your Blood Type, which has since sold over 7 million copies. D'Adamo contends that each of the four primary blood types--A, B, AB, and O--fares best when following a particular dietary protocol. Type O, says D'Adamo, should eat like hunter-gatherers--lean meat, poultry, fish, and vegetables (with some restrictions). Type A, conversely, should be vegetarians, eating plenty of cereals, legumes, and vegetables. Type B thrives on dairy, meat, and vegetables (with restrictions on certain cereals and legumes), while type AB falls somewhere in between types A and B. Roughly 40 percent of the population has blood type A, and thus, according to D'Adamo, should become vegetarians to avoid degenerative disease.

Despite its popularity, the blood type diet has never been backed by published scientific research, and has therefore endured heavy criticism. In July 0f 2013, the American Journal of Clinical Nutrition published a systematic review of the Cochrane Libray, MEDLINE, and Embase biomedical databases searching for scientific evidence supporting the notion that diets based on blood type can improve health and decrease disease risks. The conclusion?

"No evidence currently exists to validate the purported health benefits of blood type diets. To validate these claims, studies are required that compare the health outcomes between participants adhering to a particular blood type diet (experimental group) and participants continuing a standard diet (control group) within a particular blood type population."

In other words, not only was there no research supporting blood type diets, there hadn't even been any research conducted comparing blood type diets with standard diets. That's a rather significant point and a good reminder that popularity, especially in the world of nutrition, doesn't necessarily correlate with evidence. Finally, earlier this year, a team of University of Toronto researchers studied the blood type diet as nobody had previously done. "Based on the data of 1,455 study participants, we found no evidence to support the 'blood-type' diet theory," said the senior author of the study, Dr. Ahmed El-Sohemy. They did find, however, that adherence to certain blood type diet protocols does improve health, particularly cardiovascular health, "but these associations were independent of an individual's ABO genotype, so the findings do not support the 'Blood-Type' diet hypothesis."

And herein lies the reason why the blood type diet remains popular. Its dietary recommendations are generally healthy, and for those switching from highly processed foods diets, it's no surprise they feel better. Although the blood type diet works for some people, this has nothing to do with blood type. Similarly, we could assign the blood type protocols based on hair color. For example, we could assign blood type A's protocol to people with brown hair, blood type B to those with blond hair, etc. Inevitably, some people with brown hair following the 'brown hair' protocol would enjoy success, but certainly not because they have brown hair. Quite simply, the blood type diet isn't scientific and the sooner its proponents acknowledge this, the sooner the diet and the false hope it engenders can dissipate.