Skeptics often confront various challenging situations. One is sophisticated spurious knowledge (or SSK – OK, I just made that up). These are elaborate systems that purport to describe some body of knowledge but which are ultimately based on nothing – they are completely disconnected from reality.

Astrology is a great example of this. There are several astrological systems that are very complex, include charts and diagrams, and follow a certain internal paradigm or logic and underlying philosophy. None of this prevents astrology from being 100% fantasy. There is no relationship between the positions of the planets and stars and the personalities or fates of humans, and no mechanism by which such a relationship might exist.

The problems with systems of SSK is that they can give the powerful illusion of real knowledge. Further, people can become highly invested (in many ways) in such systems making it difficult to abandon them. They can even become institutionalized.

Another type of claim that presents a challenge are those for which there is simply no evidence one way or the other. As promoters of science, we like to appeal to high quality scientific evidence to support our positions. When there simply isn’t any science, then we have to resort to arguments of plausibility. These are perfectly legitimate, just not as satisfying.

We can also appeal to the very legitimate position that the person making the claim has the burden of proof, especially if they are selling something or making medical claims. But still, saying there is no evidence is not as rhetorically effective as being able to point to evidence that disproves the claim.

The Blood Type Diet

The blood type diet was invented by naturopath Peter D’Adamo, following what appears to me to be the naturopathic tradition of just making stuff up. He created an elaborate system of hand-waving explanations, with lots of sciencey references to genetics and metabolism.

Ultimately, however, the blood type diet was based on nothing. A 2013 systematic review concluded: “No studies that showed the health effects of ABO blood type diets were identified.”

This did not stop D’Adamo from writing a best selling book (selling 7 million copies according to his website). He has built a career out of the blood type diet, without actually bothering to publish any clinical evidence that his diet works.

The appalling lack of evidence has also not stopped Dr. Oz from promoting the diet. (Apparently eating right for his type gives him “vital energy.”)

The blood type diet, in essence, is the astrology of nutrition.

Fortunately, researchers have finally decided to put the popular but unfounded diet to a rigorous clinical test, and the results are just published in PLOS One – ABO Genotype, ‘Blood-Type’ Diet and Cardiometabolic Risk Factors.

Researchers looked at 1,455 subjects on each of the four blood type diets, and followed several outcomes like cholesterol, BMI, and waist circumference. Not surprisingly, for the A, AB, and O type diets subjects had some improvement in some of the parameters. Actually I found it most surprising that the B diet showed no improvement in any parameter.

Improvement is not surprising because every diet will tend to work, in that people following a diet will tend to eat less overall, or avoid the worst indulgences, and this will reduce cholesterol or weight.

The real outcome in this study was to look at the relationship between the blood type of the subjects and their response to the various diets. Not surprisingly (to skeptics, anyway) there was absolutely no relationship. It mattered not one bit what blood type the subjects were.

Senior author, El-Sohemy, is quoted:

We can now be confident in saying that the blood type diet hypothesis is false.

So what happens now? If this were a medical intervention, and a clinical study of this rigor and statistical power were published, it would result in abandoning the intervention (maybe not as quickly as we would like, but that is what generally happens). Proponents might still do further research, hoping for a different result, but it would be unethical to continue to use an intervention when the only evidence we have shows that it does not work.

I suspect that D’Adamo, who has never bothered to test his multi-million dollar scheme, will shrug off the results without batting an eye. Perhaps he may look up long enough to offer some ad-hoc rationalization for dismissing the study. You can always find some flaw or weakness to point to, and pretend that it’s a fatal flaw, if you don’t like the results. Or he may point to the fact that most subjects had improved parameters on the diets (except the B diet) and proclaim that his diets work.

I wonder if Dr. Oz will now publicly pull his support from the blood type diet and inform his audience that the best evidence we have (the only evidence) shows that it does not work.

In any case, I can now say to those who ask (and this is a frequent question) that the blood type diet has been studied and it simply does not work.