I suspect that readers of the blog on Randi.org will not be shocked – Shocked! – to hear that a fad diet lacks scientific evidence or even rationale. I would not consider this a bias, but reasonable expectation. If you press a button and get an electric shock 20 times in a row, you will probably at least flinch on the 21st time.

This results from process. Science and skepticism are not about specific beliefs or conclusions, but process. If someone comes up with an idea about how a particular diet affects health, a scientific process would involve first doing some background research. Is the idea plausible, and has anyone studied it before. What does current evidence say about the probability that this new idea is correct?

If the idea still seems promising, then do some preliminary research, and then more and more rigorous research as long as the idea still seems to be holding up. Eventually you need to do studies that directly address the core claim – the net health effects of the specific dietary factors that you are proposing.

The process of the fad diet is a tiny bit different. First, you come up with a new idea about diet. Then you publish your ideas in the popular press and promote and market the hell out of it. Done and done.

As skeptics we don’t say, “that diet does not work,” we say, “that process is not valid, therefore there is no justification for claiming that the diet works.” We can go further and do the background research the promoters should have done themselves – is the idea even plausible?

Criticizing process and plausibility is appropriate and satisfying to the skeptic, but not always to the general public. It is therefore always nice when we can say, “not only is this not supported by evidence and highly implausible, we have evidence to show that it doesn’t work.”

I would like to say that the blood type diet has evidence for lack of efficacy, but a recent systematic review found that there is simply a lack of evidence. The notion of blood type diet was nonsensical from the start. The idea is that people with different ABO blood types would benefit from different diets.

The diet was pioneered (read – invented out of whole cloth) by naturopath Peter J. D'Adamo. Naturopaths, as a profession, are not science-based. Their practice seems to be little more than an amalgamation of various dubious practices that only have in common their lack of a scientific basis. D’Adamo plays off the “individualization” marketing meme popular in alternative medicine. His website declares:

“One of the hallmarks of alternative medicine is the recognition of the biochemical uniqueness of each individual and the need to tailor treatments and prescriptions to match that individual variability. While a person's genetic code, ultimately, is the basis of this individuality, basing treatments on genetic factors is too broad an approach and not consistent with alternative medicine.”

Let me correct that:

One of the hallmarks of alternative medicine is the exploitation of scientific illiteracy, which allows proponents to use terms like “biochemical uniqueness” in a vacuous but effective way. We pretend that each individual is unique and that we tailor treatments and prescriptions to match that individual variability, when in fact we use cookie-cutter pseudoscience. While a person's genetic code, ultimately, is the basis of this individuality (if you ignore developmental, epigenetic, and environmental factors), basing treatments on genetic factors is far too much work, involving lots of science and research and stuff, and is not consistent with alternative medicine. 

Along those lines, here is some BS that D’Adamo made up:

“According to naturopath Peter J. D'Adamo, N.D., in his book Eat Right 4 Your Type, the missing link might be the four basic blood types: O, A, B, and AB. "There had to be a reason why there were so many paradoxes in dietary studies and disease survival," why some people lose weight and others do not on the same diet or why some people keep their vitality as they age, and others do not, says Dr. D'Adamo.”

The ABO blood type is essentially genetically determined proteins on blood cells that are important because of their implications for immunity in the context of transfusions or organ transplants. This represents diversity through genetic drift, and there is no reason to suspect that ABO proteins have any connection to diet or health. The plausibility here is fairly close to zero. (I won’t say actually zero, because they may be markers for other genes that do have implications for diet and health, but there is no evidence this is the case.)

D’Adamo’s site further claims:

“Following decades of ongoing research and clinical work, Dr. D'Adamo created The Blood Type Diets™, a way of eating and living that has transformed the health of millions.

The researchers scoured the medical literature, and found:

Sixteen articles were identified from a total of 1415 screened references, with only one article that was considered eligible according to the selection criteria. The identified article studied the variation between LDL-cholesterol responses of different MNS blood types to a low-fat diet. However, the study did not directly answer the current question. No studies that showed the health effects of ABO blood type diets were identified.

And concluded:

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.

D’Adamo claims “decades of research.” Independent researchers find “No evidence.”

How does D’Adamo answer his critics? By calling into question their motivations, their background, and by linking to articles attacking skeptics and skepticism. Not with evidence –because apparently there isn’t any.


Steven Novella, M.D. is the JREF's Senior Fellow and Director of the JREF’s Science-Based Medicine project.