In 1996 a naturopath published “Eat Right 4 Your Type”, a diet book purporting that people with different blood types would benefit from different diets. There are a lot more people who want to lose weight than who want to exercise skepticism, so the book became a multi-million dollar success.

As an aside, the proliferation of myriad different diets on the market should make us suspect that none of them are very effective. For example, there were countless ineffective but widely used remedies for pneumonia before the discovery of penicillin. Afterwards, there was only one treatment.

I wrote in 2011 that the blood type diet had two very important flaws. The first is that it makes absolutely no sense physiologically. That is, there is absolutely no reason to suppose that blood types, which are proteins on the surfaces of our red blood cells, have anything to do with the way we burn calories or use micronutrients. This is not a fatal flaw. Just because something doesn’t mesh with our current understanding doesn’t mean it’s false. The effectiveness of this diet would be easy to show in a rigorous randomized trial, and if proven effective this would trump the first objection. That is the second flaw – that this diet was completely unproven. There was absolutely no evidence that people eating their blood-type-specified diet did any better than those eating a diet for some other blood type.

Now just because something is unproven doesn’t mean it’s not true, but that should be our assumption. Most things have nothing to do with most other things. Given any pair of things – the position of the planets and your romantic fate on Valentine’s Day, the last four digits of your social security number and the winning lottery ticket, your blood type and what you should eat –a scientist would (in the absence of evidence) assume the null hypothesis, that thing A is totally unrelated to thing B.

So it’s pretty safe to assume that something unproven is ineffective. And when that unproven thing would be easy to test and is a big money maker, we should be very suspicious that the people making the money would prefer to keep it untested.

So why is “Eat Right 4 Your Type” such a big success? As I suggested in my prior post, each of the diets it recommends for each blood type is quite sensible. Any diet that results in the consumption of fewer calories will result in weight loss. You could pick one of the four diets randomly and do pretty well.

The new chapter in this story is that last month investigators at the University of Toronto published a study in PLOS ONE testing the blood-type diet. I’ll spare you the details of the study, but it showed that people who followed most of the diets lost weight independently of whether they were following the diet suggested for their blood type or for some other blood type.

The study wasn’t randomized. It just looked at the diets that people were already eating. My regular readers know that I don’t give observational studies much weight. I would never recommend a new medication or surgery based on a non-randomized study (because I would cling to the null hypothesis). But given a diet that already had a lot going against it and no evidence for it, this is another suggestion that you should choose what you eat based on your belt size not your blood type.

Learn more:

Blood type diet not based in science, new study says (Today Health)
Blood Type Diet – Disproved (Neurologica Blog)
ABO Genotype, ‘Blood-Type’ Diet and Cardiometabolic Risk Factors (PLOS ONE article)
Eat Right for Your Belt Size, Not Your Blood Type (my post from 2011)