Pseudosciences are in many ways like cargo cults. When observed from afar they seem to have all the trappings of legitimacy. Astrologers, as an example, have their own professional associations and journals. Over the hundreds of years astrology has accumulated an impressive amount of knowledge that’s taught in dedicated colleges.

The impressive sounding trappings don’t change the fact that astrology amounts to little more than made-up nonsense. The location of the stars during birth has no known way to affect personality. And when tested objectively, astrology fails to deliver.

• Astrologers performed as well as guessing in a double-blind study of astrology
• In another blinded study (PDF) astrologers failed to beat either random change or laypeople in matching horoscopes to personality types. Furthermore, there was no internal consistency in astrologers’ predictions – predictions by different astrologers didn’t match any better than would be expected from random chance.

In other words, there is no cargo cult alchemy that transforms pure bunk into knowledge, regardless of how impressive the trappings.

Now, with the short introduction alchemically transformed into long detour, we get to the point of the post. PLOS One recently published a study evaluating the popular Blood Type Diet on heart disease risk factors by Wang et al.

I occasionally get emails asking if Blood Type Diet helps acne, so I thought to take a moment to talk about the study and the diet in relation to acne.

Blood Type Diet primer

Let’s start by looking at what the diet is. Blood Type Diet was invented by a naturopathic doctor Peter D’Adamo, following the time-honored naturopathic tradition of just making stuff up.

It seems the theory is based on interesting data regarding differences in susceptibility of different blood groups to diseases as well as the possible interaction of blood type to dietary lectins. Looking at the deceptively names ‘Scientific Basis’ section of Dr. D’Adamo’s website reveals a lot of ‘could be’, ‘possibly’ and ‘perhaps’ type of information.

In addition to differences in disease susceptibility between different blood types, he presents information about possible differences in how red blood cells from different blood types react to lectins. He also points to some observations where people of different blood types showed somewhat different digestive responses to fats, proteins and carbohydrates.

This kind of data is what I call “Oh, that’s interesting” type of results. These are very preliminary findings and we really have no idea whether these have any significance to real world health outcomes. For example, it’s possible that people with different blood groups digest food somewhat differently, yet this difference has no real health effects. It’s equally possible that the findings Dr. D’Adamo points to are just ‘scientific noise’, observations that turned out to be false or unreliable.

What’s completely missing from Dr. D’Adamo’s scientific basis is actual, real science. He presents no data showing that adhering to blood type diet yields better results than adhering to non-blood type, healthy diet.

In other words, the ‘scientific basis’ consists entirely of preliminary and questionable findings that can be interpreted in many ways.

A systemic review from 2013 concluded that

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.

Blood type diets lack supporting evidence: a systematic review.

However, this doesn’t stop Dr. D’Adamo from employing alt-med alchemy to transform a legitimate and common sense concepts (ancestry affects your dietary sensitivities, personalized medicine) into utter nonsense (you can use blood type to predict your optimal diet and personalize medicine).

Blood Type Diet studied

None of this of course means the diet wouldn’t or couldn’t work. To determine whether adhering to the blood type diet has an effect on heart disease risk factors analyzed 1455 participants to the Toronto Nutrigenomics and Health study. Each participant was asked to track their diet for 1 month using a diet questionnaire that tracked the type and amount of foods eaten. From this data the investigators determined how well each participant adhered to each of the blood type specific diets, and for each participant an adherence score for each blood type specific diets was calculated. So one individual could have adherence score of 50 (out of 100) for diet for blood type A and 20 for diet for blood type C, as an example. Blood type of each participant was determined from a blood sample.

The researchers then asked several questions from this data:

1. Is adherence to a particular blood type diet associated with better health, regardless of blood type. So for example, are people with higher adherence score to blood type diet A in better health than people with low adherence score?
2. Does adhering to correct blood type diet improve health outcomes? In other words, are people with high adherence scores to correct blood type diet in better health than people with low adherence scores? So are people with blood type A who stick to the diet for blood type A in better health than people who don’t stick to it or stick to a wrong diet?

Here are the main findings for the question #1:

• People who adhered to the diet for blood type A (lots of fruits and vegetables and grains, minimal dairy and meat) had lower BMI, blood pressure, insulin and blood sugar levels.
• Adherence to diet for blood type B (fruits and veggies and moderate amounts of grains, dairy and eggs) was associated with slight improvements in HDL cholesterol levels but nothing else.
• Adherence to AB diet (similar to type A diet but with more restrictions) was associated with lower insulin, cholesterol and triglyceride levels.
• Adherence to O diet (lots of fruits and vegetables and meat, similar to paleo) was associated with lower triglyceride levels but nothing else.

There’s nothing surprising about these results. All the blood type diets call for large amounts of fruits and vegetables and reductions in processed and junk foods. So it’s not surprising that higher adherence scores are linked to better health.

So what about the question #2? The real money question.

Here’s what the investigators wrote:

In order to examine whether individuals would benefit more from following their own ‘Blood-Type’ diet, the levels of cardiometabolic disease risk factors were compared between individuals with the matched blood group and the unmatched blood group while sharing similar diet adherence. However, no significant interaction effects were observed between diet adherence and blood group for most of the risk factors, suggesting that effects of following ‘Blood-Type’ diets is independent of an individual’s blood group.

In other words, blood group had no effect on health outcomes.

Study criticism

To be honest, this was not the strongest of studies and suffers from many problems. For example, the participants weren’t asked to follow the blood type diet, instead the data was drawn from another study and analyzed again for the purpose of this study. As a consequence, most participants didn’t receive high adherence scores for any diet.

Dr. D’Adamo called the study flawed and outlined many other problems with it. I would agree with him that we shouldn’t dismiss the blood type diet idea based on this study alone. However, when you combine a very implausible idea with negative study results, weak as they may be, defending the very implausible idea becomes so much harder.

What about acne? Can Eating Right For Your Type help acne?

And finally we come to acne. I’m sure you’ve read stories at acne forums where people swear that the blood type diet cured their acne.

I actually don’t doubt those stories. It’s very much possible that going on blood type diet helps your skin, but this has nothing to do with ‘eating right for your type’.

Another recently published study shows that people with moderate to severe acne eat much more sugar and junk food than people with mild or no acne.

It’s well-known that eating a lot of sugar spikes acne-causing hormones and that reducing sugar and other bad carbohydrates also reduces acne. The diets for all blood types make some very sensible recommendations, such as reducing sugar, fast carbohydrates and processed food, and recommends eating more fruits and vegetables.

Anyone who makes such diet changes will improve both their overall and skin health. There’s no need to complicate things by adding blood type on top of that.

Conclusion

There’s no question that the Blood Type Diet is popular. Dr. D’Adamo has sold over 7 million copies of his various ‘Eat Right For Your Type’ books. But the popularity has less to do with the scientific merits of the idea and more to do with people’s desire for simple solutions to complex problems, i.e., all the answers can be found with a simple blood test. And it helps to have Dr. Oz and other celebrity peddlers of nonsense singing praise for your book.

Perhaps time will prove Dr. D’Adamo correct, but as things stand now there’s no evidence that blood type has any effect on the optimal diet for you.

‘Eating right for your type’ can indeed help acne. Not because you are somehow optimizing diet for your body, but because you are implementing common sense dietary recommendations and eating fewer foods that are known to aggravate acne – for all body types.