I have a question for the blood type diet folks: if obesity is related to blood type, why is the obesity epidemic such a recent phenomenon?
The mix of blood types remains fairly constant by region, yet obesity rates have been increasing rapidly in the past forty years.
Blood Type Diet
Our blood type is determined by the presence or absence of certain antigens that are found on the surface of red blood cells. The makeup determines whether we are Type A, B, AB, or O.
The blood-type diet was popularized by naturopath Peter D’Adamo in his book Eat Right 4 Your Blood Type. He proposed different diets for each blood type because he thought different blood types process certain foods differently.
Are people buying into this idea? Well, the book has been translated into 52 languages and has sold over 7 million copies, and, some Hollywood stars swear by it.
So, it must be true, right?
What Do Red Blood Cells Have To Do With Metabolism
Red blood cells have one function: they carry oxygen in hemoglobin molecules for respiration. That is their only function. They have no part in metabolic functions, nor do they have anything to do with digestion.
Which begs the question: how can they be responsible for obesity?
Evidence Supporting The Blood Type Diet
In 2013, the American Journal of Clinical Nutrition published the results of a study that searched for any evidence that would substantiate the health claims of blood type diets. They reviewed over 1,400 studies and references and concluded: “No evidence currently exists to validate the purported health benefits of blood type diets.”
What about more recent studies?
A recent study by scientists at the University of Toronto that involved almost 1,500 healthy, young adults concluded: “we found no evidence to support the ‘blood-type’ diet theory… The way an individual responds to any one of these diets has absolutely nothing to do with their blood type and has everything to do with their ability to stick to a sensible vegetarian or low-carbohydrate diet,”
About Correlation And Causation
What about the studies that claim certain blood types are more likely to be obese than others?
Here’s what you need to understand about statistics:
correlation is NOT causation.
What that means is, when a study finds a relationship between A and B, it does not mean that A causes B or vice versa. It simply means we are finding cases where both A and B are found together, and that is all we can infer from that observation.
If the relationship comes up very often (a high correlation), it then gives us a reason to look into it and find out why the two show up together so often and what, if anything, we can learn from it.
Even in cases where we always find A when we find B, it still doesn’t mean that one causes the other.
What it tells us is, when we find one, we can predict with some certainty that we will find the other. That is not the same as saying one causes or is responsible for the other.
Here is a simple example to show how correlation is Not causation:
Observation: Thirty eight percent of the people in the United States are in the type O-positive blood group.
We have a correlation, but do we have causation?
Would you conclude: if you moved to the United States, your blood type would change to O-positive?
Would you conclude: if you are type O-positive, your family will likely want to move to the United States, or would be more successful if you did move there?
Of course not – yet that is exactly the kinds of conclusions some people are making when interpreting statistics.
It is true that type O-positive individuals are more likely to be obese than type A-negative, but it is not because their blood type causes or predisposes them to being obese.
It is simply because there are more type O-positive individuals in the general population than type A-negative (38% vs 6%). That is simply a correlation, it is not causation.