Your blood literally gives you life (yes, literally). So it's understandable that some folks have come to believe that your blood — specifically your blood type — has other powers, such as determining your personality and giving hints about your ideal nutrition. But it turns out, as important as blood is, there's not much evidence supporting these ideas.
Let's start with the basics: What is it that determines what kind of blood you have? As we wrote last month, your ABO blood type is determined by the presence of antigens (protein molecules located on the surface of red blood cells) and antibodies, which hang out in your plasma.
These are part of what determines whether or not your blood donation is compatible with someone else's body and won't cause a dangerous reaction. So being able to figure out patients' blood types — a discovery that earned Karl Landsteiner a Nobel Prize in 1930 — made blood transfusions a whole lot safer.
Blood typing is a pretty amazing thing (it literally saves lives), so it's not totally surprising that we humans, who tend to glom on to anything we think reveals secrets about our inner selves, would want to extrapolate those blood type differences to infer a lot more. But there's really no truth to those conclusions. For instance, the 1990s were full of talk of the blood type diet (a.k.a. Eat Right 4 Your Type). According to the diet, we should all eat different types of foods based on our blood type. If you're, say, Type A, you should stick with vegetarian nutrition while your Type O friends should eat as much meat-based protein as they can.
However, as our own Kelsey Miller explains, there's simply no evidence to suggest your blood type has anything to do with the best nutrition for your body. In fact, the Eat Right 4 Your Type plans for every blood type emphasize eating vegetables, fruit, and lean sources of protein, which are all good ideas for pretty much everyone. So you might still feel better when following the meal plan for your blood type, but it really has nothing to do with your ABO.
Another popular idea is that your personality is somehow correlated with your blood type. This one is particularly widespread in Japan, to the point that it's used to match potential dates. And some people have even tried to prove the connection. However, their results are definitely not conclusive, with other researchers heavily disputing those findings.
Still, knowing your blood type can help speed up your treatment in emergency situations, so it's a good idea — even if it won't help you find your soulmate.