A Footnote on Evidence: Cravings - Island of Sanity

Island of Sanity


A Footnote on Evidence: Cravings


You probably get cravings for some food now and then. You suddenly have a strange desire to eat strawberries or pickles or whatever.

Why do we get cravings? One popular theory is that it is our body's way of telling us that we need a certain nutrient. Like if you're short on vitamin C, maybe your body will create a craving for orange juice. I've always thought this theory sounded plausible.

I read an article recently that claimed that scientists had proved that theory false.

I think this makes a good example to discuss evidence because it's not a subject where many people have strong opinions. I mean, I don't think anyone's religious faith will be shaken or their most fundamental political beliefs shattererd by the answer to this question.

The Refutation

So here was the experiment: They got some number of people who had a craving for orange juice. To some of them they gave a synthetic drink that tasted like orange juice but which did not contain vitamin C. To others they gave a drink that contained vitamin C but did not taste like orange juice. The result: The people who were given the drink that tasted like orange juice found the craving went away. The people who were given the drink that had vitamin C still had a craving for orange juice.

The researchers reasoned, if what created the craving was a shortage of vitamin C, those who got vitamin C should find the craving satisfied and so it would go away, while those who got the orange-flavored drink that did not contain vitamin C should find the craving not satisfield and so it would continue. But in fact they got the opposite result. And so, they concluded, shortages of a nutrient are not what causes cravings.

Minor Objections

I see several flaws in these conclusions. Let me repeat that I am not emotionally attached to the "nutrient shortage" theory. If someone came up with evidence that answered my objections my response would basically be, "Huh, and it seemed so plausible."

I have two minor objections:

1. How did they find people with a craving for orange juice? The article I read didn't say. I mean, did they get a random group of people, and then just sit around waiting for each of them to crave orange juice? But that could take years. Did they do something to create this craving? If so, what did they do? If they did something to create the craving, that introduces another factor that could invalidate the experiment. As I have no idea what they did, if anything, I don't know if this was a problem or not.

2. They assumed that the only nutrient that orange juice contains is vitamin C. This is clearly and obviously false. Just read the marketing literature from the Orange Growers Association! Maybe what triggered the craving was not a shortage of vitamin C but of some other nutrient that is contained in orange juice, and which their fake drink also has.

Major Objections

But my more fundamental objection is this: Their argument is only valid if you make two key assumptions about how this craving mechanism would work. And both of those assumptions are unlikely.

1. The system cannot be fooled. By this reasoning, depth perception is a myth. That is, the idea that people can tell whether something is close up or far away by looking at it must be false. There are optical illusions that fool our senses on this. Artists routinely draw pictures that make it look like one object is near and another is far away, when in reality both are on the same sheet of paper, exactly the same distance from our eyes. Does the fact that our eyes can be fooled by an optical illusion prove that depth perception doesn't really exist? Of course not. It just proves that the mechanism is not infallible. It is possible to draw a picture that makes it look like an object is far away when it really isn't. So maybe in the same way, the craving/taste mechanism is not infallible. It could generally work, but at the same time it could be possible to fool it with something that tastes like the food being craved.

2. It assumes that the system is fixed rather than learned. That is, it assumes that we are all born with a lack of nutrient X leading to a craving for food Y, rather than our bodies developing this association over time.

Have you ever wondered: Do people who have never had a certain food -- perhaps they live in a part of the world where that food is unknown, or they just never happen to have tried it -- have cravings for that food? Like, do Eskimos have cravings for watermelons? I don't recall ever having a crave that left me thinking, "I really want ... someting. But I'm not sure what it is." Like I'll get a craving for beef or chicken now and then. But I don't think I've ever had a craving for snake or kangaroo. Well, as I've never eaten those things, how would I know if I did? But I've never had a craving where I thought I wanted some kind of meat but I couldn't identify what kind. Does anyone today have cravings for mammoth or dodo bird?

Maybe the body has some mechanism to connect foods you've eaten with the nutrients they supply, that the system learns as it goes along. If that's the case, then you'd never have a craving for a food you'd never eaten. But it would also mean that if you ate the food that the system thought you needed, but it didn't supply the expected nutrient, the system would have to re-evaluate. It couldn't know the instant you took a bite that it didn't work. It would have to wait until the food was digested and nutrients extracted. How would such a system work? I have no idea.

My Point

My point being this: They presented evidence that they claimed prove a certain theory was false. But what they rebutted was a version of the theory that they just made up, not the real theory. At least, not the real theory as a thoughtful person would construct it if they spent some time thinking about it.

This is the classic logic fallacy of the "strawman". In this case, I think the researchers just didn't think it through. (Or maybe they did and the writer of the article that I read just didn't explain it well enough.) On controversial issues, people will do this very deliberately. Person A makes, say, a political argument. Person B can't rebut the actual argument, so he makes up an argument that sort of kind of sounds like what A said, but isn't really the same, and is full of holes. Then B points out the holes in the argument that B made up, and pretends that that is what A said, and claims to have refuted A.

© 2022 by Jay Johansen


No comments yet.

Add Comment