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There’s certainly no shortage of advice out there about what you should do when you’re sick. And if you’ve ever asked your grandparents for their tried-and-true remedies, or you’ve been up at 2 AM Googling “how to cure a cold,” you might have stumbled upon this popular nugget of wisdom:.

You should feed a cold, but starve a fever. It kinda sounds like something Ben Franklin would’ve said, but it’s actually much older than that. And even though it’s an old saying that’s somewhat scoffed at nowadays, there might be some truth to it.

We’re not sure when the idea started, but people have associated cold symptoms with being cold for a long time. So over two thousand years ago when the so-called Father of Medicine, Hippocrates, noted that eating raises a person’s body temperature, a treatment naturally emerged. Since colds were thought to be similar to or literally stem from being cold, it seemed only logical that warming yourself up with some extra food would help.

And in turn, since your body is too hot during a fever, eating less could theoretically cool things down. And lo, an adage was born—and it’s stuck around for centuries, perhaps because it’s simple and cheap advice that doesn’t require any bloodletting, leeches, or co-pays. You can find it in medieval medical texts, and when fevers and colds reached epidemic levels in nineteenth century England, it was the remedy of choice.

This advice wasn’t just popular in Europe, either. There’s a similar proverb from India that also recommends fasting when you’re sick. But it seems like in all that time, nobody really tested the idea.

It wasn’t until 1942 that the validity of “feeding a cold, starving a fever” was formally questioned. And while the author of that paper didn’t do any new science, he did at least ponder things deliberately to arrive at his “Ehh, sounds good to me” conclusion. To his credit, at the time, doctors still didn’t know exactly what caused colds— rhinoviruses, the viruses responsible for the majority of colds, were first identified in the 1950s.

And they didn’t know nearly as much about fevers as we do today. But even without all that info, he wasn’t wrong. “Feed a cold” isn’t terrible advice. Like all the cells in your body, your immune cells need fuel to survive, let alone fight infections.

And studies have found that your energy needs increase when you’re ill, so it kinda makes intuitive sense that you’d need to increase food intake to account for that. And eating when you’re sick might do more than provide extra fuel. A 2002 study tested volunteers’ blood before and after a 1200 calorie liquid meal and found eating increased the production of interferon gamma.

That’s a cellular signal that boosts a kind of immune response called cell-mediated immunity, which is especially good at attacking cells infected with pathogens, like those harboring viruses. But more surprisingly, starving a fever might not be terrible advice, either. That’s because studies have found that reducing calorie intake or basically not eating at all alters the immune system in ways that might actually help you fight off fever-causing infections.

When people are really sick, they often don’t feel like eating, and some scientists now think that loss of appetite is beneficial. A review paper from 1997 concluded that fasting during an infection can actually help the body fight it off, in spite of the whole calories-needed-for-cells thing. And that 2002 study found that subjects given water instead of food produced more of a different cellular signal, interleukin-4.

It helps activate a different kind of immune response called humoral immunity, which is especially good against pathogens that can be detected more directly, such as bacteria. Fasting has also been shown to increase the body’s resistance to stress and toxins, like the ones produced by some bacteria. And ultimately, that may mean that whether you should eat more or less when sick may depend on what you’re infected with— which kind of lines up with the whole feed colds but starve fevers thing.

This has even been shown directly in a 2016 study in mice. Researchers found that extra food helped the mice fight the flu virus, but it made a bacterial infection much worse. So if you think of “colds” as referring to viral infections and “fevers” as bacterial infections, then the whole “feed a cold, starve a fever” thing actually kind of works—at least for mice.

Similar studies would need to be done in people and on lots of different viruses and bacteria to see if the pattern holds. And there’s another catch: a high fever is a classic symptom of the flu virus, and you can even sometimes get fevers from cold viruses, which kind of muddies things up a bit. “Feed a cold, starve a fever, unless it’s from a cold,” starts to get confusing real fast. And good advice is even tougher to make pithy because no matter what you’re sick with, doctors say you need to stay hydrated.

So maybe it should be “Feed a cold, starve a fever, unless it’s from a cold, and drink lots of fluids either way.” But that just doesn’t roll off the tongue the same way. Unfortunately, it seems like there isn’t really a simple saying that covers the complexities of the human body. But the fact that eating more or less may both be helpful at times does explain why “feed a cold, starve a fever” has stood the test of time—even if it’s not the most perfect medical advice.

Thanks for watching this episode of SciShow! If you liked learning about how this old saying might actually be right, you might like our episode about 6 popular home remedies that aren’t all that awesome for you. [OUTRO ♪].