Maybe you had a granny whose raspy voice you still hear rattling around your head every time you feel a cold coming on. “Feed a cold, starve a fever.” Or, wait, is it the other way around?

Maybe every time you stick a hand in the fridge, foraging for something yummy, you hear that ringing won’t-go-away rhyme, the one about the apple-a-day and the doctor.

So here’s the burning question: What’s with all those aphorisms anyway? Is there any truth to the bits of medical folk wisdom we all know by heart, if not by practice? We dialed up a wise and jolly family physician, Dr. Jeffrey Cain, who happens to be president-elect of the American Academy of Family Physicians and chief of family medicine at Children’s Hospital Colorado.

We’ll take it one by one:

“Feed a cold, starve a fever.” The skinny: This one has medieval roots, the good doctor tells us, first appearing in print in a 1574 dictionary. “When people have a fever, they don’t want to eat,” says Cain. “There’s no science behind it, but there’s no harm behind it either. The most important thing, though, is to give your body a rest; let it heal.”

“Sweat it out.” The skinny: “Sometimes when someone has a fever they think that if they pile on the pajamas, and get under lots of blankets, they will sweat out the virus. There’s no science to it. When you break a fever, you will sweat, and as your fever goes down, you’ll get chills. But you can’t sweat out a virus,” says Cain, who has practiced family medicine for 26 years. “Another interpretation is that it means to be sure to exercise when you have a cold or fever. What we know from research is that regular exercisers have fewer colds. It gives our body a boost. But once you’re sick, there’s no data that exercise will help you. In fact, it might hurt you. …”

“Walking is the best exercise.” The skinny: This goes back to that ol’ healer, Hippocrates, who would have flunked the Athenian P.C. Exam since he claimed it was “man’s best exercise.” Says the modern-day Cain: “When people ask, what’s the best exercise, the answer is, ‘The one you’ll do.’ Walking is great, it’s easy and you don’t need any equipment.”

“An apple a day keeps the doctor away.” The skinny: “We know Americans don’t get enough fruits or vegetables,” says Cain, “so I’m all for an apple a day. If you want to take it up a notch, try, ‘Be sure your plate is filled with lots of colors, flavors and textures.’ Stick to whole foods, in forms as close to all-natural as you can get.”