Feeding a cold seems like common sense: we all need energy to fight an infection, so making sure we eat something is surely a good idea. And if it’s in the form of soup, it also fits with the familiar advice from doctors about staying hydrated.
In fact, there’s some evidence that feeding a cold might have another, even more direct, benefit. In 2002, researchers at the Academic Medical Centre in Amsterdam reported that eating helps the immune system’s ability to fight viruses, which cause colds. They found that levels of interferon gamma, a protein involved in the body’s infection-fighting system, quadrupled after a meal.
In contrast, avoiding food actually lowered interferon gamma levels. At least, that’s what this study found, but it only involved six participants, and – perhaps surprisingly – has never been properly followed up. So given the lack of compelling evidence, you’re probably best off being guided by your appetite when you have a cold – eat for energy, but don’t force yourself to overeat.
And what about fasting to treat a fever? This one dates back to the Ancient Greeks, but there’s no good evidence for it. Indeed, if it’s true that eating does boost interferon gamma, then avoiding food could even be counterproductive. As with colds, the best things you can do for a fever are to keep yourself hydrated and get plenty of rest.