The popular proverb that we should eat when we have a cold, but not when we have a fever makes some sense intuitively. Since fevers usually only last a day a two and you tend not to have much of an appetite anyway, eating little isn’t difficult. But colds tend to last between seven and 10 days so you’re inclined to eat and would be left feeling pretty weak and wretched if you didn’t.

So it’s doable, but is there any evidence that following the saying makes you feel better more quickly? Liquids are, of course, essential and it’s the nutrients from food which enable cells to function. Yet illness commonly makes people lose their appetite altogether, and it has been suggested that this so-called “infection-induced anorexia” helps boost the immune system. But if this is the case, why should not eating have this effect only when we are ill?

One study dating back to 2002 gave rise to many a headline stating that “feed a cold, starve a fever” wasn’t an old wives’ tale after all. Dutch scientists asked volunteers to fast overnight before visiting their laboratory for tests on two separate occasions. On the first visit, they were given a liquid meal and on the second they received only water. Blood tests showed that levels of gamma interferon, a substance important in triggering immune responses against infection, particularly by viruses, increased by an average of 450% after participants had been given the meal, and decreased after consuming only water.

Meanwhile, fasting appeared to increase levels of another immune system signalling chemical called interleukin-4 on average fourfold – much more than the smaller increase seen in the study participants after they were given the liquid meal. Interleukin-4 plays a key role in fighting bacterial infection; it’s main role is in regulating immune reactions to infectious agents that have entered the blood and tissues, but that have yet to infiltrate individual cells.

So continuing to eat promotes the type of immunity that is particularly effective in combating the type of virus-based infection of cells you would have with a cold. And a fever might be caused by infectious bacteria, in which case starving yourself could promote the other type of immunity. So far, so good for those who saw the Dutch research as supportive of the “Feed a cold, starve a fever” maxim. Except that it’s not as neat as that. 

A common cause of fever is flu which is caused by a virus, so the theory doesn’t quite fit. Moreover this study was tiny, with just six volunteers taking part. Even lead study author Gijis van den Brink warned people not to change their eating habits in response to illness on the basis of the study. Then there’s other evidence, admittedly only from mice, that when only 40% of the normal calories for a day are consumed, infection with flu was not only more likely, but symptoms were worse and the mice took longer to recover.

Although there has been work showing that calorie restriction both extends the lifespan of mice and rats by between 20% and 30%, and reduces the incidence of tumours, when it comes to flu, the evidence suggests that mice are better off eating.  

But back to people. Not only is there a lack of scientific evidence to back up the proverb, but there are also historical and linguistic debates about its origins. Many credit Geoffrey Chaucer’s Canterbury Tales. Others insist the phrase does not appear in the late 14th century collection of stories. There have been suggestions that it’s a mistranslation and that the intended meaning was that feeding a cold would “stave off” a fever.

Those wanting a definitive answer will have to wait until we know more about the complexities of the immune system. Until then, appetite is probably your best guide. Whether you have a cold or a fever, it is important to keep taking fluids. As for food, you do want to keep your strength up if you can, but it depends on what you can stomach.

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