“Feed a cold, starve a fever” is an adage that has been around for centuries. Now a new study in mice finds that it might actually have some truth — but it depends what exactly is the cause of your fever.

Why it matters:

Loss of appetite is common with sickness and Ruslan Medzhitov, an immunologist at Yale University, and his colleagues wanted to know why. Is it just a consequence of illness, or does it have some protective benefit we don’t fully understand?

The nitty gritty:

Researchers infected mice with either a bacteria that causes food poisoning or a flu virus.

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All the mice began to eat less after falling ill, but some were force-fed food or given pure glucose. After 10 days all the bacteria-infected mice who had continued being fed had died, while more than half that had avoided food lived. But it was the opposite in those infected with the flu: More than 75 percent lived if they had been force-fed, while only about 10 percent lived if they hadn’t. Food was protective against the virus, but detrimental to the bacterial infection.

“To our complete surprise we found that force feeding was protective” in viral infections, Medzhitov said.

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Intrigued, the team conducted more experiments, and found that glucose, but not proteins or fats, was the dangerous component of foods during a bacterial infection. The study was published Thursday in the journal Cell.

But keep in mind:

The work was done in mice, not people. But a 2002 study in humans found similar results: eating stimulates the kind of immune response needed to combat viral infections, while fasting might stimulate the immune response that takes down unfriendly bacteria.

What they’re saying:

“What it shows is that if we understand the infection, there might be simple ways that we can improve outcome,” said David Schneider, an immunomicrobiologist at Stanford University who wasn’t involved in the work.

But he noted more needs to be done before we know how far to generalize these findings, which used only one strain of mice and might not apply to every infection.

“We don’t want to say, ‘Ok, bacteria means we don’t feed patients.’ It’s not time for that yet,” he said. “There are always going to be exceptions.”

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You’ll want to know:

The differences in nutrition seemed to influence survival not through a direct impact on the pathogen, but rather by changing the ability of the mice’s own tissue to withstand the metabolic stress that came with illnesses, said Janelle Ayres, an immunomicrobiologist at the Salk Institute in La Jolla, Calif., who was not involved in the study.

“Conventional wisdom among most scientists and the general public is you have an infection, you have to take an antibiotic or you have to take an antiviral and you just have to kill it,” she said. “This nicely demonstrates that we need to be able to deal with metabolic stresses, or we can compromise our ability to defend against infection.”

The bottom line:

Fevers can be caused by both bacteria and viruses — so the adage “Feed a cold, starve a fever” is an oversimplification. But knowing whether to feed patients based on the infection they have could be useful not just for chicken soup remedies but also for more serious infections like sepsis, which can be caused by both types of pathogens.