Think back to the last time you came down with a cold and what it felt like to be sick. For most people, the feeling of sickness is a set of psychological and behavioral changes including fatigue, lethargy, changes in appetite, changes in sleep patterns and a desire to be away from others.

Of course, none of these changes feel particularly good, but what if they are actually good for us in terms of recovering from the infection?

Interestingly, these infection-induced behavioral changes, collectively known as “sickness behaviors,” occur in most other animals – from your pet dogs and cats to the worms in your backyard. Because so many animals exhibit sickness behaviors during infection, scientists have thought for decades that these behaviors may protect us from infections.

In our immunobiology lab at Yale University, we are interested in these sickness behaviors and most recently have focused on the aspect of appetite loss during infection. If all sickness behaviors indeed help us survive infections, then how does loss of appetite specifically fit in?

One common theory is that although we are starving ourselves, starvation is worse for the bacteria or virus than it is for us. Some scientific evidence supports this theory, but a lot does not.

Recently we ventured to reexamine why we lose our appetites when we get sick.

Why your appetite matters when you get an infection

The question of whether or not we should eat when we get sick is commonly argued, both at home and in the hospital. Every family has its own beliefs about how to address appetite loss during infection.

Some believe it’s best to keep well-fed regardless of desire to eat, some swear by old adages like “feed a fever, starve a cold” and few suggest letting the sick individual’s appetite guide their food consumption. Determining which of these is the right approach – or if it even matters – could help people recover better from mild infections.

Another, perhaps more important, reason to understand appetite changes during infection is to improve survival of critically ill patients in intensive care units across the world. Critically ill patients often cannot feed themselves, so doctors generally feed them during the time of critical illness.

But how much food is the right amount of food? And what type of food is best? And which patients should we feed? Doctors have struggled with these questions for decades and have performed many clinical trials to test different feeding regimens, but no definitive conclusions have been reached.

If we could understand the role of appetite in infection, we could provide more rational care for infected patients at home and in the hospital.

Is losing your appetite a good thing when you’re sick?

Based on our recent findings, it depends.

Like humans, lab mice lose their appetite when infected. When we infected mice with the bacteria Listeria monocytogenes and fed them, they died at a much higher frequency.

In stark contrast, when we infected mice with the flu virus and fed them, they survived better than their unfed counterparts.