Why is loss of appetite a symptom of illness, if you need energy to recover?


Why is loss of appetite a symptom of illness, if you need energy to recover?

In: Biology

Digestion requires a lot of energy. In fact, if you eat a big meal you might notice that you feel sleepy afterwards. When fighting an illness, the body wants to focus all of its energy on the problem at hand, not waste some of it on digestion. While digestion will be needed to take more energy in eventually, the body still has a lot of energy stored already so it doesn’t really need to waste that effort on new food while it is getting rid of the urgent issue of the illness.

For the short term, this is fine. However for longer illnesses, like cancer, it can lead to dangerous weight loss.

Digestion used energy and is not an “essential” function. You need food obviously but digestion can shut down for weeks and you’d still survive. It’s not essential like circulation, respiration, immune response etc.

In fact, you could in theory get most of your food intravenously and not need these organs. The body would rather divert all focus into fighting invaders than digesting food, especially if said food could introduce more pathogens. If you’ve been well fed and of good health, you likely have large enough energy reserves to fight most invaders

Is it? I always get very hungry when I am sick.

Others have answered very well, but to add to it: your immune response is VERY callous. It will straight up kill you. It can starve you, like you said, it can raise your body temperature high enough to kill you, it can swell your throat so much you cant breath. Most of the symptoms you associate with having a disease are things your immune response is doing to you, to try and cure you.

A lot of illnesses of the gi tract – poisoning/infection cause nausea and lack of appetite to convince your body to get rid of whatever the problem is.

Many other illnesses can trigger this same pathway despite having nothing to do with the digestive system. Common example – motion sickness is your vestibular (inner ear) system mismatching your vision – telling your brain that something is out of whack – and that *something* would likely only happen if you were poisoned. Your body convinces your digestion to want to vomit because of the inner ear. Lots of other illnesses can trigger this or a similar response.

In addition to the other good answers, if you get a cold, it often involves loss of olfactory senses and that means food will taste bland, because much of its taste is actually in its smell.