system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is loss of appetite a symptom of illness, if you need energy to recover?
gpt
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.