Just look at people with cancer. Many simply cannot eat. At all. They just cannot do it. It's not depression, loads of other diseases and conditions are depressing, yet people can still eat.

And while the person is clearly sick, every loved one tries to help by hoping to find food they can eat. Yet it just doesn't work. I've personally seen feeding tubes put in, to help in this regard.

There's some mechanism involved, and I've often wondered if the body knows food will also feed the tumor and is attempting to starve it. Regardless, the 'switch' is there.