It means, in brief, that the subject is not eating normally. Lance proffers a bagel with cream cheese. The subject refuses it.
Lance proffers a chunk of challah bread.
At most, the subject will try a bite, announce that it tastes like sawdust, and refuse to eat any more. At times, the subject will choke down a pre-measured quantity of food, complaining all the while, moved to such heroism by the knowledge that one must eat to live. At times, the subject eats normally. The subject was known in the past to be a healthy eater.