A neurological syndrome of gross overeating. Also known as hyperphagia. This condition can be caused by very normal things like an increased exercise regimin or a growth spurt, but can also be caused by more severe issues like depression.
Example: A brain-injured man gains over one hundred pounds in the first year after his injury; he seems unable to control his food intake.’
Here is a very in-depth web overview of the condition, it’s causes and what to look for if you think someone has developed this issue: http://www.medicineonline.com/articles/p/2/polyphagia/excessive-hunger.html