The disease that causes constant hunger and means you're always hungry even though you've eaten is known as Prader Willie Syndrome.
Prader Willie Syndrome is caused by a flaw on the Maribel's Chromosome 15 and it can cause occasional violent behavior, Short Stature and Intellectual Impairment.
It's a rare disorder but Prader Willie Syndrome causes someone to feel hungry all the time without there stomach being satisfied after eating.