The presence of constant hunger in cats is called polyphagia. Moreover, the term implies violation of food behavior, if before the appearance of such a symptom was all right.
This does not apply to those animals whose owners simply underfeed their Pets, not really paying attention to them and don’t care about them. To fill the lost energy requires more nutrients. Possible help will be vitamin and mineral supplements to the daily diet.
Perhaps they are not enough youngsters, and to obtain the necessary amounts of vitamins, the cat tries to eat twice the normal serving. It is known that in the cold season, trying to keep warm, the body spends the entire stock of calories that comes from food.