Food is produced from farms each turn, as long as there is enough population to work in the fields. Each one of the troops' and population's members eats some food per turn, according to the race of the kingdom. Lack of enough food for the next turn, gives a message of "not enough food" at the top of the screen if you have the advisor option turned on. If a kingdom takes a turn with not enough food, the result will be that many troops and population from those that didnt eat to starve.