You may now eat Food equal to the Level of the monster you have just defeated — this is known as your food requirement. You gain one Small Effort for each Food that you eat.
If you cannot (or choose not to) meet the food requirement, you must spend one Small Effort for the first Food you do no eat, two Small Effort for the second Food, three Small Effort for the third Food, etc. If you do not have enough Effort to spend for Food you do not eat, you die of starvation and lose the game.
If you have plenty of effort remaining, you can decide to eat less food than the current requirement, in order to save the food for a more crucial moment later on.