This is a simple method for penalizing player characters who neglect a basic survival necessity, and rewarding those who make the effort to do more than the minimum. It's compatible with nearly any iteration of old school Dungeons & Dragons, its retro-clones, its OSR relatives, and maybe more.
Characters must eat to survive, and they must eat regularly to function normally. For each day that a character fails to eat at least one substantial meal, the character suffers a -1 cumulative penalty to all actions and saving throws, i.e. -1 on the first day, -2 on the second day, -3 on the third day. The DM will determine, based on the character's Constitution and species, how many days the character may survive until starvation ends the character's life. Penalties commence each day as soon as the characters become active until they have their first meal.
If a character manages to have three meals in a single day and the third meal is consumed whilst resting, he or she recovers 1d3 hit points upon finishing it.
Water, or an acceptable liquid substitute, is also a necessity, and failure to drink also results in a -1 cumulative penalty to all actions and saving throws for each day of deprivation as above. The DM will determine how many days the character may survive until death by thirst occurs. Penalties commence each day as soon as the characters become active until they have their first drink.
Characters who fail to consume both food and water will suffer the penalties for both.
Starting on the fourth day of deprivation of either, characters will begin to lose 1dx hit points per day, where x equals the hit die type of the character's class.