Characters who don’t eat or drink suffer the Effects of Exhaustion. A single level of Exhaustion caused by lack of food or water can be removed if the character eats and drinks the full required amount The next day.

Food

A character needs 1lb of food per day and can make food last longer by subsisting on half Rations. Eating 0.5lb of food in a day counts as half a day without food. A character can survive on a half day’s food for a number of days equal to his or her endurance score -5 (with a minimum of 1 day). At the end of each day beyond that limit, a character automatically suffers one level of Exhaustion. After 2 or more Days on half food a character must eat an extra 0.5lb, for a total of 1.5lb within a single day to return to 0 days without enough Food. This rule still applies if the character has already gained exhaustion from only eating half a days food. 

If a character does not eat at least 0.5lb in a day they gain 1 level of Exhaustion.

Water

A character needs one gallon of liquid (1lb) per day, or two gallons (2lb) per day if the weather is hot. A character who drinks only half that in a day, must succeed on a DC 75 survival saving throw or suffer one level of Exhaustion at the end of the day. A character who drinks less than half of their daily needs automatically suffers one level of Exhaustion at the end of the day. If the character already has one or more levels of Exhaustion From Dehydration, the character takes 2 additional levels of exhaustion instead of one.