Lack of food, or even proper nutrition has a profound effect on anyone. I have worked up a table to indicate the effects of malnutrition on PCs. For simplicity’s sake, an adventurer out on campaign should have about 2 lbs of food per day, with 1/3 to 1/2 of that being Carbs (potatoes, breads, etc.), 1/3 to 1/2 of it “Greens” (being fruits, vegetables, etc.), and the remaining 1/4 to 1/3 of the diet being Protein (meat, fish, fowl, etc.). Beans have the honor of counting as both greens and protein. Water should be consumed in quantities of 3 liters or more per day. On a low activity day (hanging around in town, for example), only one pound of food is required.
The table indicates the cumulative effects of lacking any or all of the above nutritional elements. Of course, adverse conditions will aggravate the conditions listed, and should be adapted as necessary (extreme heat greatly accelerating water depletion, for example)
|2||-1 Dex||-1 Str||Fatigued|
|3||-1 Cha||-1 Int||-1 Con||Exhausted|
|4||-1 Con||-1 Str||Disabled|
|5||-1 Dex||-1 Wis||-1 Con||Disabled|
|6||-1 Cha||-1 Str||Dead|
|7||-1 Int||-1 Con|
|8||-1 Dex||-1 Con||-1 Str|
|9||-1 Cha||-1 Wis||-1 Con|
As you can see, the table progresses in a linear fashion and may be interpolated out as far as necessary. Each of these penalties are cumulative, and add up with each other. So, if you have a party that forgets to bring along turnips in their field kit, they will be fine for a couple of days. However, lacking all those vitamins and minerals will start affecting their health, both mental and physical. They will be more susceptible to disease, and will begin to lose cognitive ability as well.
Granted, some foods will account for more than one category. Beans function as protein and veggies, potatoes act as carbs and veg, etc. Also, if there is decent rationing going on, a half day’s worth of food will only count for 1/2 of a day on the table. So it will take twice as long for any given condition to take effect if some food is being eaten.
However, I am assuming in this table that adventurers are undertaking normal actions: combat, hiking, arguing, carrying heavy stuff all over creation, wearing armor, and such like. This is a tool for you to use, to give a baseline for something that is not covered in the DMG. Unlike typical ability damage, starvation damage will recover at 2 points per day, once normal nutrition is restored.
Lacking adequate water is, obviously, a dangerous proposition. 3 litres a day is necessary for normal activities, and every day you have no water at all severely depletes your ability to perform. Less than 3 litres, but more than 1 a day will halve the rate of declining health, and restoring full water intake will bring a character back up the table two day’s worth at a time. But strenuous activities will increase the need for water. Say every hour of intense physical activity (combat), or two hours of moderate physical activity (hiking), or for every 10 degrees over 70 the temperature is will increase the daily need for water by 1 litre each. So if it’s 90 degrees, and the PCs are hiking across fields all day (8 hours), and then have to fight goblins for an hour, they will require 10 litres of water that day, each one of them. Frankly, it sounds like a terrible day to me, too. But there it is. If they only brought 5 litres of water apiece, they would do alright for one day. But two days like that, and they would wake up the next day feeling sick. Look up heat stroke sometime. It isn’t fun.
So there’s that. Who says wilderness adventures can’t be fun?