Low hunger and high nutrition are essential for ensuring your character's survival. The hunger bar is lowered by consuming food items, such as pike. To consume a food item, press shift+E to enter the Foods menu. Select the item you wish to consume. As your Hunger bar diminishes, your Nutrition level will increase. The rate at which it increases depends on the amount and type of food consumed. For instance, eating enough roaches to empty your hunger will keep you full, but isn't near as nourishing as, say, a pike, or stag. The nutrition scale goes as follows, from worst to best: Starving, Malnourished, Insufficient, Scarce, Temperate, Good, Nourished, Abundant. It is important to remember that your nutrition level is more important than the hunger bar, and that just feeling hungry is not an indicator that you need to eat since it is possible to feel very hungry while still being well-fed.
NOTE: If you are at Scarce-to-Abundant for Nourishment, you can only eat cooked or prepared foods.
Thirst is another life-requirement for your character. However, Thirst does not affect Nutrition, and anything can be quaffed regardless of your Nutrition level. Thirst increases slowly compared to hunger, which can increase rapidly during the course of a few hours. Thirst is diminished either by drinking from water stored in a container or by drinking from a body of water. When drinking directly from water, your thirst bar is emptied entirely.
Warmth is the final necessity for survival. To gain warmth, you can (A. Build a fire, and wait/perform an activity nearby, (B. Put on heavy clothes and perform an activity, (C. Enter a heated building and perform an activity/wait, or (D. Enter an use a sauna. If warmth is not maintained in one of the red or grey levels, your character may get Frostbite.
Vigour affects your ability to perform tasks in that if your vigour is low, you are, as displayed, Tired. If you have a restful sleep, your Vigour will be at Vigorous, or possibly Lively. When your Vigour is low or completely expended (Ready To Drop), you had best hurry to bed or shelter, or pass out and face the consequences.