A player will automatically fast when the hunger bar drops between 6%-8%. However, once a player loses their fat reserves/layers, they will no longer be able to fast and will enter a state of starvation.
You will NOT die from not eating in wurm! Do not panic when your food and nutrition is low. The only problems caused by not eating in wurm is slower stamina regain and not being able to carry out heavy labour actions, such as mining or digging.