Your body keeps a food pool and a water pool, and both drain steadily as the day passes. Neglect shows up in bands: a full body notices nothing, a hungry one wears the badge, and a starving one starts losing vigour — which ends with you prone, not dead. Starvation is deliberately forgiving: it stops you, it does not kill you outright.
Eating
Eating is an action like any other — it takes a few seconds and can be interrupted. What a food restores is a property of the food itself, and cooked food restores considerably more than raw. A fish straight off the line is a poor meal; the same fish cleaned at a butcher's block and cooked over a fire is a good one.
Where food comes from
Today, mostly out of the water: fishing feeds the kitchen, and the kitchen — butchery and cooking, two crafts that work together — turns the catch into meals. See crafting for how stations work.