The regeneration is pretty slow, so you might not notice it for a while. You only really need to eat a can of food and a couple swigs of water from a canteen as long as your character isn't already complaining, but you also need your energy up for your blood to regenerate. Sprint around for a minute to warm up (not sure if temperature is working yet but it doesn't hurt just in case), duck into a building, and give your character a few minutes to rest.