(All of this is outdated) Hunger has five levels: Starving (0-49%), Very Hungry (50-79%), Hungry (80-89%), Full (90-100%), Overstuffed (>100%), where the number represents the right most number in the mouse over. 10 units of hunger represent 1%, thus a food that recovers 40 units of hunger will increase the right most number by 4%.
If you are at the Very Hungry level, your character cannot regenerate SHP, if it's below maximum.
If you are at the Starving level, your character will slowly lose HHP. If you are at the Overstuffed level, you will only be able to move at a crawl. 1% of hunger can replace 5% of stamina.
Also when your SHP is low and starts to slowly regenerate it will reduce your hunger level.