Your regeneration rates for health, stamina and mana are directly affected by how hungry you are. When you begin to get hungry, your regenerative abilities are slowly decreased until you are starving. Once you get to that point you stop regenerating health and mana entirely. You will still regenerate stamina but at a very slow rate in order to allow you some movement.
If you are poisoned, your stat regeneration rates are affected. Your hit points, stamina and mana will not regenerate at their normal levels (as adjusted by hunger) while you are poisoned.