I would suggest something like this:
Have thirst influence max stamina at a 1:1 ratio, where 0% thirst means you can't sprint. Keeping thirst topped up at 100% could grant stamina bonuses, allowing you to sprint for 20% longer, for example. Alternatively, it could enable you to sprint for as long as thirst remains at 100%. This mechanic would tie into combat situations nicely, making juice boxes analogous to potions in an RPG—valuable resources to have on hand when you need to outmaneuver or outrun an opponent.
Hunger could affect strength instead, reducing melee damage when you're hungry. The rest of its effects could mimic the thirst mechanic, but for melee damage—providing a damage buff when hunger is at 100%. This would work well if resources in the final game are scarce, incentivizing players to rely on melee combat against NPC mobs to conserve precious ammo for encounters with Z-players.
Another alternative is to have hunger influence HP as well—or instead. At 80%, hunger would equate to 100 health, 100% hunger would equate to 120 health for example, while at 0% hunger (with some stop gap for negative hp), they would start taking damage at set intervals due to starvation.
--edit.
Being topped up at 100% could heal you one point at a set interval, though might diminish some of the medical items of which there are a lot by the way.
