You maximum health determines how big of a hit you can take.

A player gets 40% of their maximum health as the stagger bar. Taking or blocking hits that fill the stagger bar result in you staggering. In the case of blocking, if the stagger bar is filled after the shield dmg reduction/stagger calculation is done your block or parry FAILS and then its up to your armor to mitigate the entire hit. If your block/parry succeeds your shield mitigates the dmg and then your armor mitigates it even further.

So if you try to block a 2* fuling which does around 160 dmg with a blackmetal shield and 90 hp, your dmg calculation is going to end up like below. I don't what what your block skill is but for the sake of the calculation im just going to assume your shield is max upgraded (90) and your actual block value with the shield is 110.

(160^2)/(4*110) = 58 dmg converted to stagger if blocked
(160^2)(4*160) = 38 dmg converted to stagger if parried.

40% of 90 = 36.

This means that once the food you have chosen starts to wear off even just a little bit, you lose the required maximum health to parry a 2* fuling. It also means that you cannot block a 2* fuling with that food.