Join any Hardcore Server (Should work on normal aswell)
Get some food (I used canned Tuna, not sure if others will work)
Get another Item (one slot pref.)
Rightlick food an choose eat all (not sure again if normally eating works aswell)
Exchange the food you´re eating with another Item on the ground (just dropping might also work fine.)
Wait eat animation to stop and go again.
---> Food will always stay at 100%, not sure about water, lemon, Spite, fruit and so on. Sorry didnt had much time to test. If someone has feel free to create a more detailed report or add informations under this report and I will edit.