How to deduct hit points after each move to a new cell (grid based movement)

Hello all,

I was wondering how one would reproduce this effect:

lparchive.org/Ultima-1/Update%2004/39-ultima_2_41_dragon_turtle_loop.gif

Each time the Avatar moves to another cell his total food decreases.

I haven't been able to figure it out. Closest I came is the "food" decreasing as long as the player doesn't hold down a key. If the player holds down a key to move smoothly across the grid then the deductions don't happen...