Typing in this command in peaceful mode:
/effect [user] minecraft:hunger 1 3
should reduce the hunger bar, and it works in 1.8.3 and earlier.
In 1.8.7, however, it won't work.
I even tried changing the duration, and the level of the effect, and it still won't do it.
MC-82389
Hunger Effect Doesn't Work in Peaceful Difficulty
Resolved
Works As Intended
1
I think this is intended, to make it so on peaceful you always regenerate. (low food = no natural regen)