Try to get your Food Metre to 0 by doing any action or type in /effect @p hunger 10 100. After the hunger effect has worn off, switch your difficulty to Peaceful. Your Food Metre will attempt to saturate itself but you will still get hurt as if you have no food left and the Food Metre will go back down to 0 and refill again. When replicating the above bug, any damage instead of the slow hunger damage will decrease the food metre.