Hunger is used while moving in a bubble column. Hunger1.png shows my starting food at y64, satiation is already fully depleted and food is at 18. Hunger2.png shows my food at the top of the bubble column, y318, food at 12. The only action between those two screenshots was walking into the bubble column.

It seems to be using it at the same rate as swimming. A test swim from z170 to z424 used the exact same amount of hunger. I also tested being pulled down a bubble column via a magma block and hunger also depletes at swim cost.

This seems unintentional.