Using testfor command to constantly fill hunger bar and hunger

Solution 1:

The best and easiest way to do this would be to give all players the saturation effect constantly, instead of trying to do it only when a player is hungry.

/effect @a 23 1000000 1 1

Which gives all players Saturation I for 1000000 seconds, and hides the particles given off by the effect. The advantage to setting the time to such a large number is so that the effect remaining time displays as xx:xx to indicate to the player that the saturation effect is permanent, and that they don't need to worry about food.

Run this command on a clock input of some sort and hide command block output with /gamerule commandBlockOutput false and you're all set.