Don't want to write this by hand? GodCraft sets minecraft:player.saturation for you with a switch.Get GodCraftOverview
minecraft:player.saturation is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Stores the player's hidden saturation, which is used up before the hunger bar starts to drop. Used by the player entity.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:player.saturation": {}
}