With the better quality foods that are high in protein, they don't need to eat as much because it doesn't contain all of the fillers,grains etc., that the cheaper quality foods do. It probably also contains more calories per cup so Mac should get plenty of calories while still eating less. You'll also notice less stool waste because their bodies will use most of the food. They should also have more energy and will probably feel a lot better now too.
Bookmarks