A couple years ago, we switched the Chihuahuas to a grain-free food. We could see a difference almost overnight. The doggies just looked healthy and their coats were noticeably more shiny. Around that time, people started complementing us, and the vet even remarked that our dogs were healthier than hers.
Here we are about a month on the Chicken & Rice meal I've been making for them, as well as the regular vitamin supplement. Their coats are even shinier and they just exude health. They even have more energy on this diet, not to mention the level of enthusiasm has skyrocketed since they started getting real food.
There is a weird side effect, though, from the new diet. I realize the temperatures have dropped, but it's still a little disconcerting that the entire pack hardly drinks any water. They pee normally, and they look like they are in fantastic health ... they just hardly drink.
My theory is that the really moist food now gives them most of the water they need. There's no secret supply of water here, so their bodies have to be getting it from somewhere.