I've never tried that line of their food. About a decade ago when I first got my shih-tzu, she was 3-5 years old, she had a very dull grey and white coat. She was fed a mix of people food, white rice, and some dry kibbles (not sure on brand as we were just given some in a zip-lock bag). Almost as soon as I got her, I picked up a new dog food recommended by a shop owner. It was Diamond brand dog food. Can't quite recall the formula, but definitely not a "natural" line. The first year we had her while she was on that food, her coat got noticeably better! Of course, we had her shaved down, and when it grew out again, it was vibrantly black and white. Her whites had a nice shine to it too! I'm pretty sure it was the food, not just the extra sun and outdoors she got from being with us versus being with her previous owners. I can't recall our reason for changing the food on her other than the fact that after she had finished that large bag of food, by then, the place that sold it was no longer. Her hair never got the same shine with the other two brands we went with later, but it was never as "dull" again either.
I'm sorry, this may not be relevant to the specific line you're asking about. I was just sharing my experience with Diamond themselves, albeit a VERY long time ago, before this generation of recalls, etc.
