I also did a lot of research on this topic. I went with lowest calories ranked against more or less good quality and availability to me.
What I found is that looking at the Kcal per cup, most diet foods had as much or more calories than what I was feeding (TOTW high prairie which is the only thing my shepherd tolerates).
The only accessble to me one that I found lower in calories than the TOTW I feed, was Avoderm. I bought a bag. I found that ALL that, Mel didn't tolerate well but at least some TOTW mixed in helped keep the solid stools. I also found TOTW high prairie puppy has right about equivalent calories to the diet so we switched to TOTW puppy instead of adult and it works marvously for both dogs and Mel has indeed lost weight. We went from 38 pounds to 35 in a couple months. She's chubby still but if I do anything too dramatic in reduction of amount of food she becomes violent and horrible to live with.
Yes, most diet foods have a lot of fillers. Not sure of your brand though, I don't think I saw it. Just evaluate the kcal per cup because a lot of diet foods aren't so diet when compared to regular foods in calorie count. That was my issue, actual calories, versus the idea of "don't feed diet food" blanket comment, per se, though I wanted fairly high quality ingredients too.
