My dog's on a low protein diet right now, has been for years but now even more so. The older she gets, the more trouble I have keeping weight on her, and also with age, I've had to put her on a mostly dry food diet because the wet food has been messing with her stomach, and coincidentally more messes for me to clean. I'm a vegetarian, so I don't keep meat in the house, therefore go the canned or dry route with supplements.
But I do wish there was a high-calorie, low protein "senior" food available ready made. They all seem to be geared towards roly poly older dogs. Mines a bony-butt, always has been. She's 12 pounds and poops like a labrador retriever though
I supplement her food though with pureed pumpkin (of late, made from organic local pumpkin from my CSA), home made peanut butter for pills, fish oil and some cranberry powder from time to time. I find treats to be more of an issue than food, and ESPECIALLY chewies.