When my dog (Labrador) was a puppy I fed him Eukanuba (top end) dog food for the first year or two. Then I graduated to the cheapest dog food. Now he turns 11 next week and a month ago my vet wanted me to get him on a senior food (Purina One). It is more expensive about $26 for the large bag instead of $12-$15. He's been on it now for a month or so. I swear that my dog has WAY more energy than he did on the cheap food. I've been very, very surprised at the change I've seen. He's not overweight, just starting to get old, arthritis for the last 3-4 years. Has anyone else seen this in their dogs?