We changed Charlie from purina to pedigree puppy when he was a tiddler - did not agree with him at all! Until we changed his food again (to naturediet, which he loves), we thought we just had a very active puppy. Then we took him to puppy classes, and we were quite surprised to discover he was the bounciest puppy there (especially considering he was a lhasa in a class with a cocker, a boxer, and a staff!

). We also found that he was forever scratching, and although most lhasa's have quite 'teary' eyes, his were constantly gunked up; he also very really had a solid poop, which he had the delightful habit of munching it unless we were there to swoop it up immediately.
Puppy trainer recommended that we tried a more 'natural' type of food, so we swopped to naturediet and found that we actually now had a chirpy (rather than manic!) puppy, the scratching stopped almost altogether (apart from when he's been bitten by summat when rooting in the woods :rolleyes: ) , and we are cleaning his eyes once a day rather than five or six times. Also, he now has 'normal' poops, and prefers dog biscuits to doings as a treat ;)
Obviously every dog is different, and some foods agree with some more than others, but we've also found that our experiences seem to reflect those of other owners. Charlie is now fed on Naturediet, with Burns either as a mixer, or for a bit of variety, and he loves both! :)