My dog had loose stools, so I switched to a more natural dog food brand and he doesn't have problems now.
I've heard a lot of vets telling that dog food is bad and blaming it for the raise of cancers in dogs (when you read the ingredient list carefully you can see why). My dog's trainer suggests to his customers to give them real food (raw meat, a bit of steamed rice and vegs, a bit of fruits and vitamin/mineral supplements) with some blood tests at first to fix deficiencies (like humans).
I lived in France and now in Japan and get local brands. Most brands in Japan are imported from America and here's a list of brands I found were kind of good: acana, evo, california natural, argyle dish
I search for food without corn, wheat and with real animal protein (not weird animal by-products they put in big commercial brands). The most protein the better.