I fed my cats Science Diet.
Why? The bag says it's healthy (omg Marissa, if a bag of chocolate cookies said it was healthy would you believe that?), the company sponsors animal events, it's sold in most vet's offices...
Yes. Science Diet donates their food to animal shelters through a program, they sponsor continuing education for vets and vet techs...How could it be so unhealthy??
Well, the ingredients...are mostly by-products. What are by-products? The parts of meat not fit for human consumption. Beaks for example.
There is Corn, which isn't easy for animals to digest, but works as a filler.
There are chemicals to preserve the shelf life. I read about a dog who was diagnosed with a cancer that was caused by a chemical in his dog food. That chemical isn't in the food now, but for 10 years of that dog's life, before they knew what it could do, it was.
No amount of healthy things they toss in and include on the front of the package can make up for that.
Rather than confuse you further I will recommend websites...
The Truth About Petfood