science based information http://skeptvet.com/Blog/2018/08/grain-free-diets-and-heart-disease-in-dogs/
(excerpt below, click on link for full article)
Bottom Line
Nutrition and metabolism are complicated, and the exact relationship between dietary composition, breed genetics, and other factors leading to DCM is not yet clear. It is too early to say with certainty whether the diets are the primary cause of DCM in these dogs or whether other breeds may also be at risk. However, it is clear that the idea behind the health claims for grain-free diets is speculative at best and very likely untrue. Extreme diet fads hardly ever turn out to be a good idea in people, and the same is probably true for pets.
If you are feeding a grain-free diet, there is no need to panic. If you own a golden retriever or other breed that has been shown to be develop DCM in the past, it makes sense to talk to your vet and potentially have taurine levels tested or other diagnostics done depending on the circumstances. The diet you are feeding may be perfectly fine, but it is also probable not any better than any other diet with more conventional ingredients, and there is now some small indication that it may place some dogs at greater risk for this preventable disease.
The links above to the FDA and UC Davis Vet School will provide more information.
Also: https://www.mspca.org/angell_services/choosing-the-right-diet-for-your-pet/
Animal by-products (excerpt below)
In addition to grain, animal by-products have become “dirty words” on the ingredient list. Although not necessarily appealing to humans (particularly in the USA), the definition of a by-product in pet food is a part of the animal that is not skeletal muscle. This includes organ meats and intestines (not intestinal contents). AAFCO specifically excludes hair, hooves, horns, hide, manure, etc… as acceptable by-products. So in reality, by-products are perfectly healthy and full of nutrients. And you can be sure that a wild wolf or mountain lion is eating “by-products” in nature.
