http://skeptvet.com/Blog/2016/07/more-nonsense-from-holistic-vets-about-commercial-therapeutic-diets/ (excerpt below)
Here’s what one nutritionist I’ve talked to has say about by-products:
A by-product only means that it was not the intended main product of the industry. It gives no indication on nutrient profile, digestibility and bioavailability, etc. Many people who dislike by-products will happily buy wheat bran (a by-product of the baking industry). Moreover, by-products vary according to country and culture. Liver, an excellent source of nutrition, is considered a by-product in the US because skeletal meat is the primary product of slaughtering an animal and many people do not eat organs any more. By-products can be excellent ingredients in pet food and it would be wasteful (and terribly self-centered) to not use it to nourish humans or animals.
