Actually byproducts, meat byproducts more specifically include: euthanized dogs and cats, without any plastic ID tags removed, roadkill, slaughterhouse rejects due to disease, old age, or otherwise unfit for human consumption, and animal control pick ups. If you don't believe that to be true, there's a guy on Youtube who is a holistic veterinarian where I got this information from, and I've had this same information quoted to me by a homoepathic counselor. And there's many others out there who have actually taken the courses to find this stuff out, and they know their sh*t, unlike all the other people and especially, all the pet food companies who will tell you there's nothing wrong with byproducts because they've been "cleaned" which is just flat-out wrong. These companies also believe cats and dogs need high amounts of carbohydrates to store as energy for later which is also flat-out wrong, cats and dogs get their energy from the meat they eat, there is no storing involved, and when it does get stored, it just gets manufactured into sugar and thus fat that they waste away because of.
Now; dogs and cats will eat roadkill, but they should not be eating food that's comprised of euthanized dogs and cats, especially since the chemical used to euthanize does NOT deteriorate in heat during the cooking process.
Science Diet, Iams, and Purina are all horrible foods, with no quality to their ingredients whatsoever. The companies do not care about the health of the animals eating their food, they care only about getting ingredients for cheap and selling their products off to pet owners who don't know any better. The foods are inexpensive because they're inexpensive to make and jeopardize the health of your pet. The whole, "100% complete and balanced" is bullsh*t too, you wouldn't feed your child the same food every day, you should not be feeding your pet the same food every day either; they were designed to eat varying sources of meat protein - cats don't hunt and find mice each and every day to eat, and dogs don't hunt and find chickens every day to eat. Feeding the same food, and thus, same protein source and grains(which they shouldn't be eating anyway), gives way to food allergies, and medical ailments such as kidney failure, liver failure, dehydration, and a list of others because the cat or dog's body begins refusing the ingredients in the food.
Purina, Iams, and Science Diet contain no nutrition suitable enough for carnivores to be eating it. Sorry, I'm sure I'll get h*ll for this but that's just how it is. If you don't believe me, do your own research, but first and foremost do not buy into what pet food companies say. Follow what people who have actually DONE the research say. A major study to look into is the Pottenger study; which tested two groups of cats, one fed on cooked meat, the other group fed on raw meat, and it details the medical ailments the cat fed on cooked meat experienced. And your average-day commercial pet food company like Iams, Purina, and Science Diet cook any meat in their products down. Food cooked beyond the temperature of I think...117 degrees starts losing nutrients, and I can assure you all the cooking that goes into baking kibble, and even canned meat reaches MUCH higher temperature than that. My cat puked 2 - 3 times a week on Purina, which I thought was normal, until I switched her to Natural Balance, and then to Wysong, and she hasn't puked once. Her dandruff is gone, she's no longer fat, and she looks half her age, and looks as if she visits the groomer at least once a week. My other cat, who had a much more iron-clad stomach, didn't puke once on Purina, but she had massive amounts of dandruff and was ill-tempered. When I switched her to Natural Balance, and then to Wysong, she lost the dandruff, her fat turned to pure muscle(can't even pick her up with one arm, you need two, she has that much muscle mass), and her temper has...well, she doesn't have a temper. Now she's the sweetest thing. But all the chemicals and preservatives and ingredients in pet food from companies like Purina, Science Diet and Iams, has a lot of ill-mannered and medically ill affects on cats and dogs. Natural Balance and Wysong I've both had amazing luck with; I only switched them off of Natural Balance because the one cat is not spayed, and requires high caloric food, and the only high caloric food Natural Balance has is chicken-based, which the cat is allergic to thanks to feeding her the same food day in and day out.
I will post the source to the holistic veterinarian of whom I speak; generally I don't buy into veterinarian mumbo jumbo, because veterinarians are not nutritionists and most will support the big name-brand companies because they fund their veterinarian schools. There are only a few veterinarians out there who won't fall into the pet industry's hands, and find other, and in my opinion, more honest ways for funding. Veterinarians KNOW what meat byproducts are and how bad they are, but they are more concerned with funding from these big name-brands and as far I'm concerned, there is no right reason to do that.
ImADogLover - you may think your dogs aren't having any problems, but there's probably medical issues dwelling inside them that have yet to make a physical show. Dogs and cats, and all domesticated critters are still very good at hiding illness.