Can I ask why science diet is bad food? how come vets always recommend it if it's bad food?
Some vets are reimbursed by the food companies to recommend and sell the products. Remember, there will always be some people in the world who value money over their patients' health... and some who simply are unaware, for whatever reason, that the foods are not really that healthy for animals. Science Diet isn't the worst food, but it's a far cry from anything good.
I will do a comparison for you of three different foods that have the same "flavor" of food. One will be generic Purina (you didn't say exactly which formula of theirs he is currently on), another will be Science Diet Adult, and the third will be Orijen. Anything in red is considered a controversial ingredient.
Purina Dog Chow Complete and Balanced
Whole grain corn, meat and bone meal, corn gluten meal, animal fat preserved with mixed tocopherols, soybean meal, poultry by-product meal, egg and chicken flavor, whole grain wheat, animal digest, salt, calcium carbonate, potassium chloride, dicalcium phosphate, choline chloride, zinc sulfate, yellow 6, vitamin E supplement, l-lysine monohydrochloride, ferrous sulfate, yellow 5, red 40, manganese sulfate, niacin, blue 2, vitamin A supplement, copper sulfate, calcium pantothenate, garlic oil, pyridoxine hydrochloride, vitamin B12 supplement, thiamine mononitrate, vitamin D3 supplement, riboflavin supplement, calcium iodate, menadione sodium bisulfite complex, folic acid, biotin, sodium selenite.
Hill's Science Diet Adult 6+ Large Breed
Chicken, whole grain wheat, cracked pearled barley, whole grain sorghum, whole grain corn, corn gluten meal, chicken meal, chicken liver flavor, pork fat, dried beet pulp, soybean oil, lactic acid, flaxseed, potassium chloride, iodized salt, calcium carbonate, choline chloride, vitamins (vitamin E supplement, l-ascorbyl-2-polyphosphate (source of vitamin C), niacin supplement, thiamine mononitrate, vitamin A supplement, calcium pantothenate, vitamin B12 supplement, pyridoxine hydrochloride, riboflavin supplement, folic acid, biotin, vitamin D3 supplement), minerals (ferrous sulfate, zinc oxide, copper sulfate, manganous oxide, calcium iodate, sodium selenite), oat fiber, taurine, mixed tocopherols for freshness, natural flavors, beta-carotene, dried apples, dried broccoli, dried carrots, dried cranberries, dried peas.
Orijen Dog Food
Boneless chicken, chicken meal, chicken liver, whole herring, boneless turkey, turkey meal, turkey liver, whole eggs, boneless walleye, whole salmon, chicken heart, chicken cartilage, herring meal, salmon meal, chicken liver oil, red lentils, green peas, green lentils, sun-cured alfalfa, yams, pea fibre, chickpeas, pumpkin, butternut squash, spinach greens, carrots, red delicious apples, bartlett pears, cranberries, blueberries, kelp, licorice root, angelica root, fenugreek, marigold, sweet fennel, peppermint leaf, chamomile, dandelion, summer savory, rosemary, Enerococcus faecium, supplements: vitamin A, vitamin D3, vitamin E, niacin, riboflavin, folic acid, biotin, vitamin B12, zinc proteinate, iron proteinate, manganese proteinate, copper proteinate, selenium yeast.
Paying attention not only to the number of controversial ingredients, but also what the ingredients in general are... it's a bit easy to make a distinction from a really horrible food (Purina, 1/5), an "okay but not great" food (Hill's, 3/5), and a high-quality food (Orijen, 5/5).
A lot of controversial ingredients are fillers (basically anything derived from plants aside from entire fruits and vegetables). Products from corn, wheat, and soy are very common allergens in dogs and basically are put into food to make the dog feel full. They are also not very nutritional, and a dog's digestive system is not designed to absorb plant-based nutrients so it is more difficult for them to benefit from these ingredients.
Anonymous meat-type products are also not very good. Things like meat and bone meal, animal fat, and animal digest. Anything that doesn't have a named source (like chicken, lamb, salmon, etc.). These products can come from literally anywhere and nobody is the wiser. Animal digest in particular is something I run far away from. The FDA found sodium pentobarbital in it (that is the drug used for euthanization), which I wouldn't want my animals ingesting no mater how minute the traces. This means the source for animal digest could be anything that has been euthanized. There have also been some reports of pet collars being found in the machinery that processes and renders the sources for the ingredients... so... yeah.
A lot of foods have nixed using coloring agents (yellow, blue, red). Really their only purpose is to make the food more aesthetically pleasing to us
. Our pets don't care what color their food is. Food coloring agents can also cause reactions in some animals and people.
Garlic oil is controversial because of its link to Heinz body aenemia. However, some claim that it takes a large amount of garlic to cause any adverse effects. I am not a vet or animal dietician, so I cannot make any definitive claims. Some owners claim garlic, in moderation, has many health benefits. While not related, I personally cannot consume garlic or anything containing garlic, so I generally stay away from it regardless.
Beet pulp is slightly controversial because while it may have intestinal and blood sugar benefits, it is also largely a filler and has relatively high sugar content. In moderation, this ingredient is reasonable.
While that was incredibly long-winded, I hope that clarified a bit for you... sorry for that huge wall of text XD