To start breaking down the problem with dry food, I’ve listed the ingredients for Purina and Royal Canin below. You’ll see they contain a lot of cereals like wheat, soya and rice. Since cats are carnivores, we try to avoid giving them these, especially in high amounts; they are sources of carbohydrates (which are harmful) and these grains are often difficult for cats to digest.
Actually, the trend nowadays is more towards grain-free. It isn't perfect either, as the cereals are replaced by starches like potato, sweet potato, or peas, which are added for technical reasons. These are starch sources that give the kibble its consistency; otherwise, it would just be powder.
Generally speaking, grain-free is preferred because more care is usually taken with the protein sources (which are mostly animal-based) in terms of both quantity and quality.
I'll give you the ingredient list for Carnilove as an example.
One small downside is that grain-free brands can be hard to find in high street shops; you usually have to go online to sites like Zooplus...
Purina One (pink bag)
chicken (17%), dried poultry protein, whole grain wheat (13%), maize gluten meal, wheat gluten meal, animal fat, soya meal, maize starch, dried chicory root, maize, minerals.
Royal Canin Sterilised
poultry protein (dried), maize, vegetable protein isolate*, lignocellulose, animal fats, animal proteins (hydrolysed), rice, wheat, dried beet pulp, maize gluten, yeasts, minerals, fish oil.
Carnilove
wild boar meat meal (35%), lamb meat meal (24%), yellow peas (19%), chicken fat (preserved with tocopherols) (6%), apples (4%), chicken liver (3%), tapioca starch (3%), salmon oil (2%), carrots (1%), flaxseed (1%), chickpeas (1%).
(Notice how the Carnilove ingredients are fully detailed with a percentage for each item, which isn't the case for Purina and even less so for Royal Canin).
Best,
