Dog and cat food is made separately for a reason. The core reason is that while cats need more protein, certain acids, and some vitamins, dogs can lead a perfectly healthy life without consuming such high levels.
Cat food is designed to be smaller because they have small mouths. Moreover, their bodies cannot produce certain essential nutrients, so they need a considerably balanced diet to compensate for the lack of those nutrients and boost their metabolism. On the other hand, dog food is not that concentrated and provides the required amount of nutrients for them, which is much less compared to cat food.

Although dog food is generally considered safe for healthy cats, it is still not as recommended for them as is cat food. But of course, every cat is different, so it should not be considered an excellent alternative to food specifically designed for them.
Another essential thing to note is that their foods should never be interchanged because while a cat's body can handle Vitamin D, it can prove fatal for dogs. A cat eating dog food would result in several deficiencies and possible health problems. There is also the risk of cats developing illness due to repetitive consumption of large quantities of dog food like vomiting, diarrhea, or other adverse signs.
Like any creature, cats require specific nutrients to keep them healthy and playful. Unlike humans or dogs, they cannot produce some essential vitamins and acids, and readymade cat food is made keeping in mind these requirements only. While eating dog food accidentally is alright for them, it is never recommended as a permanent option as it cannot fulfill the lack of many essential nutrients. Suppose it is given to cats for an extended time. In that case, they will develop many deficiencies as not only is dog food harmful to cats but is also highly under-nourishing and non-specific to the needs of a feline.
Some other foods that are not good for cats are:-
Milk
What could be wrong with offering your kitty a nice bowl of milk? Nothing, right? Many people are unaware that most cats are lactose intolerant, meaning that a food that is so commonly given to dogs can lead to multiple digestive problems. Their bodies cannot break down dairy products, which may lead to illnesses like diarrhoea.
Grapes and raisins
Some signs that your cat may be reacting to these foods are hyperactivity or repeated vomiting. Although there is no specific reason why these can be harmful, they are proven to cause kidney failure in cats. Not all cats will show these signs, so it's best to take no risks and not feed these products to them.
Raw meat and fish
Cats eat meat and fish but giving them raw food as meat, fish, or eggs can lead to food poisoning. Also, an enzyme is present in raw fish that destroys thiamine, an essential B vitamin for cats. The lack of thiamine has severe consequences like convulsions or coma.
Caffeine
This one should be obvious, but caffeine is fatal to cats. What makes it worse is that there is no antidote for this either. Caffeine poisoning can lead to rapid breathing, restlessness, heart palpitations, and muscle tremors. It is not just found in coffee or tea, but also chocolate, cocoa beans, cola, and drinks like Red Bull. It's also found in some painkillers and cold medicines. So it is advised to avoid any food item that involves caffeine.
Each animal has very different needs, and the food sold in the market is created keeping in mind the requirements of both bodies. It isn't a marketing strategy that wants to manipulate you into spending money on dog food instead of cat food. It is as simple as dog food cannot provide the essential nutrients that cat food can.
Animals tend to overfeed by scavenging food when they don't feel full after the meal provided to them. To prevent an interchange of foods between your cat and dog, you should feed them treats. It will keep their tummy full and also increase their favoritism for you. If they are still not satisfied and constantly eating from bowls that are not their own, you should consult your veterinarian to change their diet plan.
Conclusion
To conclude, dog food is harmful to cats, and to decrease the chances of your cat consuming dog food; you can refer to the many brands available in the market that are designed specifically for cats. They are a complete and balanced commercial diet available at affordable rates. Not only will they keep your cat healthy, but they will also keep its taste buds always asking for more!