The vitamin and mineral mix added to dog food differs from the mix added to cat food. These premixes are fully fine-tuned to the needs of your cat or dog. If your dog occasionally has a bit of cat food, then that’s not going to be an issue. In principle, dogs won't be missing out on any nutrition if they eat cat food, but the ratios in cat food aren’t geared towards dogs’ needs. Hence giving your dog cat food long-term is not a good idea. It’s best to avoid giving your cat dog food. After all, a cat needs certain substances, such as taurine. Dog food often doesn't contain these substances in the right quantities.