This is specifically for a small breed dog My friend feeds her adult dogs puppy food along with her 9 month old puppy. All the dogs are the Havanese breed. At first she was feeding the 9 month old adult food for a while because of the other adult dogs. Now she is feeding them all puppy food. I tried to tell her it's not good but she doesn't believe me. So what will it do?
You are right. Puppy food is formulated for growing dogs that have higher nutritional requirements and burn more calories due to their activity level and metabolism. The protein and carbs in it are not in balance for a mature dog. Simply compare the analysis table on the puppy food with that on regular dog food and you'll see a striking difference. Simply tell your friend that if she cares about the health of her dog, that her dog would do much better on adult dog food.
It depends on the formulation, but most likely, the adult dog eating puppy food would just get fat if it eats enough to satisfy its appetite.