What would happen to a puppy if instead of puppy food he eats adult dog food?

We got a puppy a couple of months ago, he's now 4 months old. We have another dog who is 8 years old, i've noticed that recently my older dog has started eating the puppy's food and the little one is eating the adult dog food, I've tried to feed them separately, in different rooms, at different... show more We got a puppy a couple of months ago, he's now 4 months old. We have another dog who is 8 years old, i've noticed that recently my older dog has started eating the puppy's food and the little one is eating the adult dog food, I've tried to feed them separately, in different rooms, at different times, in very different looking plates, but the baby is still eating the adult dog food and the older one keeps eating puppy food, they will sit and stare at the pplate until i let them go for the other one... it's like they do it on purpose! I'm worried because i don't know how it can affect the pup, he's an Amstaff, and so far (after about a month or so of this behavior starting) i haven't seen any symptoms like diarrhea or constipation, he's growing nice and big, and i don't see any signs of a skin allergy... the adult dog food is ''Alpo Prime Cuts'' by Purina, and the puppy food is Purina dog chow for puppies (formerly know as ''puppy chow'') i have noticed, however, that my adult dog has gotten significantly fatter, is it that the puppy food has more fat or some kind of hormones? i KNOW i have to stop this as soon as possible so don't tell me ''stop it right now'', believe me, i have tried but i ultimately think it's better that they eat each other's food rather than just not eating anything at all... but what could happen if it continues? will my puppy have some kind of growth problem? does the puppy food have something in it that adult dog food doesn't and he needs as part of his development? or is it just marketing?
Update: OK people seem not to get the question. I do have shuttable doors, but if i lock them in separate rooms so they will each eat their food, they just won't eat. period. the reason i feed them that brand is because it's the one i have available, i don't live in the States so i don't have access to the... show more OK people seem not to get the question. I do have shuttable doors, but if i lock them in separate rooms so they will each eat their food, they just won't eat. period. the reason i feed them that brand is because it's the one i have available, i don't live in the States so i don't have access to the brands that people are recommending.Because i know of the high content of grain in commercial dog food I try to compliment their nutrition with chicken giblets at least once a week (no bones) and once or twice a week i'll mix cooked ground beef with their food. i have tried all of the tricks you are suggesting, if you read the details above you will know that i AM trying to stop this behavior, i know that that's not the food they should be eating, what i want to know is WHAT SPECIFICALLY CAN HAPPEN TO THE PUPPY FOR EATING ADULT DOG FOOD i know that puppy food makes the adult dog fat, if you can read, you'll see i added it in the previous details. PLEASE READ BEFOR
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