From what I know, puppies feed on breast milk.
Are there any healthy foods that I can feed a puppy along with breast milk, or does breast milk contain all the nutrients they need to grow to full maturity?
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You need to make sure the mother is on a lactating diet. I've learned in the pet food business that lactating mothers have the highest need of nutrition over all other life stages. Also when you start the puppies on solid food, try wetting the food first and make sure its a puppy specified diet. An 'All life stages' food is okay but not optimal for the calories and building block nutrients puppies need. As well large breed puppy food is different then small dog puppy food so I would go to a pet store and ask to make sure they get the right food.
So, as a general rule, you should expect that the puppy is getting sufficient nutrition in the early stages of growth from the milk supplied by the mother. However, the health and general condition of the mother plays a role here. It's important to ensure that she is sufficiently looked after to ensure that she produces sufficient milk for the litter and that the the milk she produces contains the necessary nutrients.
Also important, especially if the litter is large, is that you monitor the puppies to ensure proper feeding is happening, look for signs that the puppies are nursing and weigh them daily to see evidence of growth and weight gain. In general, your puppies should be feeding about every 2 to 2.5 hours.
If the female is not in good health and properly nourished or is producing insufficient milk for the litter, then you may need supplemental options.
Now, this only goes on for about a month, then you can start to transition them to regular food (as the linked ASPCA article mentions). Obviously, the milk from the mother is not sufficient to bring the puppies to full growth, but it should be sufficient to bring them to a point that they can get nutrients from other sources.