Puppies go through great changes in their bodies from the time of birth to the time that they are weaned from their mother. These changes are so tremendous that they can be seen taking place on a day-to-day basis. During this period of growth the type of food that they receive should be changing to keep up with the needs of their growing body.

A puppy's first stage of eating takes place in their mother's womb. This food comes in a form of nutrients that are found through the mother's blood and are carried over to the puppy's blood without him having to do anything. After their birth, a puppy's fragile digestive tract are only able to absorb nutrients that are easily digested. These well absorbed nutrients are provided by their mother in the form of milk.

During the following weeks of nursing the puppies, ingredients found in the mother's milk is also changing. During the first 24 hours after the birth of the puppies (whelping), the mother's milk is largely made of a substance called colostrum. This substance is very high in proteins, vitamins, and calories, and provide the puppies with protective anti-bodies that help to build up their immune system. A healthy immune system protects their delicate and susceptible bodies from a number of infections and illnesses. This type of milk also provides them with the energy that they need for the first 24 hours of their lives.

After the initial burst of nutrients from the colostrum is transferred to the puppies, the mother then stops producing the substance from her milk. During the following days, the mother's milk changes in concentration. It becomes more mature, more concentrated, and has more nutrients to accommodate the puppies' changing bodies. As they approach the weaning stage, the milk changes again and becomes more diluted as the puppies diet changes and eats more solid foods.

The first two to five days after birth is considered to be the most crucial period that occurs in a dog's life. The other critical period happens at weaning and two to three weeks following that. This is the time when the puppy's digestive tract changes in order to adapt from their mother's milk to their new diet of solid foods.

The transition from mother's milk to solid food should take place gradually, and automatically. Forcing this process to happen in a shorter amount of time causes more stress and more difficulty for the puppies. Puppies' sensitive digestive tract should be trained to handle different types of food by slowly decreasing the amount of milk and at the same time increasing the amount of solid food.