There are several reasons to delay the introduction of cow's milk until your baby reaches his first birthday.
Babies can't digest cow's milk as completely or easily as breast milk or formula. Cow's milk contains high concentrations of protein and minerals, which can tax your baby's immature kidneys. In addition, cow's milk doesn't have the right amounts of iron, vitamin C, and other nutrients for infants. It may even cause iron-deficiency anemia in some babies, since cow's milk protein can irritate the lining of the digestive system, leading to blood in the stools. Finally, cow's milk doesn't provide the healthiest types of fat for growing babies.
Once your child's ready to digest it, though, milk becomes an important part of his diet. It's a rich source of calcium, which builds strong bones and teeth and helps regulate blood clotting and muscle control. And it's one of the few sources of vitamin D, which helps the body absorb calcium and is crucial for bone growth. Almost all milk in the U.S. is fortified with vitamin D. (Ultraviolet rays are another source, but they're blocked by sunscreen.)
Milk also provides protein for growth, as well as carbohydrates, which will give your child the energy he needs to toddle all day. And if your child gets enough calcium from the get-go, there's evidence that he'll have a lower risk of high blood pressure, stroke, colon cancer, and hip fractures later in life.
According to the American Academy of Pediatrics (AAP), most kids will get enough calcium and vitamin D if they drink 16 to 20 ounces (2 to 2 1/2 cups) of cow's milk a day. Offer 1-year-olds whole milk (unless they're at high risk for obesity).
Don't offer more than 3 cups of milk a day or your child may not have room for the other foods she needs to round out her diet. If your toddler's still thirsty, offer water.