Sodium is one of the minerals found in human breast milk. An infant can easily process it in that form.
http://www.breastfeedingbasics.org/cgi-bin/deliver.cgi/content/Anatomy/com_minerals.html
Can I put salt in my baby’s food?
“There’s no need to add salt to your baby’s food. Babies need only a very small amount of salt: less than 1g (0.4g sodium) a day until they are 12 months. Your baby’s kidneys can’t cope with more salt than this.
Before your baby is six months old, he will get all the sodium he needs from breastmilk or infant formula milk.
Once your baby begins eating solid food, you don’t need to add any salt to home-cooked baby food or commercial baby food, even if you think it tastes bland.
Toddlers also need very little salt. After your baby’s first birthday, the daily recommended maximum amount of salt for him until he is three years old is 2g a day (0.8g sodium).
Be careful not to give your baby any pre-prepared foods that aren’t specially made for babies. Foods such as adult breakfast cereals and pasta sauces can be high in salt.”