One of the (very few) drawbacks of a baby being purely breastfed for the first six months of life is that the baby could become deficient in Vitamin D.
Breast milk is not rich in Vitamin D which is necessary for proper calcium absorption in the gut and to prevent childhood diseases like rickets.
Vitamin D is not manufactured by the body; the best source of Vitamin D is the sun. It is recommended that about 10 to 15 minutes of sun exposure a day is adequate for baby’s skin to absorb the requisite Vitamin D. Sun exposure on the arms and face for this length of time is adequate.
However, if you live in a part of the world that makes sun exposure difficult, either because of higher latitudes, cold or because baby is required to be in several layers of clothes, then it is recommended that babies be given a Vitamin D supplement. Discuss this requirement with your pediatrician and start as soon as possible for your infant.
When baby starts on solids, he can be fed foods rich in Vitamin D and also products that have been fortified with Vitamin D in order to combat any deficiency.