Final answer:
Exclusively breastfed infants should be given a daily supplement of vitamin D starting in the first 2 months of life to ensure adequate bone development and overall health, due to the limited natural dietary sources and sun exposure.
Step-by-step explanation:
Exclusively breastfed infants should be given a daily supplement of vitamin D beginning in the first 2 months of life. Breast milk provides many nutrients and immunoglobulins for a baby's growth and protection against infections, but it may not always provide adequate levels of vitamin D, which is crucial for bone development and general health. Since vitamin D is not found naturally in many foods and is primarily obtained from sun exposure, infants, who have limited sun exposure, are especially at risk for deficiency.
The daily requirement of vitamin D for lactating women is 400 units to ensure they can provide sufficient nutrition to their infants. However, to ensure infants get the appropriate intake, a daily supplement is recommended. Foods such as fish (salmon and tuna), fortified milk, and cereal can provide vitamin D, but for exclusively breastfed infants, a direct supplement is the most reliable method to meet their needs.